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DBI - Database independent interface for Perl |
DBI - Database independent interface for Perl
use DBI;
@driver_names = DBI->available_drivers; @data_sources = DBI->data_sources($driver_name, \%attr);
$dbh = DBI->connect($data_source, $username, $auth, \%attr);
$rv = $dbh->do($statement); $rv = $dbh->do($statement, \%attr); $rv = $dbh->do($statement, \%attr, @bind_values);
$ary_ref = $dbh->selectall_arrayref($statement); $hash_ref = $dbh->selectall_hashref($statement, $key_field);
$ary_ref = $dbh->selectcol_arrayref($statement); $ary_ref = $dbh->selectcol_arrayref($statement, \%attr);
@row_ary = $dbh->selectrow_array($statement); $ary_ref = $dbh->selectrow_arrayref($statement); $hash_ref = $dbh->selectrow_hashref($statement);
$sth = $dbh->prepare($statement); $sth = $dbh->prepare_cached($statement);
$rc = $sth->bind_param($p_num, $bind_value); $rc = $sth->bind_param($p_num, $bind_value, $bind_type); $rc = $sth->bind_param($p_num, $bind_value, \%attr);
$rv = $sth->execute; $rv = $sth->execute(@bind_values);
$rc = $sth->bind_param_array($p_num, $bind_values, \%attr); $rv = $sth->execute_array(\%attr); $rv = $sth->execute_array(\%attr, @bind_values);
$rc = $sth->bind_col($col_num, \$col_variable); $rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);
@row_ary = $sth->fetchrow_array; $ary_ref = $sth->fetchrow_arrayref; $hash_ref = $sth->fetchrow_hashref;
$ary_ref = $sth->fetchall_arrayref; $ary_ref = $sth->fetchall_arrayref( $slice, $max_rows );
$hash_ref = $sth->fetchall_hashref( $key_field );
$rv = $sth->rows;
$rc = $dbh->begin_work; $rc = $dbh->commit; $rc = $dbh->rollback;
$quoted_string = $dbh->quote($string);
$rc = $h->err; $str = $h->errstr; $rv = $h->state;
$rc = $dbh->disconnect;
The synopsis above only lists the major methods and parameters.
If you have questions about DBI, you can get help from the dbi-users@perl.org mailing list. You can get help on subscribing and using the list by emailing dbi-users-help@perl.org.
(To help you make the best use of the dbi-users mailing list, and any other lists or forums you may use, I strongly recommend that you read ``How To Ask Questions The Smart Way'' by Eric Raymond: http://www.catb.org/~esr/faqs/smart-questions.html.)
The DBI home page at http://dbi.perl.org/ is always worth a visit and includes an FAQ and links to other resources.
Before asking any questions, reread this document, consult the
archives and read the DBI FAQ. The archives are listed
at the end of this document and on the DBI home page.
An FAQ is installed as a the DBI::FAQ manpage module so
you can read it by executing perldoc DBI::FAQ.
However the DBI::FAQ module is currently (2003) outdated relative
to the online FAQ on the DBI home page.
This document often uses terms like references, objects, methods. If you're not familar with those terms then it would be a good idea to read at least the following perl manuals first: the perlreftut manpage, the perldsc manpage, the perllol manpage, and the perlboot manpage.
Please note that Tim Bunce does not maintain the mailing lists or the web page (generous volunteers do that). So please don't send mail directly to him; he just doesn't have the time to answer questions personally. The dbi-users mailing list has lots of experienced people who should be able to help you if you need it. If you do email Tim he's very likely to just forward it to the mailing list.
This is the DBI specification that corresponds to the DBI version 1.39
($Date: 2003/11/27 23:29:06 $).
The DBI is evolving at a steady pace, so it's good to check that you have the latest copy.
The significant user-visible changes in each release are documented
in the the DBI::Changes manpage module so you can read them by executing
perldoc DBI::Changes.
Some DBI changes require changes in the drivers, but the drivers can take some time to catch up. Newer versions of the DBI have added features that may not yet be supported by the drivers you use. Talk to the authors of your drivers if you need a new feature that's not yet supported.
Features added after DBI 1.21 (February 2002) are marked in the text with the version number of the DBI release they first appeared in.
Extensions to the DBI API often use the DBIx::* namespace.
See Naming Conventions and Name Space. DBI extension modules
can be found at http://search.cpan.org/search?mode=module&query=DBIx%3A%3A.
And all modules related to the DBI can be found at
http://search.cpan.org/search?query=DBI&mode=all.
The DBI is a database access module for the Perl programming language. It defines a set of methods, variables, and conventions that provide a consistent database interface, independent of the actual database being used.
It is important to remember that the DBI is just an interface. The DBI is a layer of ``glue'' between an application and one or more database driver modules. It is the driver modules which do most of the real work. The DBI provides a standard interface and framework for the drivers to operate within.
|<- Scope of DBI ->|
.-. .--------------. .-------------.
.-------. | |---| XYZ Driver |---| XYZ Engine |
| Perl | | | `--------------' `-------------'
| script| |A| |D| .--------------. .-------------.
| using |--|P|--|B|---|Oracle Driver |---|Oracle Engine|
| DBI | |I| |I| `--------------' `-------------'
| API | | |...
|methods| | |... Other drivers
`-------' | |...
`-'
The API, or Application Programming Interface, defines the call interface and variables for Perl scripts to use. The API is implemented by the Perl DBI extension.
The DBI ``dispatches'' the method calls to the appropriate driver for actual execution. The DBI is also responsible for the dynamic loading of drivers, error checking and handling, providing default implementations for methods, and many other non-database specific duties.
Each driver contains implementations of the DBI methods using the private interface functions of the corresponding database engine. Only authors of sophisticated/multi-database applications or generic library functions need be concerned with drivers.
The following conventions are used in this document:
$dbh Database handle object $sth Statement handle object $drh Driver handle object (rarely seen or used in applications) $h Any of the handle types above ($dbh, $sth, or $drh) $rc General Return Code (boolean: true=ok, false=error) $rv General Return Value (typically an integer) @ary List of values returned from the database, typically a row of data $rows Number of rows processed (if available, else -1) $fh A filehandle undef NULL values are represented by undefined values in Perl \%attr Reference to a hash of attribute values passed to methods
Note that Perl will automatically destroy database and statement handle objects if all references to them are deleted.
To use DBI, first you need to load the DBI module:
use DBI; use strict;
(The use strict; isn't required but is strongly recommended.)
Then you need to connect to your data source and get a handle for that connection:
$dbh = DBI->connect($dsn, $user, $password,
{ RaiseError => 1, AutoCommit => 0 });
Since connecting can be expensive, you generally just connect at the start of your program and disconnect at the end.
Explicitly defining the required AutoCommit behaviour is strongly
recommended and may become mandatory in a later version. This
determines whether changes are automatically committed to the
database when executed, or need to be explicitly committed later.
The DBI allows an application to ``prepare'' statements for later
execution. A prepared statement is identified by a statement handle
held in a Perl variable.
We'll call the Perl variable $sth in our examples.
The typical method call sequence for a SELECT statement is:
prepare,
execute, fetch, fetch, ...
execute, fetch, fetch, ...
execute, fetch, fetch, ...
for example:
$sth = $dbh->prepare("SELECT foo, bar FROM table WHERE baz=?");
$sth->execute( $baz );
while ( @row = $sth->fetchrow_array ) {
print "@row\n";
}
The typical method call sequence for a non-SELECT statement is:
prepare,
execute,
execute,
execute.
for example:
$sth = $dbh->prepare("INSERT INTO table(foo,bar,baz) VALUES (?,?,?)");
while(<CSV>) {
chomp;
my ($foo,$bar,$baz) = split /,/;
$sth->execute( $foo, $bar, $baz );
}
The do() method can be used for non repeated non-SELECT statement
(or with drivers that don't support placeholders):
$rows_affected = $dbh->do("UPDATE your_table SET foo = foo + 1");
To commit your changes to the database (when AutoCommit is off):
$dbh->commit; # or call $dbh->rollback; to undo changes
Finally, when you have finished working with the data source, you should disconnect from it:
$dbh->disconnect;
The DBI does not have a concept of a ``current session''. Every session
has a handle object (i.e., a $dbh) returned from the connect method.
That handle object is used to invoke database related methods.
Most data is returned to the Perl script as strings. (Null values are
returned as undef.) This allows arbitrary precision numeric data to be
handled without loss of accuracy. Beware that Perl may not preserve
the same accuracy when the string is used as a number.
Dates and times are returned as character strings in the current default format of the corresponding database engine. Time zone effects are database/driver dependent.
Perl supports binary data in Perl strings, and the DBI will pass binary data to and from the driver without change. It is up to the driver implementors to decide how they wish to handle such binary data.
Most databases that understand multiple character sets have a default global charset. Text stored in the database is, or should be, stored in that charset; if not, then that's the fault of either the database or the application that inserted the data. When text is fetched it should be automatically converted to the charset of the client, presumably based on the locale. If a driver needs to set a flag to get that behaviour, then it should do so; it should not require the application to do that.
Multiple SQL statements may not be combined in a single statement
handle ($sth), although some databases and drivers do support this
(notably Sybase and SQL Server).
Non-sequential record reads are not supported in this version of the DBI. In other words, records can only be fetched in the order that the database returned them, and once fetched they are forgotten.
Positioned updates and deletes are not directly supported by the DBI.
See the description of the CursorName attribute for an alternative.
Individual driver implementors are free to provide any private
functions and/or handle attributes that they feel are useful.
Private driver functions can be invoked using the DBI func() method.
Private driver attributes are accessed just like standard attributes.
Many methods have an optional \%attr parameter which can be used to
pass information to the driver implementing the method. Except where
specifically documented, the \%attr parameter can only be used to pass
driver specific hints. In general, you can ignore \%attr parameters
or pass it as undef.
The DBI package and all packages below it (DBI::*) are reserved for
use by the DBI. Extensions and related modules use the DBIx::
namespace (see http://www.perl.com/CPAN/modules/by-module/DBIx/).
Package names beginning with DBD:: are reserved for use
by DBI database drivers. All environment variables used by the DBI
or by individual DBDs begin with ``DBI_'' or ``DBD_''.
The letter case used for attribute names is significant and plays an important part in the portability of DBI scripts. The case of the attribute name is used to signify who defined the meaning of that name and its values.
Case of name Has a meaning defined by ------------ ------------------------ UPPER_CASE Standards, e.g., X/Open, ISO SQL92 etc (portable) MixedCase DBI API (portable), underscores are not used. lower_case Driver or database engine specific (non-portable)
It is of the utmost importance that Driver developers only use
lowercase attribute names when defining private attributes. Private
attribute names must be prefixed with the driver name or suitable
abbreviation (e.g., ``ora_'' for Oracle, ``ing_'' for Ingres, etc).
Most DBI drivers require applications to use a dialect of SQL (Structured Query Language) to interact with the database engine. The SQL Standards Reference Information section provides links to useful information about SQL.
The DBI itself does not mandate or require any particular language to be used; it is language independent. In ODBC terms, the DBI is in ``pass-thru'' mode, although individual drivers might not be. The only requirement is that queries and other statements must be expressed as a single string of characters passed as the first argument to the prepare or do methods.
For an interesting diversion on the real history of RDBMS and SQL, from the people who made it happen, see:
http://ftp.digital.com/pub/DEC/SRC/technical-notes/SRC-1997-018-html/sqlr95.html
Follow the ``Full Contents'' then ``Intergalactic dataspeak'' links for the SQL history.
Some drivers support placeholders and bind values. Placeholders, also called parameter markers, are used to indicate values in a database statement that will be supplied later, before the prepared statement is executed. For example, an application might use the following to insert a row of data into the SALES table:
INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)
or the following, to select the description for a product:
SELECT description FROM products WHERE product_code = ?
The ? characters are the placeholders. The association of actual
values with placeholders is known as binding, and the values are
referred to as bind values.
Note that the ? is not enclosed in quotation marks, even when the
placeholder represents a string. Some drivers also allow placeholders
like :name and :n (e.g., :1, :2, and so on)
in addition to ?, but their use is not portable.
With most drivers, placeholders can't be used for any element of a statement that would prevent the database server from validating the statement and creating a query execution plan for it. For example:
"SELECT name, age FROM ?" # wrong (will probably fail) "SELECT name, ? FROM people" # wrong (but may not 'fail')
Also, placeholders can only represent single scalar values. For example, the following statement won't work as expected for more than one value:
"SELECT name, age FROM people WHERE name IN (?)" # wrong "SELECT name, age FROM people WHERE name IN (?,?)" # two names
When using placeholders with the SQL LIKE qualifier, you must
remember that the placeholder substitutes for the whole string.
So you should use ``... LIKE ? ...'' and include any wildcard
characters in the value that you bind to the placeholder.
Null Values
Undefined values, or undef, can be used to indicate null values.
However, care must be taken in the particular case of trying to use
null values to qualify a SELECT statement. Consider:
SELECT description FROM products WHERE product_code = ?
Binding an undef (NULL) to the placeholder will not select rows
which have a NULL product_code! Refer to the SQL manual for your database
engine or any SQL book for the reasons for this. To explicitly select
NULLs you have to say ``WHERE product_code IS NULL'' and to make that
general you have to say:
... WHERE (product_code = ? OR (? IS NULL AND product_code IS NULL))
and bind the same value to both placeholders. Sadly, that more general
syntax doesn't work for Sybase and MS SQL Server. However on those two
servers the original ``product_code = ?'' syntax works for binding nulls.
Performance
Without using placeholders, the insert statement shown previously would have to
contain the literal values to be inserted and would have to be
re-prepared and re-executed for each row. With placeholders, the insert
statement only needs to be prepared once. The bind values for each row
can be given to the execute method each time it's called. By avoiding
the need to re-prepare the statement for each row, the application
typically runs many times faster. Here's an example:
my $sth = $dbh->prepare(q{
INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)
}) or die $dbh->errstr;
while (<>) {
chomp;
my ($product_code, $qty, $price) = split /,/;
$sth->execute($product_code, $qty, $price) or die $dbh->errstr;
}
$dbh->commit or die $dbh->errstr;
See execute and bind_param for more details.
The q{...} style quoting used in this example avoids clashing with
quotes that may be used in the SQL statement. Use the double-quote like
qq{...} operator if you want to interpolate variables into the string.
See Quote and Quote-like Operators in the perlop manpage for more details.
See also the bind_column method, which is used to associate Perl
variables with the output columns of a SELECT statement.
In this section, we cover the DBI class methods, utility functions, and the dynamic attributes associated with generic DBI handles.
Constants representing the values of the SQL standard types can be
imported individually by name, or all together by importing the
special :sql_types tag.
The names and values of all the defined SQL standard types can be produced like this:
foreach (@{ $DBI::EXPORT_TAGS{sql_types} }) {
printf "%s=%d\n", $_, &{"DBI::$_"};
}
These constants are defined by SQL/CLI, ODBC or both.
SQL_BIGINT is (currently) omitted, because SQL/CLI and ODBC provide
conflicting codes.
See the type_info, type_info_all, and bind_param methods for possible uses.
Note that just because the DBI defines a named constant for a given data type doesn't mean that drivers will support that data type.
The following methods are provided by the DBI class:
connect
$dbh = DBI->connect($data_source, $username, $password)
or die $DBI::errstr;
$dbh = DBI->connect($data_source, $username, $password, \%attr)
or die $DBI::errstr;
Establishes a database connection, or session, to the requested $data_source.
Returns a database handle object if the connection succeeds. Use
$dbh->disconnect to terminate the connection.
If the connect fails (see below), it returns undef and sets both $DBI::err
and $DBI::errstr. (It does not set $!, etc.) You should generally
test the return status of connect and print $DBI::errstr if it has failed.
Multiple simultaneous connections to multiple databases through multiple
drivers can be made via the DBI. Simply make one connect call for each
database and keep a copy of each returned database handle.
The $data_source value must begin with ``dbi:driver_name:''.
The driver_name specifies the driver that will be used to make the
connection. (Letter case is significant.)
As a convenience, if the $data_source parameter is undefined or empty,
the DBI will substitute the value of the environment variable DBI_DSN.
If just the driver_name part is empty (i.e., the $data_source
prefix is ``dbi::''), the environment variable DBI_DRIVER is
used. If neither variable is set, then connect dies.
Examples of $data_source values are:
dbi:DriverName:database_name dbi:DriverName:database_name@hostname:port dbi:DriverName:database=database_name;host=hostname;port=port
There is no standard for the text following the driver name. Each
driver is free to use whatever syntax it wants. The only requirement the
DBI makes is that all the information is supplied in a single string.
You must consult the documentation for the drivers you are using for a
description of the syntax they require. (Where a driver author needs
to define a syntax for the $data_source, it is recommended that
they follow the ODBC style, shown in the last example above.)
If the environment variable DBI_AUTOPROXY is defined (and the
driver in $data_source is not ``Proxy'') then the connect request
will automatically be changed to:
$ENV{DBI_AUTOPROXY};dsn=$data_source
DBI_AUTOPROXY is typically set as ``dbi:Proxy:hostname=...;port=...''.
If $ENV{DBI_AUTOPROXY} doesn't begin with 'dbi:' then ``dbi:Proxy:''
will be prepended to it first. See the DBD::Proxy documentation
for more details.
If $username or $password are undefined (rather than just empty),
then the DBI will substitute the values of the DBI_USER and DBI_PASS
environment variables, respectively. The DBI will warn if the
environment variables are not defined. However, the everyday use
of these environment variables is not recommended for security
reasons. The mechanism is primarily intended to simplify testing.
See below for alternative way to specify the username and password.
DBI->connect automatically installs the driver if it has not been
installed yet. Driver installation either returns a valid driver
handle, or it dies with an error message that includes the string
``install_driver'' and the underlying problem. So DBI->connect
will die
on a driver installation failure and will only return undef on a
connect failure, in which case $DBI::errstr will hold the error message.
The $data_source argument (with the ``dbi:...:'' prefix removed) and the
$username and $password arguments are then passed to the driver for
processing. The DBI does not define any interpretation for the
contents of these fields. The driver is free to interpret the
$data_source, $username, and $password fields in any way, and supply
whatever defaults are appropriate for the engine being accessed.
(Oracle, for example, uses the ORACLE_SID and TWO_TASK environment
variables if no $data_source is specified.)
The AutoCommit and PrintError attributes for each connection default to
``on''. (See AutoCommit and PrintError for more information.)
However, it is strongly recommended that you explicitly define AutoCommit
rather than rely on the default. Future versions of
the DBI may issue a warning if AutoCommit is not explicitly defined.
The \%attr parameter can be used to alter the default settings of
PrintError, RaiseError, AutoCommit, and other attributes. For example:
$dbh = DBI->connect($data_source, $user, $pass, {
PrintError => 0,
AutoCommit => 0
});
The username and password can also be specified using the attributes
Username and Password, in which case they take precedence
over the $username and $password parameters.
You can also define connection attribute values within the $data_source
parameter. For example:
dbi:DriverName(PrintError=>0,Taint=>1):...
Individual attributes values specified in this way take precedence over
any conflicting values specified via the \%attr parameter to connect.
The dbi_connect_method attribute can be used to specify which driver
method should be called to establish the connection. The only useful
values are 'connect', 'connect_cached', or some specialized case like
'Apache::DBI::connect' (which is automatically the default when running
within Apache).
Where possible, each session ($dbh) is independent from the transactions
in other sessions. This is useful when you need to hold cursors open
across transactions--for example, if you use one session for your long lifespan
cursors (typically read-only) and another for your short update
transactions.
For compatibility with old DBI scripts, the driver can be specified by
passing its name as the fourth argument to connect (instead of \%attr):
$dbh = DBI->connect($data_source, $user, $pass, $driver);
In this ``old-style'' form of connect, the $data_source should not start
with ``dbi:driver_name:''. (If it does, the embedded driver_name
will be ignored). Also note that in this older form of connect,
the $dbh->{AutoCommit} attribute is undefined, the
$dbh->{PrintError} attribute is off, and the old DBI_DBNAME
environment variable is
checked if DBI_DSN is not defined. Beware that this ``old-style''
connect will be withdrawn in a future version of DBI.
connect_cached
$dbh = DBI->connect_cached($data_source, $username, $password)
or die $DBI::errstr;
$dbh = DBI->connect_cached($data_source, $username, $password, \%attr)
or die $DBI::errstr;
connect_cached is like connect, except that the database handle
returned is also
stored in a hash associated with the given parameters. If another call
is made to connect_cached with the same parameter values, then the
corresponding cached $dbh will be returned if it is still valid.
The cached database handle is replaced with a new connection if it
has been disconnected or if the ping method fails.
Note that the behaviour of this method differs in several respects from the behaviour of persistent connections implemented by Apache::DBI.
Caching connections can be useful in some applications, but it can also cause problems, such as too many connections, and so should be used with care.
The cache can be accessed (and cleared) via the CachedKids attribute.
available_drivers@ary = DBI->available_drivers; @ary = DBI->available_drivers($quiet);
Returns a list of all available drivers by searching for DBD::* modules
through the directories in @INC. By default, a warning is given if
some drivers are hidden by others of the same name in earlier
directories. Passing a true value for $quiet will inhibit the warning.
installed_versionsDBI->installed_versions; @ary = DBI->installed_versions; %hash = DBI->installed_versions;
Calls available_drivers() and attempts to load each of them in turn
using install_driver(). For each load that succeeds the driver
name and version number are added to a hash. When running under
the DBI::PurePerl manpage drivers which appear not be pure-perl are ignored.
When called in array context the list of successfully loaded drivers is returned (without the 'DBD::' prefix).
When called in scalar context a reference to the hash is returned
and the hash will also contain other entries for the DBI version,
OS name, etc.
When called in a void context the installed_versions() method will
print out a formatted list of the hash contents, one per line.
Due to the potentially high memory cost and unknown risks of loading in an unknown number of drivers that just happen to be installed on the system, this method is nor recommended for general use. It is primarily intended as a quick way to see from the command line what's installed. For example:
perl -MDBI -e 'DBI->installed_versions'
The installed_versions() method was added in DBI 1.38.
data_sources@ary = DBI->data_sources($driver); @ary = DBI->data_sources($driver, \%attr);
Returns a list of data sources (databases) available via the named
driver. If $driver is empty or undef, then the value of the
DBI_DRIVER environment variable is used.
The driver will be loaded if it hasn't been already. Note that if the
driver loading fails then data_sources() dies with an error message
that includes the string ``install_driver'' and the underlying problem.
Data sources are returned in a form suitable for passing to the
connect method (that is, they will include the ``dbi:$driver:'' prefix).
Note that many drivers have no way of knowing what data sources might be available for it. These drivers return an empty or incomplete list or may require driver-specific attributes.
There is also a data_sources() method defined for database handles.
traceDBI->trace($trace_level) DBI->trace($trace_level, $trace_filename)
DBI trace information can be enabled for all handles using the trace
DBI class method. To enable trace information for a specific handle, use
the similar $h->trace method described elsewhere.
Trace levels are as follows:
0 - Trace disabled.
1 - Trace DBI method calls returning with results or errors.
2 - Trace method entry with parameters and returning with results.
3 - As above, adding some high-level information from the driver
and some internal information from the DBI.
4 - As above, adding more detailed information from the driver.
5 and above - As above but with more and more obscure information.
Trace level 1 is best for a simple overview of what's happening. Trace level 2 is a good choice for general purpose tracing. Levels 3 and above (up to 9) are best reserved for investigating a specific problem, when you need to see ``inside'' the driver and DBI.
The trace output is detailed and typically very useful. Much of the trace output is formatted using the neat function, so strings in the trace output may be edited and truncated.
Initially trace output is written to STDERR. If $trace_filename is
specified and can be opened in append mode then all trace
output (including that from other handles) is redirected to that file.
A warning is generated is the file can't be opened.
Further calls to trace without a $trace_filename do not alter where
the trace output is sent. If $trace_filename is undefined, then
trace output is sent to STDERR and the previous trace file is closed.
The trace method returns the previous tracelevel.
See also the $h->trace and $h->trace_msg methods and the
DEBUGGING section
for information about the DBI_TRACE environment variable.
In addition to the methods listed in the previous section, the DBI package also provides these utility functions:
neat$str = DBI::neat($value); $str = DBI::neat($value, $maxlen);
Return a string containing a neat (and tidy) representation of the supplied value.
Strings will be quoted, although internal quotes will not be escaped.
Values known to be numeric will be unquoted. Undefined (NULL) values
will be shown as undef (without quotes).
If the string is flagged internally as utf8 then double quotes will be used, otherwise single quotes are used and unprintable characters will be replaced by dot (.).
For result strings longer than $maxlen the result string will be
truncated to $maxlen-4 and ``...''' will be appended. If $maxlen is 0
or undef, it defaults to $DBI::neat_maxlen which, in turn, defaults to 400.
This function is designed to format values for human consumption. It is used internally by the DBI for trace output. It should typically not be used for formatting values for database use. (See also quote.)
neat_list$str = DBI::neat_list(\@listref, $maxlen, $field_sep);
Calls DBI::neat on each element of the list and returns a string
containing the results joined with $field_sep. $field_sep defaults
to ", ".
looks_like_number@bool = DBI::looks_like_number(@array);
Returns true for each element that looks like a number.
Returns false for each element that does not look like a number.
Returns undef for each element that is undefined or empty.
hash$hash_value = DBI::hash($buffer, $type);
Return a 32-bit integer 'hash' value corresponding to the contents of $buffer. The $type parameter selects which kind of hash algorithm should be used.
For the technically curious, type 0 (which is the default if $type isn't specified) is based on the Perl 5.1 hash except that the value is forced to be negative (for obscure historical reasons). Type 1 is the better ``Fowler / Noll / Vo'' (FNV) hash. See http://www.isthe.com/chongo/tech/comp/fnv/ for more information. Both types are implemented in C and are very fast.
This function doesn't have much to do with databases, except that it can be handy to store hash values in a database.
Dynamic attributes are always associated with the last handle used
(that handle is represented by $h in the descriptions below).
Where an attribute is equivalent to a method call, then refer to the method call for all related documentation.
Warning: these attributes are provided as a convenience but they do have limitations. Specifically, they have a short lifespan: because they are associated with the last handle used, they should only be used immediately after calling the method that ``sets'' them. If in any doubt, use the corresponding method call.
$DBI::err$h->err.
$DBI::errstr$h->errstr.
$DBI::state$h->state.
$DBI::rows$h->rows. Please refer to the documentation
for the rows method.
$DBI::lasth
The following methods can be used by all types of DBI handles.
err$rv = $h->err;
Returns the native database engine error code from the last driver method called. The code is typically an integer but you should not assume that.
The DBI resets $h->err to undef before most DBI method calls, so the value only has a short lifespan. Also, most drivers share the same error variables across all their handles, so calling a method on one handle will typically reset the error on all the other handles that are children of that driver.
If you need to test for individual errors and have your program be portable to different database engines, then you'll need to determine what the corresponding error codes are for all those engines and test for all of them.
errstr$str = $h->errstr;
Returns the native database engine error message from the last driver method called. This has the same lifespan issues as the err method described above.
state$str = $h->state;
Returns an error code in the standard SQLSTATE five character format.
Note that the specific success code 00000 is translated to ''
(false). If the driver does not support SQLSTATE (and most don't),
then state will return S1000 (General Error) for all errors.
The driver is free to return any value via state, e.g., warning
codes, even if it has not declared an error by returning a true value
via the err method described above.
set_err$rv = $h->set_err($err, $errstr); $rv = $h->set_err($err, $errstr, $state, $method); $rv = $h->set_err($err, $errstr, $state, $method, $rv);
Set the err, errstr, and state values for the handle.
This will trigger the normal DBI error handling mechanisms,
such as RaiseError and HandleError, if they are enabled.
This method is typically only used by DBI drivers and DBI subclasses.
The $method parameter provides an alternate method name, instead
of the fairly unhelpful 'set_err', for the
RaiseError/PrintError error string.
The set_err method normally returns undef. The $rv parameter
provides an alternate return value. The HandleError subroutine
can access and alter this value.
trace$h->trace($trace_level); $h->trace($trace_level, $trace_filename);
DBI trace information can be enabled for a specific handle (and any
future children of that handle) by setting the trace level using the
trace method.
Trace level 1 is best for a simple overview of what's happening.
Trace level 2 is a good choice for general purpose tracing. Levels 3
and above (up to 9) are best reserved for investigating a
specific problem, when you need to see ``inside'' the driver and DBI.
Set $trace_level to 0 to disable the trace.
The trace output is detailed and typically very useful. Much of the trace output is formatted using the neat function, so strings in the trace output may be edited and truncated.
Initially, trace output is written to STDERR. If $trace_filename is
specified, then the file is opened in append mode and all trace
output (including that from other handles) is redirected to that file.
Further calls to trace without a $trace_filename do not alter where
the trace output is sent. If $trace_filename is undefined, then
trace output is sent to STDERR and the previous trace file is closed.
See also the DBI->trace method, the $h->{TraceLevel} attribute,
and DEBUGGING for information about the DBI_TRACE environment variable.
trace_msg$h->trace_msg($message_text); $h->trace_msg($message_text, $min_level);
Writes $message_text to the trace file if trace is enabled for $h or
for the DBI as a whole. Can also be called as DBI->trace_msg($msg).
See trace.
If $min_level is defined, then the message is output only if the trace
level is equal to or greater than that level. $min_level defaults to 1.
func$h->func(@func_arguments, $func_name) or die ...;
The func method can be used to call private non-standard and
non-portable methods implemented by the driver. Note that the function
name is given as the last argument.
It's also important to note that the func() method does not clear
a previous error ($DBI::err etc.) and it does not trigger automatic
error detection (RaiseError etc.) so you must check the return
status and/or $h->err to detect errors.
(This method is not directly related to calling stored procedures. Calling stored procedures is currently not defined by the DBI. Some drivers, such as DBD::Oracle, support it in non-portable ways. See driver documentation for more details.)
See also install_method for how you can avoid needing to
use func() and gain.
can$is_implemented = $h->can($method_name);
Returns true if $method_name is implemented by the driver or a default method is provided by the DBI. It returns false where a driver hasn't implemented a method and the default method is provided by the DBI is just an empty stub.
These attributes are common to all types of DBI handles.
Some attributes are inherited by child handles. That is, the value of an inherited attribute in a newly created statement handle is the same as the value in the parent database handle. Changes to attributes in the new statement handle do not affect the parent database handle and changes to the database handle do not affect existing statement handles, only future ones.
Attempting to set or get the value of an unknown attribute is fatal, except for private driver specific attributes (which all have names starting with a lowercase letter).
Example:
$h->{AttributeName} = ...; # set/write
... = $h->{AttributeName}; # get/read
Warn (boolean, inherited)Warn attribute enables useful warnings for certain bad practices. Enabled by default. Some
emulation layers, especially those for Perl 4 interfaces, disable warnings.
Since warnings are generated using the Perl warn function, they can be
intercepted using the Perl $SIG{__WARN__} hook.
Active (boolean, read-only)Active attribute is true if the handle object is ``active''. This is rarely used in
applications. The exact meaning of active is somewhat vague at the
moment. For a database handle it typically means that the handle is
connected to a database ($dbh->disconnect sets Active off). For
a statement handle it typically means that the handle is a SELECT
that may have more data to fetch. (Fetching all the data or calling $sth->finish
sets Active off.)
Kids (integer, read-only)Kids is the number of currently existing database
handles that were created from that driver handle. For a database
handle, Kids is the number of currently existing statement handles that
were created from that database handle.
For a statement handle, the value is zero.
ActiveKids (integer, read-only)Kids, but only counting those that are Active (as above).
CachedKids (hash ref)CachedKids returns a reference to the cache (hash) of
statement handles created by the prepare_cached method. For a
driver handle, returns a reference to the cache (hash) of
database handles created by the connect_cached method.
CompatMode (boolean, inherited)CompatMode attribute is used by emulation layers (such as
Oraperl) to enable compatible behaviour in the underlying driver
(e.g., DBD::Oracle) for this handle. Not normally set by application code.
It also has the effect of disabling the 'quick FETCH' of attribute values from the handles attribute cache. So all attribute values are handled by the drivers own FETCH method. This makes them slightly slower but is useful for special-purpose drivers like DBD::Multiplex.
InactiveDestroy (boolean)InactiveDestroy attribute can be used to disable the database
engine related effect of DESTROYing a handle (which would normally
close a prepared statement or disconnect from the database etc).
The default value, false, means a handle will be fully destroyed
when it passes out of scope.
For a database handle, this attribute does not disable an explicit
call to the disconnect method, only the implicit call from DESTROY
that happens if the handle is still marked as Active.
Think of the name as meaning 'treat the handle as not-Active in the DESTROY method'.
This attribute is specifically designed for use in Unix applications
that ``fork'' child processes. Either the parent or the child process,
but not both, should set InactiveDestroy on all their shared handles.
Note that some databases, including Oracle, don't support passing a
database connection across a fork.
To help tracing applications using fork the process id is shown in
the trace log whenever a DBI or handle trace() method is called.
The process id also shown for every method call if the DBI trace
level (not handle trace level) is set high enough to show the trace
from the DBI's method dispatcher, e.g. >= 9.
PrintError (boolean, inherited)PrintError attribute can be used to force errors to generate warnings (using
warn) in addition to returning error codes in the normal way. When set
``on'', any method which results in an error occuring will cause the DBI to
effectively do a warn("$class $method failed: $DBI::errstr") where $class
is the driver class and $method is the name of the method which failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
By default, DBI->connect sets PrintError ``on''.
If desired, the warnings can be caught and processed using a $SIG{__WARN__}
handler or modules like CGI::Carp and CGI::ErrorWrap.
RaiseError (boolean, inherited)RaiseError attribute can be used to force errors to raise exceptions rather
than simply return error codes in the normal way. It is ``off'' by default.
When set ``on'', any method which results in an error will cause
the DBI to effectively do a die("$class $method failed: $DBI::errstr"),
where $class is the driver class and $method is the name of the method
that failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
If you turn RaiseError on then you'd normally turn PrintError off.
If PrintError is also on, then the PrintError is done first (naturally).
Typically RaiseError is used in conjunction with eval { ... }
to catch the exception that's been thrown and followed by an
if ($@) { ... } block to handle the caught exception. In that eval
block the $DBI::lasth variable can be useful for diagnosis and reporting.
For example, $DBI::lasth->{Type} and $DBI::lasth->{Statement}.
If you want to temporarily turn RaiseError off (inside a library function
that is likely to fail, for example), the recommended way is like this:
{
local $h->{RaiseError}; # localize and turn off for this block
...
}
The original value will automatically and reliably be restored by Perl,
regardless of how the block is exited.
The same logic applies to other attributes, including PrintError.
HandleError (code ref, inherited)HandleError attribute can be used to provide your own alternative behaviour
in case of errors. If set to a reference to a subroutine then that
subroutine is called when an error is detected (at the same point that
RaiseError and PrintError are handled).
The subroutine is called with three parameters: the error message
string that RaiseError and PrintError would use,
the DBI handle being used, and the first value being returned by
the method that failed (typically undef).
If the subroutine returns a false value then the RaiseError
and/or PrintError attributes are checked and acted upon as normal.
For example, to die with a full stack trace for any error:
use Carp;
$h->{HandleError} = sub { confess(shift) };
Or to turn errors into exceptions:
use Exception; # or your own favourite exception module
$h->{HandleError} = sub { Exception->new('DBI')->raise($_[0]) };
It is possible to 'stack' multiple HandleError handlers by using closures:
sub your_subroutine {
my $previous_handler = $h->{HandleError};
$h->{HandleError} = sub {
return 1 if $previous_handler and &$previous_handler(@_);
... your code here ...
};
}
Using a my inside a subroutine to store the previous HandleError
value is important. See the perlsub manpage and the perlref manpage for more information
about closures.
It is possible for HandleError to alter the error message that
will be used by RaiseError and PrintError if it returns false.
It can do that by altering the value of $_[0]. This example appends
a stack trace to all errors and, unlike the previous example using
Carp::confess, this will work PrintError as well as RaiseError:
$h->{HandleError} = sub { $_[0]=Carp::longmess($_[0]); 0; };
It is also possible for HandleError to hide an error, to a limited
degree, by using set_err to reset $DBI::err and $DBI::errstr,
and altering the return value of the failed method. For example:
$h->{HandleError} = sub {
return 0 unless $_[0] =~ /^\S+ fetchrow_arrayref failed:/;
return 0 unless $_[1]->err == 1234; # the error to 'hide'
$h->set_err(0,""); # turn off the error
$_[2] = [ ... ]; # supply alternative return value
return 1;
};
This only works for methods which return a single value and is hard to make reliable (avoiding infinite loops, for example) and so isn't recommended for general use! If you find a good use for it then please let me know.
ShowErrorStatement (boolean, inherited)ShowErrorStatement attribute can be used to cause the relevant Statement text to be
appended to the error messages generated by the RaiseError and
PrintError attributes. Only applies to errors on statement handles
plus the prepare(), do(), and the various select*() database handle methods.
(The exact format of the appended text is subject to change.)
If $h->{ParamValues} returns a hash reference of parameter
(placeholder) values then those are formatted and appended to the
end of the Statement text in the error message.
TraceLevel (integer, inherited)TraceLevel attribute can be used as an alternative to the trace method
to set the DBI trace level for a specific handle.
FetchHashKeyName (string, inherited)FetchHashKeyName attribute is used to specify whether the fetchrow_hashref()
method should perform case conversion on the field names used for
the hash keys. For historical reasons it defaults to 'NAME' but
it is recommended to set it to 'NAME_lc' (convert to lower case)
or 'NAME_uc' (convert to upper case) according to your preference.
It can only be set for driver and database handles. For statement
handles the value is frozen when prepare() is called.
ChopBlanks (boolean, inherited)ChopBlanks attribute can be used to control the trimming of trailing space
characters from fixed width character (CHAR) fields. No other field
types are affected, even where field values have trailing spaces.
The default is false (although it is possible that the default may change). Applications that need specific behaviour should set the attribute as needed. Emulation interfaces should set the attribute to match the behaviour of the interface they are emulating.
Drivers are not required to support this attribute, but any driver which
does not support it must arrange to return undef as the attribute value.
LongReadLen (unsigned integer, inherited)LongReadLen attribute may be used to control the maximum length of long fields
(``blob'', ``memo'', etc.) which the driver will read from the
database automatically when it fetches each row of data. The
LongReadLen attribute only relates to fetching and reading long values; it
is not involved in inserting or updating them.
A value of 0 means not to automatically fetch any long data. (fetch
should return undef for long fields when LongReadLen is 0.)
The default is typically 0 (zero) bytes but may vary between drivers. Applications fetching long fields should set this value to slightly larger than the longest long field value to be fetched.
Some databases return some long types encoded as pairs of hex digits.
For these types, LongReadLen relates to the underlying data length and not the
doubled-up length of the encoded string.
Changing the value of LongReadLen for a statement handle after it
has been prepare'd will typically have no effect, so it's common to
set LongReadLen on the $dbh before calling prepare.
Note that the value used here has a direct effect on the memory used by the application, so don't be too generous.
See LongTruncOk for more information on truncation behaviour.
LongTruncOk (boolean, inherited)LongTruncOk attribute may be used to control the effect of fetching a long
field value which has been truncated (typically because it's longer
than the value of the LongReadLen attribute).
By default, LongTruncOk is false and so fetching a long value that
needs to be truncated will cause the fetch to fail.
(Applications should always be sure to
check for errors after a fetch loop in case an error, such as a divide
by zero or long field truncation, caused the fetch to terminate
prematurely.)
If a fetch fails due to a long field truncation when LongTruncOk is
false, many drivers will allow you to continue fetching further rows.
See also LongReadLen.
TaintIn (boolean, inherited)TaintIn attribute is set to a true value and Perl is running in
taint mode (e.g., started with the -T option), then all the arguments
to most DBI method calls are checked for being tainted. This may change.
The attribute defaults to off, even if Perl is in taint mode. See the perlsec manpage for more about taint mode. If Perl is not running in taint mode, this attribute has no effect.
When fetching data that you trust you can turn off the TaintIn attribute, for that statement handle, for the duration of the fetch loop.
The TaintIn attribute was added in DBI 1.31.
TaintOut (boolean, inherited)TaintOut attribute is set to a true value and Perl is running in
taint mode (e.g., started with the -T option), then most data fetched
from the database is considered tainted. This may change.
The attribute defaults to off, even if Perl is in taint mode. See the perlsec manpage for more about taint mode. If Perl is not running in taint mode, this attribute has no effect.
When fetching data that you trust you can turn off the TaintOut attribute, for that statement handle, for the duration of the fetch loop.
Currently only fetched data is tainted. It is possible that the results of other DBI method calls, and the value of fetched attributes, may also be tainted in future versions. That change may well break your applications unless you take great care now. If you use DBI Taint mode, please report your experience and any suggestions for changes.
The TaintOut attribute was added in DBI 1.31.
Taint (boolean, inherited)Taint attribute is a shortcut for TaintIn and TaintOut (it is also present
for backwards compatibility).
Setting this attribute sets both TaintIn and TaintOut, and retrieving it returns a true value if and only if TaintIn and TaintOut are both set to true values.
Profile (inherited)Profile attribute enables the collection and reporting of method call timing statistics.
See the the DBI::Profile manpage module documentation for much more detail.
The Profile attribute was added in DBI 1.24.
private_your_module_name_*private_''.
It is strongly recommended that you use just one private
attribute (e.g., use a hash ref) and give it a long and unambiguous
name that includes the module or application name that the attribute
relates to (e.g., ``private_YourFullModuleName_thingy'').
Because of the way the Perl tie mechanism works you cannot reliably
use the ||= operator directly to initialise the attribute, like this:
my $foo = $dbh->{private_yourmodname_foo} ||= { ... }; # WRONG
you should use a two step approach like this:
my $foo = $dbh->{private_yourmodname_foo};
$foo ||= $dbh->{private_yourmodname_foo} = { ... };
This attribute is primarily of interest to people sub-classing DBI.
This section covers the methods and attributes associated with database handles.
The following methods are specified for DBI database handles:
clone$new_dbh = $dbh->clone(); $new_dbh = $dbh->clone(\%attr);
The clone method duplicates the $dbh connection by connecting
with the same parameters ($dsn, $user, $password) as originally used.
The attributes for the cloned connect are the same as those used for the original connect, with some other attribute merged over them depending on the \%attr parameter.
If \%attr is given then the attributes it contains are merged into the original attributes and override any with the same names. Effectively the same as doing:
%attribues_used = ( %original_attributes, %attr );
If \%attr is not given then it defaults to a hash containing all the attributes in the attribute cache of $dbh excluding any non-code references, plus the main boolean attributes (RaiseError, PrintError, AutoCommit, etc.). This behaviour is subject to change.
The clone method can be used even if the database handle is disconnected.
The clone method was added in DBI 1.33. It is very new and likely
to change.
data_sources@ary = $dbh->data_sources(); @ary = $dbh->data_sources(\%attr);
Returns a list of data sources (databases) available via the $dbh
driver's data_sources() method, plus any extra data sources that
the driver can discover via the connected $dbh. Typically the extra
data sources are other databases managed by the same server process
that the $dbh is connected to.
Data sources are returned in a form suitable for passing to the
connect method (that is, they will include the ``dbi:$driver:'' prefix).
The data_sources() method, for a $dbh, was added in DBI 1.38.
do$rows = $dbh->do($statement) or die $dbh->errstr; $rows = $dbh->do($statement, \%attr) or die $dbh->errstr; $rows = $dbh->do($statement, \%attr, @bind_values) or die ...
Prepare and execute a single statement. Returns the number of rows
affected or undef on error. A return value of -1 means the
number of rows is not known, not applicable, or not available.
This method is typically most useful for non-SELECT statements that
either cannot be prepared in advance (due to a limitation of the
driver) or do not need to be executed repeatedly. It should not
be used for SELECT statements because it does not return a statement
handle (so you can't fetch any data).
The default do method is logically similar to:
sub do {
my($dbh, $statement, $attr, @bind_values) = @_;
my $sth = $dbh->prepare($statement, $attr) or return undef;
$sth->execute(@bind_values) or return undef;
my $rows = $sth->rows;
($rows == 0) ? "0E0" : $rows; # always return true if no error
}
For example:
my $rows_deleted = $dbh->do(q{
DELETE FROM table
WHERE status = ?
}, undef, 'DONE') or die $dbh->errstr;
Using placeholders and @bind_values with the do method can be
useful because it avoids the need to correctly quote any variables
in the $statement. But if you'll be executing the statement many
times then it's more efficient to prepare it once and call
execute many times instead.
The q{...} style quoting used in this example avoids clashing with
quotes that may be used in the SQL statement. Use the double-quote-like
qq{...} operator if you want to interpolate variables into the string.
See Quote and Quote-like Operators in the perlop manpage for more details.
last_insert_id$rv = $dbh->last_insert_id($catalog, $schema, $table, $field); $rv = $dbh->last_insert_id($catalog, $schema, $table, $field, \%attr);
Returns a value 'identifying' the row just inserted, if possible. Typically this would be a value assigned by the database server to a column with an auto_increment or serial type. Returns undef if the driver does not support the method or can't determine the value.
The $catalog, $schema, $table, and $field parameters may be required
for some drivers (see below). If you don't know the parameter values
and your driver does not need them, then use undef for each.
There are several caveats to be aware of with this method if you want to use it for portable applications:
* For some drivers the value may only available immediately after the insert statement has executed (e.g., mysql, Informix).
* For some drivers the $catalog, $schema, $table, and $field parameters are required (e.g., Pg), for others they are ignored (e.g., mysql).
* Drivers may return an indeterminate value if no insert has been performed yet.
* For some drivers the value may only be available if placeholders have not been used (e.g., Sybase, MS SQL). In this case the value returned would be from the last non-placeholder insert statement.
* Some drivers may need driver-specific hints about how to get the value. For example, being told the name of the database 'sequence' object that holds the value. Any such hints are passed as driver-specific attributes in the \%attr parameter.
* If the underlying database offers nothing better, then some
drivers may attempt to implement this method by executing
``select max($field) from $table''. Drivers using any approach
like this should issue a warning if AutoCommit is true because
it is generally unsafe - another process may have modified the table
between your insert and the select. For situations where you know
it is safe, such as when you have locked the table, you can silence
the warning by passing Warn => 0 in \%attr.
* If no insert has been performed yet, or the last insert failed, then the value is implementation defined.
Given all the caveats above, it's clear that this method must be used with care.
The last_insert_id method was added in DBI 1.38.
selectrow_array@row_ary = $dbh->selectrow_array($statement); @row_ary = $dbh->selectrow_array($statement, \%attr); @row_ary = $dbh->selectrow_array($statement, \%attr, @bind_values);
This utility method combines prepare, execute and
fetchrow_array into a single call. If called in a list context, it
returns the first row of data from the statement. The $statement
parameter can be a previously prepared statement handle, in which case
the prepare is skipped.
If any method fails, and RaiseError is not set, selectrow_array
will return an empty list.
If called in a scalar context for a statement handle that has more
than one column, it is undefined whether the driver will return
the value of the first column or the last. So don't do that.
Also, in a scalar context, an undef is returned if there are no
more rows or if an error occurred. That undef can't be distinguished
from an undef returned because the first field value was NULL.
For these reasons you should exercise some caution if you use
selectrow_array in a scalar context.
selectrow_arrayref$ary_ref = $dbh->selectrow_arrayref($statement); $ary_ref = $dbh->selectrow_arrayref($statement, \%attr); $ary_ref = $dbh->selectrow_arrayref($statement, \%attr, @bind_values);
This utility method combines prepare, execute and
fetchrow_arrayref into a single call. It returns the first row of
data from the statement. The $statement parameter can be a previously
prepared statement handle, in which case the prepare is skipped.
If any method fails, and RaiseError is not set, selectrow_array
will return undef.
selectrow_hashref$hash_ref = $dbh->selectrow_hashref($statement); $hash_ref = $dbh->selectrow_hashref($statement, \%attr); $hash_ref = $dbh->selectrow_hashref($statement, \%attr, @bind_values);
This utility method combines prepare, execute and
fetchrow_hashref into a single call. It returns the first row of
data from the statement. The $statement parameter can be a previously
prepared statement handle, in which case the prepare is skipped.
If any method fails, and RaiseError is not set, selectrow_hashref
will return undef.
selectall_arrayref$ary_ref = $dbh->selectall_arrayref($statement); $ary_ref = $dbh->selectall_arrayref($statement, \%attr); $ary_ref = $dbh->selectall_arrayref($statement, \%attr, @bind_values);
This utility method combines prepare, execute and fetchall_arrayref into a single call. It returns a reference to an array containing a reference to an array for each row of data fetched.
The $statement parameter can be a previously prepared statement handle,
in which case the prepare is skipped. This is recommended if the
statement is going to be executed many times.
If RaiseError is not set and any method except fetchall_arrayref
fails then selectall_arrayref will return undef; if
fetchall_arrayref fails then it will return with whatever data
has been fetched thus far. You should check $sth->err
afterwards (or use the RaiseError attribute) to discover if the data is
complete or was truncated due to an error.
The fetchall_arrayref method called by selectall_arrayref
supports a $max_rows parameter. You can specify a value for $max_rows
by including a 'MaxRows' attribute in \%attr.
The fetchall_arrayref method called by selectall_arrayref
also supports a $slice parameter. You can specify a value for $slice by
including a 'Slice' or 'Columns' attribute in \%attr. The only
difference between the two is that if Slice is not defined and
Columns is an array ref, then the array is assumed to contain column
index values (which count from 1), rather than perl array index values.
In which case the array is copied and each value decremented before
passing to /fetchall_arrayref.
selectall_hashref$hash_ref = $dbh->selectall_hashref($statement, $key_field); $hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr); $hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr, @bind_values);
This utility method combines prepare, execute and
fetchall_hashref into a single call. It returns a reference to a
hash containing one entry for each row. The key for each row entry is
specified by $key_field. The value is a reference to a hash returned by
fetchrow_hashref.
The $statement parameter can be a previously prepared statement handle,
in which case the prepare is skipped. This is recommended if the
statement is going to be executed many times.
If any method except fetchrow_hashref fails, and RaiseError is not set,
selectall_hashref will return undef. If fetchrow_hashref fails and
RaiseError is not set, then it will return with whatever data it
has fetched thus far. $DBI::err should be checked to catch that.
selectcol_arrayref$ary_ref = $dbh->selectcol_arrayref($statement); $ary_ref = $dbh->selectcol_arrayref($statement, \%attr); $ary_ref = $dbh->selectcol_arrayref($statement, \%attr, @bind_values);
This utility method combines prepare, execute, and fetching one column from all the rows, into a single call. It returns a reference to an array containing the values of the first column from each row.
The $statement parameter can be a previously prepared statement handle,
in which case the prepare is skipped. This is recommended if the
statement is going to be executed many times.
If any method except fetch fails, and RaiseError is not set,
selectcol_arrayref will return undef. If fetch fails and
RaiseError is not set, then it will return with whatever data it
has fetched thus far. $DBI::err should be checked to catch that.
The selectcol_arrayref method defaults to pushing a single column
value (the first) from each row into the result array. However, it can
also push another column, or even multiple columns per row, into the
result array. This behaviour can be specified via a 'Columns'
attribute which must be a ref to an array containing the column number
or numbers to use. For example:
# get array of id and name pairs:
my $ary_ref = $dbh->selectcol_arrayref("select id, name from table", { Columns=>[1,2] });
my %hash = @$ary_ref; # build hash from key-value pairs so $hash{$id} => name
You can specify a maximum number of rows to fetch by including a
'MaxRows' attribute in \%attr.
prepare$sth = $dbh->prepare($statement) or die $dbh->errstr; $sth = $dbh->prepare($statement, \%attr) or die $dbh->errstr;
Prepares a statement for later execution by the database engine and returns a reference to a statement handle object.
The returned statement handle can be used to get attributes of the statement and invoke the execute method. See Statement Handle Methods.
Drivers for engines without the concept of preparing a
statement will typically just store the statement in the returned
handle and process it when $sth->execute is called. Such drivers are
unlikely to give much useful information about the
statement, such as $sth->{NUM_OF_FIELDS}, until after $sth->execute
has been called. Portable applications should take this into account.
In general, DBI drivers do not parse the contents of the statement (other than simply counting any Placeholders). The statement is passed directly to the database engine, sometimes known as pass-thru mode. This has advantages and disadvantages. On the plus side, you can access all the functionality of the engine being used. On the downside, you're limited if you're using a simple engine, and you need to take extra care if writing applications intended to be portable between engines.
Portable applications should not assume that a new statement can be prepared and/or executed while still fetching results from a previous statement.
Some command-line SQL tools use statement terminators, like a semicolon, to indicate the end of a statement. Such terminators should not normally be used with the DBI.
prepare_cached$sth = $dbh->prepare_cached($statement) $sth = $dbh->prepare_cached($statement, \%attr) $sth = $dbh->prepare_cached($statement, \%attr, $allow_active)
Like prepare except that the statement handle returned will be
stored in a hash associated with the $dbh. If another call is made to
prepare_cached with the same $statement and %attr values, then the
corresponding cached $sth will be returned without contacting the
database server.
Here are some examples of prepare_cached:
sub insert_hash {
my ($table, $field_values) = @_;
my @fields = sort keys %$field_values; # sort required
my @values = @{$field_values}{@fields};
my $sql = sprintf "insert into %s (%s) values (%s)",
$table, join(",", @fields), join(",", ("?")x@fields);
my $sth = $dbh->prepare_cached($sql);
return $sth->execute(@values);
}
sub search_hash {
my ($table, $field_values) = @_;
my @fields = sort keys %$field_values; # sort required
my @values = @{$field_values}{@fields};
my $qualifier = "";
$qualifier = "where ".join(" and ", map { "$_=?" } @fields) if @fields;
$sth = $dbh->prepare_cached("SELECT * FROM $table $qualifier");
return $dbh->selectall_arrayref($sth, {}, @values);
}
Caveat emptor: This caching can be useful in some applications, but it can also cause problems and should be used with care. Here is a contrived case where caching would cause a significant problem:
my $sth = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
$sth->execute($bar);
while (my $data = $sth->fetchrow_hashref) {
my $sth2 = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
$sth2->execute($data->{bar});
while (my $data2 = $sth2->fetchrow_arrayref) {
do_stuff(...);
}
}
In this example, since both handles are preparing the exact same statement,
$sth2 will not be its own statement handle, but a duplicate of $sth
returned from the cache. The results will certainly not be what you expect.
Typically the the inner fetch loop will work normally, fetching all
the records and terminating when there are no more, but now $sth
is the same as $sth2 the outer fetch loop will also terminate.
The $allow_active parameter lets you adjust DBI's behaviour when
prepare_cached is returning a statement handle that is still active.
There are three settings:
0: A warning will be generated, and finish will be called on
the statement handle before it is returned. This is the default
behaviour if $allow_active is not passed.
1: finish will be called on the statement handle, but the
warning is suppressed.
2: DBI will not touch the statement handle before returning it.
You will need to check $sth->{Active} on the returned
statement handle and deal with it in your own code.
Because the cache used by prepare_cached() is keyed by all the
parameters, including any attributes passed, you can also avoid
this issue by doing something like:
my $sth = $dbh->prepare_cached("...", { dbi_dummy => __FILE__.__LINE__ });
which will ensure that prepare_cached only returns statements cached by that line of code in that source file.
commit$rc = $dbh->commit or die $dbh->errstr;
Commit (make permanent) the most recent series of database changes if the database supports transactions and AutoCommit is off.
If AutoCommit is on, then calling
commit will issue a ``commit ineffective with AutoCommit'' warning.
See also Transactions in the FURTHER INFORMATION section below.
rollback$rc = $dbh->rollback or die $dbh->errstr;
Rollback (undo) the most recent series of uncommitted database changes if the database supports transactions and AutoCommit is off.
If AutoCommit is on, then calling
rollback will issue a ``rollback ineffective with AutoCommit'' warning.
See also Transactions in the FURTHER INFORMATION section below.
begin_work$rc = $dbh->begin_work or die $dbh->errstr;
Enable transactions (by turning AutoCommit off) until the next call
to commit or rollback. After the next commit or rollback,
AutoCommit will automatically be turned on again.
If AutoCommit is already off when begin_work is called then
it does nothing except return an error. If the driver does not support
transactions then when begin_work attempts to set AutoCommit off
the driver will trigger a fatal error.
See also Transactions in the FURTHER INFORMATION section below.
disconnect$rc = $dbh->disconnect or warn $dbh->errstr;
Disconnects the database from the database handle. disconnect is typically only used
before exiting the program. The handle is of little use after disconnecting.
The transaction behaviour of the disconnect method is, sadly,
undefined. Some database systems (such as Oracle and Ingres) will
automatically commit any outstanding changes, but others (such as
Informix) will rollback any outstanding changes. Applications not
using AutoCommit should explicitly call commit or rollback before
calling disconnect.
The database is automatically disconnected by the DESTROY method if
still connected when there are no longer any references to the handle.
The DESTROY method for each driver should implicitly call rollback to
undo any uncommitted changes. This is vital behaviour to ensure that
incomplete transactions don't get committed simply because Perl calls
DESTROY on every object before exiting. Also, do not rely on the order
of object destruction during ``global destruction'', as it is undefined.
Generally, if you want your changes to be commited or rolled back when you disconnect, then you should explicitly call commit or rollback before disconnecting.
If you disconnect from a database while you still have active
statement handles (e.g., SELECT statement handles that may have
more data to fetch), you will get a warning. The warning may indicate
that a fetch loop terminated early, perhaps due to an uncaught error.
To avoid the warning call the finish method on the active handles.
ping$rc = $dbh->ping;
Attempts to determine, in a reasonably efficient way, if the database server is still running and the connection to it is still working. Individual drivers should implement this function in the most suitable manner for their database engine.
The current default implementation always returns true without
actually doing anything. Actually, it returns ``0 but true'' which is
true but zero. That way you can tell if the return value is genuine or
just the default. Drivers should override this method with one that
does the right thing for their type of database.
Few applications would have direct use for this method. See the specialized Apache::DBI module for one example usage.
get_info$value = $dbh->get_info( $info_type );
Returns information about the implementation, i.e. driver and data
source capabilities, restrictions etc. It returns undef for
unknown or unimplemented information types. For example:
$database_version = $dbh->get_info( 18 ); # SQL_DBMS_VER $max_select_tables = $dbh->get_info( 106 ); # SQL_MAXIMUM_TABLES_IN_SELECT
See Standards Reference Information for more detailed information about the information types and their meanings and possible return values.
The DBI curently doesn't provide a name to number mapping for the information type codes or the results. Applications are expected to use the integer values directly, with the name in a comment, or define their own named values using something like the the constant manpage pragma.
Because some DBI methods make use of get_info(), drivers are strongly encouraged to support at least the following very minimal set of information types to ensure the DBI itself works properly:
Type Name Example A Example B ---- -------------------------- ------------ ------------ 17 SQL_DBMS_NAME 'ACCESS' 'Oracle' 18 SQL_DBMS_VER '03.50.0000' '08.01.0721' 29 SQL_IDENTIFIER_QUOTE_CHAR '`' '"' 41 SQL_CATALOG_NAME_SEPARATOR '.' '@' 114 SQL_CATALOG_LOCATION 1 2
table_info$sth = $dbh->table_info( $catalog, $schema, $table, $type ); $sth = $dbh->table_info( $catalog, $schema, $table, $type, \%attr );
Returns an active statement handle that can be used to fetch information about tables and views that exist in the database.
The arguments $catalog, $schema and $table may accept search patterns
according to the database/driver, for example: $table = '%FOO%';
Remember that the underscore character ('_') is a search pattern
that means match any character, so 'FOO_%' is the same as 'FOO%'
and 'FOO_BAR%' will match names like 'FOO1BAR'.
The value of $type is a comma-separated list of one or more types of tables to be returned in the result set. Each value may optionally be quoted, e.g.:
$type = "TABLE"; $type = "'TABLE','VIEW'";
In addition the following special cases may also be supported by some drivers:
$sth = $dbh->table_info('%', '', '');
If your driver doesn't support one or more of the selection filter parameters then you may get back more than you asked for and can do the filtering yourself.
This method can be expensive, and can return a large amount of data. (For example, small Oracle installation returns over 2000 rows.) So it's a good idea to use the filters to limit the data as much as possible.
The statement handle returned has at least the following fields in the order show below. Other fields, after these, may also be present.
TABLE_CAT: Table catalog identifier. This field is NULL (undef) if not
applicable to the data source, which is usually the case. This field
is empty if not applicable to the table.
TABLE_SCHEM: The name of the schema containing the TABLE_NAME value.
This field is NULL (undef) if not applicable to data source, and
empty if not applicable to the table.
TABLE_NAME: Name of the table (or view, synonym, etc).
TABLE_TYPE: One of the following: ``TABLE'', ``VIEW'', ``SYSTEM TABLE'', ``GLOBAL TEMPORARY'', ``LOCAL TEMPORARY'', ``ALIAS'', ``SYNONYM'' or a type identifier that is specific to the data source.
REMARKS: A description of the table. May be NULL (undef).
Note that table_info might not return records for all tables.
Applications can use any valid table regardless of whether it's
returned by table_info.
See also tables, Catalog Methods and Standards Reference Information.
column_info$sth = $dbh->column_info( $catalog, $schema, $table, $column );
Returns an active statement handle that can be used to fetch information about columns in specified tables.
The arguments $schema, $table and $column may accept search patterns according to the database/driver, for example: $table = '%FOO%';
Note: The support for the selection criteria is driver specific. If the driver doesn't support one or more of them then you may get back more than you asked for and can do the filtering yourself.
The statement handle returned has at least the following fields in the order shown below. Other fields, after these, may also be present.
TABLE_CAT: The catalog identifier.
This field is NULL (undef) if not applicable to the data source,
which is often the case. This field is empty if not applicable to the
table.
TABLE_SCHEM: The schema identifier.
This field is NULL (undef) if not applicable to the data source,
and empty if not applicable to the table.
TABLE_NAME: The table identifier. Note: A driver may provide column metadata not only for base tables, but also for derived objects like SYNONYMS etc.
COLUMN_NAME: The column identifier.
DATA_TYPE: The concise data type code.
TYPE_NAME: A data source dependent data type name.
COLUMN_SIZE: The column size. This is the maximum length in characters for character data types, the number of digits or bits for numeric data types or the length in the representation of temporal types. See the relevant specifications for detailed information.
BUFFER_LENGTH: The length in bytes of transferred data.
DECIMAL_DIGITS: The total number of significant digits to the right of the decimal point.
NUM_PREC_RADIX: The radix for numeric precision.
The value is 10 or 2 for numeric data types and NULL (undef) if not
applicable.
NULLABLE: Indicates if a column can accept NULLs. The following values are defined:
SQL_NO_NULLS 0 SQL_NULLABLE 1 SQL_NULLABLE_UNKNOWN 2
REMARKS: A description of the column.
COLUMN_DEF: The default value of the column.
SQL_DATA_TYPE: The SQL data type.
SQL_DATETIME_SUB: The subtype code for datetime and interval data types.
CHAR_OCTET_LENGTH: The maximum length in bytes of a character or binary data type column.
ORDINAL_POSITION: The column sequence number (starting with 1).
IS_NULLABLE: Indicates if the column can accept NULLs. Possible values are: 'NO', 'YES' and ''.
SQL/CLI defines the following additional columns:
CHAR_SET_CAT CHAR_SET_SCHEM CHAR_SET_NAME COLLATION_CAT COLLATION_SCHEM COLLATION_NAME UDT_CAT UDT_SCHEM UDT_NAME DOMAIN_CAT DOMAIN_SCHEM DOMAIN_NAME SCOPE_CAT SCOPE_SCHEM SCOPE_NAME MAX_CARDINALITY DTD_IDENTIFIER IS_SELF_REF
Drivers capable of supplying any of those values should do so in the corresponding column and supply undef values for the others.
Drivers wishing to provide extra database/driver specific information should do so in extra columns beyond all those listed above, and use lowercase field names with the driver-specific prefix (i.e., 'ora_...'). Applications accessing such fields should do so by name and not by column number.
The result set is ordered by TABLE_CAT, TABLE_SCHEM, TABLE_NAME and ORDINAL_POSITION.
Note: There is some overlap with statement attributes (in perl) and SQLDescribeCol (in ODBC). However, SQLColumns provides more metadata.
See also Catalog Methods and Standards Reference Information.
primary_key_info$sth = $dbh->primary_key_info( $catalog, $schema, $table );
Returns an active statement handle that can be used to fetch information about columns that make up the primary key for a table. The arguments don't accept search patterns (unlike table_info()).
For example:
$sth = $dbh->primary_key_info( undef, $user, 'foo' ); $data = $sth->fetchall_arrayref;
Note: The support for the selection criteria, such as $catalog, is driver specific. If the driver doesn't support catalogs and/or schemas, it may ignore these criteria.
The statement handle returned has at least the following fields in the order shown below. Other fields, after these, may also be present.
TABLE_CAT: The catalog identifier.
This field is NULL (undef) if not applicable to the data source,
which is often the case. This field is empty if not applicable to the
table.
TABLE_SCHEM: The schema identifier.
This field is NULL (undef) if not applicable to the data source,
and empty if not applicable to the table.
TABLE_NAME: The table identifier.
COLUMN_NAME: The column identifier.
KEY_SEQ: The column sequence number (starting with 1). Note: This field is named ORDINAL_POSITION in SQL/CLI.
PK_NAME: The primary key constraint identifier.
This field is NULL (undef) if not applicable to the data source.
See also Catalog Methods and Standards Reference Information.
primary_key@key_column_names = $dbh->primary_key( $catalog, $schema, $table );
Simple interface to the primary_key_info() method. Returns a list of
the column names that comprise the primary key of the specified table.
The list is in primary key column sequence order.
foreign_key_info
$sth = $dbh->foreign_key_info( $pk_catalog, $pk_schema, $pk_table
, $fk_catalog, $fk_schema, $fk_table );
Returns an active statement handle that can be used to fetch information about foreign keys in and/or referencing the specified table(s). The arguments don't accept search patterns (unlike table_info()).
$pk_catalog, $pk_schema, $pk_table
identify the primary (unique) key table (PKT).
$fk_catalog, $fk_schema, $fk_table
identify the foreign key table (FKT).
If both PKT and FKT are given, the function returns the foreign key, if any, in table FKT that refers to the primary (unique) key of table PKT. (Note: In SQL/CLI, the result is implementation-defined.)
If only PKT is given, then the result set contains the primary key of that table and all foreign keys that refer to it.
If only FKT is given, then the result set contains all foreign keys in that table and the primary keys to which they refer. (Note: In SQL/CLI, the result includes unique keys too.)
For example:
$sth = $dbh->foreign_key_info( undef, $user, 'master'); $sth = $dbh->foreign_key_info( undef, undef, undef , undef, $user, 'detail'); $sth = $dbh->foreign_key_info( undef, $user, 'master', undef, $user, 'detail');
Note: The support for the selection criteria, such as $catalog, is
driver specific. If the driver doesn't support catalogs and/or
schemas, it may ignore these criteria.
The statement handle returned has the following fields in the order shown below. Because ODBC never includes unique keys, they define different columns in the result set than SQL/CLI. SQL/CLI column names are shown in parentheses.
PKTABLE_CAT ( UK_TABLE_CAT ):
The primary (unique) key table catalog identifier.
This field is NULL (undef) if not applicable to the data source,
which is often the case. This field is empty if not applicable to the
table.
PKTABLE_SCHEM ( UK_TABLE_SCHEM ):
The primary (unique) key table schema identifier.
This field is NULL (undef) if not applicable to the data source,
and empty if not applicable to the table.
PKTABLE_NAME ( UK_TABLE_NAME ): The primary (unique) key table identifier.
PKCOLUMN_NAME (UK_COLUMN_NAME ): The primary (unique) key column identifier.
FKTABLE_CAT ( FK_TABLE_CAT ):
The foreign key table catalog identifier.
This field is NULL (undef) if not applicable to the data source,
which is often the case. This field is empty if not applicable to the
table.
FKTABLE_SCHEM ( FK_TABLE_SCHEM ):
The foreign key table schema identifier.
This field is NULL (undef) if not applicable to the data source,
and empty if not applicable to the table.
FKTABLE_NAME ( FK_TABLE_NAME ): The foreign key table identifier.
FKCOLUMN_NAME ( FK_COLUMN_NAME ): The foreign key column identifier.
KEY_SEQ ( ORDINAL_POSITION ): The column sequence number (starting with 1).
UPDATE_RULE ( UPDATE_RULE ): The referential action for the UPDATE rule. The following codes are defined:
CASCADE 0 RESTRICT 1 SET NULL 2 NO ACTION 3 SET DEFAULT 4
DELETE_RULE ( DELETE_RULE ): The referential action for the DELETE rule. The codes are the same as for UPDATE_RULE.
FK_NAME ( FK_NAME ): The foreign key name.
PK_NAME ( UK_NAME ): The primary (unique) key name.
DEFERRABILITY ( DEFERABILITY ): The deferrability of the foreign key constraint. The following codes are defined:
INITIALLY DEFERRED 5 INITIALLY IMMEDIATE 6 NOT DEFERRABLE 7
( UNIQUE_OR_PRIMARY ): This column is necessary if a driver includes all candidate (i.e. primary and alternate) keys in the result set (as specified by SQL/CLI). The value of this column is UNIQUE if the foreign key references an alternate key and PRIMARY if the foreign key references a primary key, or it may be undefined if the driver doesn't have access to the information.
See also Catalog Methods and Standards Reference Information.
tables@names = $dbh->tables( $catalog, $schema, $table, $type ); @names = $dbh->tables; # deprecated
Simple interface to table_info(). Returns a list of matching table names, possibly including a catalog/schema prefix.
See table_info for a description of the parameters.
If $dbh->get_info(29) returns true (29 is SQL_IDENTIFIER_QUOTE_CHAR)
then the table names are constructed and quoted by quote_identifier
to ensure they are usable even if they contain whitespace or reserved
words etc.
type_info_all$type_info_all = $dbh->type_info_all;
Returns a reference to an array which holds information about each data type variant supported by the database and driver. The array and its contents should be treated as read-only.
The first item is a reference to an 'index' hash of Name => Index pairs.
The items following that are references to arrays, one per supported data
type variant. The leading index hash defines the names and order of the
fields within the arrays that follow it.
For example:
$type_info_all = [
{ TYPE_NAME => 0,
DATA_TYPE => 1,
COLUMN_SIZE => 2, # was PRECISION originally
LITERAL_PREFIX => 3,
LITERAL_SUFFIX => 4,
CREATE_PARAMS => 5,
NULLABLE => 6,
CASE_SENSITIVE => 7,
SEARCHABLE => 8,
UNSIGNED_ATTRIBUTE=> 9,
FIXED_PREC_SCALE => 10, # was MONEY originally
AUTO_UNIQUE_VALUE => 11, # was AUTO_INCREMENT originally
LOCAL_TYPE_NAME => 12,
MINIMUM_SCALE => 13,
MAXIMUM_SCALE => 14,
NUM_PREC_RADIX => 15,
SQL_DATA_TYPE => 16,
SQL_DATETIME_SUB => 17,
NUM_PREC_RADIX => 18,
INTERVAL_PRECISION=> 19,
},
[ 'VARCHAR', SQL_VARCHAR,
undef, "'","'", undef,0, 1,1,0,0,0,undef,1,255, undef
],
[ 'INTEGER', SQL_INTEGER,
undef, "", "", undef,0, 0,1,0,0,0,undef,0, 0, 10
],
];
Note that more than one row may have the same value in the DATA_TYPE
field if there are different ways to spell the type name and/or there
are variants of the type with different attributes (e.g., with and
without AUTO_UNIQUE_VALUE set, with and without UNSIGNED_ATTRIBUTE, etc).
The rows are ordered by DATA_TYPE first and then by how closely each
type maps to the corresponding ODBC SQL data type, closest first.
The meaning of the fields is described in the documentation for
the type_info method. The index values shown above (e.g.,
NULLABLE => 6) are for illustration only. Drivers may define the
fields with a different order.
This method is not normally used directly. The type_info method provides a more useful interface to the data.
Even though an 'index' hash is provided, all the field names in the index hash defined above will always have the index values defined above. This is defined behaviour so that you don't need to rely on the index hash, which is handy because the lettercase of the keys is not defined. It is usually uppercase, as show here, but drivers are free to return names with any lettercase. Drivers are also free to return extra driver-specific columns of information - though it's recommended that they start at column index 50 to leave room for expansion of the DBI/ODBC specification.
type_info@type_info = $dbh->type_info($data_type);
Returns a list of hash references holding information about one or more
variants of $data_type. The list is ordered by DATA_TYPE first and
then by how closely each type maps to the corresponding ODBC SQL data
type, closest first. If called in a scalar context then only the first
(best) element is returned.
If $data_type is undefined or SQL_ALL_TYPES, then the list will
contain hashes for all data type variants supported by the database and driver.
If $data_type is an array reference then type_info returns the
information for the first type in the array that has any matches.
The keys of the hash follow the same letter case conventions as the rest of the DBI (see Naming Conventions and Name Space). The following items should exist:
For string types, this is the maximum size of the string in bytes.
For date and interval types, this is the maximum number of characters needed to display the value.
''' for characters,
or possibly ``0x'' for binary values passed as hexadecimal. NULL (undef) is
returned for data types for which this is not applicable.
''' for characters.
NULL (undef) is returned for data types where this is not applicable.
CREATE_PARAMS for a
DECIMAL would be ``precision,scale'' if the DECIMAL type should be
declared as DECIMAL(precision,scale) where precision and scale
are integer values. For a VARCHAR it would be ``max length''.
NULL (undef) is returned for data types for which this is not applicable.
0 or an empty string = no, 1 = yes, 2 = unknown.
0 - Cannot be used in a WHERE clause 1 - Only with a LIKE predicate 2 - All comparison operators except LIKE 3 - Can be used in a WHERE clause with any comparison operator
undef) is returned
for data types for which this is not applicable.
undef) is returned for data types
for which
this is not applicable.
undef) is returned
for data types for which this is not applicable.
TYPE_NAME for use in dialog with users.
NULL (undef) is returned if a localized name is not available (in which
case TYPE_NAME should be used).
MAXIMUM_SCALE holds the same value. NULL (undef) is returned for
data types for which this is not applicable.
MINIMUM_SCALE holds the same value. NULL (undef) is returned for
data types for which this is not applicable.
DATA_TYPE column, except for interval
and datetime data types. For interval and datetime data types, the
SQL_DATA_TYPE field will return SQL_INTERVAL or SQL_DATETIME, and the
SQL_DATETIME_SUB field below will return the subcode for the specific
interval or datetime data type. If this field is NULL, then the driver
does not support or report on interval or datetime subtypes.
SQL_DATA_TYPE
field above is SQL_INTERVAL or SQL_DATETIME, this field will
hold the subcode for the specific interval or datetime data type.
Otherwise it will be NULL (undef).
Although not mentioned explicitly in the standards, it seems there is a simple relationship between these values:
DATA_TYPE == (10 * SQL_DATA_TYPE) + SQL_DATETIME_SUB
NUM_PREC_RADIX
contains the value 2 and COLUMN_SIZE holds the number of bits. For
exact numeric types, NUM_PREC_RADIX contains the value 10 and COLUMN_SIZE holds
the number of decimal digits. NULL (undef) is returned either for data types
for which this is not applicable or if the driver cannot report this information.
For example, to find the type name for the fields in a select statement you can do:
@names = map { scalar $dbh->type_info($_)->{TYPE_NAME} } @{ $sth->{TYPE} }
Since DBI and ODBC drivers vary in how they map their types into the ISO standard types you may need to search for more than one type. Here's an example looking for a usable type to store a date:
$my_date_type = $dbh->type_info( [ SQL_DATE, SQL_TIMESTAMP ] );
Similarly, to more reliably find a type to store small integers, you could
use a list starting with SQL_SMALLINT, SQL_INTEGER, SQL_DECIMAL, etc.
See also Standards Reference Information.
quote$sql = $dbh->quote($value); $sql = $dbh->quote($value, $data_type);
Quote a string literal for use as a literal value in an SQL statement, by escaping any special characters (such as quotation marks) contained within the string and adding the required type of outer quotation marks.
$sql = sprintf "SELECT foo FROM bar WHERE baz = %s",
$dbh->quote("Don't");
For most database types, quote would return 'Don''t' (including the
outer quotation marks).
An undefined $value value will be returned as the string NULL (without
single quotation marks) to match how NULLs are represented in SQL.
If $data_type is supplied, it is used to try to determine the required
quoting behaviour by using the information returned by type_info.
As a special case, the standard numeric types are optimized to return
$value without calling type_info.
Quote will probably not be able to deal with all possible input
(such as binary data or data containing newlines), and is not related in
any way with escaping or quoting shell meta-characters. The quote()
method should not be used with Placeholders and Bind Values.
quote_identifier$sql = $dbh->quote_identifier( $name ); $sql = $dbh->quote_identifier( $catalog, $schema, $table, \%attr );
Quote an identifier (table name etc.) for use in an SQL statement, by escaping any special characters (such as double quotation marks) it contains and adding the required type of outer quotation marks.
Undefined names are ignored and the remainder are quoted and then
joined together, typically with a dot (.) character. For example:
$id = $dbh->quote_identifier( undef, 'Her schema', 'My table' );
would, for most database types, return "Her schema"."My table"
(including all the double quotation marks).
If three names are supplied then the first is assumed to be a catalog name and special rules may be applied based on what get_info returns for SQL_CATALOG_NAME_SEPARATOR (41) and SQL_CATALOG_LOCATION (114). For example, for Oracle:
$id = $dbh->quote_identifier( 'link', 'schema', 'table' );
would return "schema"."table"@"link".
take_imp_data$imp_data = $dbh->take_imp_data;
Leaves the $dbh in an almost dead, zombie-like, state and returns
a binary string of raw implementation data from the driver which
describes the current database connection. Effectively it detaches
the underlying database API connection data from the DBI handle.
After calling take_imp_data(), all other methods except DESTROY
will generate a warning and return undef.
Why would you want to do this? You don't, forget I even mentioned it. Unless, that is, you're implementing something advanced like a multi-threaded connection pool.
The returned $imp_data can be passed as a dbi_imp_data attribute
to a later connect() call, even in a separate thread in the same
process, where the driver can use it to 'adopt' the existing
connection that the implementation data was taken from.
Some things to keep in mind...
* the $imp_data holds the only reference to the underlying
database API connection data. That connection is still 'live' and
won't be cleaned up properly unless the $imp_data is used to create
a new $dbh which can then disconnect() normally.
* using the same $imp_data to create more than one other new $dbh at a time may well lead to unpleasant problems. Don't do that.
The take_imp_data method was added in DBI 1.36.
This section describes attributes specific to database handles.
Changes to these database handle attributes do not affect any other existing or future database handles.
Attempting to set or get the value of an unknown attribute is fatal, except for private driver-specific attributes (which all have names starting with a lowercase letter).
Example:
$h->{AutoCommit} = ...; # set/write
... = $h->{AutoCommit}; # get/read
AutoCommit (boolean)commit or rollback
methods.
Drivers should always default to AutoCommit mode (an unfortunate
choice largely forced on the DBI by ODBC and JDBC conventions.)
Attempting to set AutoCommit to an unsupported value is a fatal error.
This is an important feature of the DBI. Applications that need
full transaction behaviour can set $dbh->{AutoCommit} = 0 (or
set AutoCommit to 0 via connect)
without having to check that the value was assigned successfully.
For the purposes of this description, we can divide databases into three categories:
Databases which don't support transactions at all. Databases in which a transaction is always active. Databases in which a transaction must be explicitly started (C<'BEGIN WORK'>).
* Databases which don't support transactions at all
For these databases, attempting to turn AutoCommit off is a fatal error.
commit and rollback both issue warnings about being ineffective while
AutoCommit is in effect.
* Databases in which a transaction is always active
These are typically mainstream commercial relational databases with
``ANSI standard'' transaction behaviour.
If AutoCommit is off, then changes to the database won't have any
lasting effect unless commit is called (but see also
disconnect). If rollback is called then any changes since the
last commit are undone.
If AutoCommit is on, then the effect is the same as if the DBI
called commit automatically after every successful database
operation. So calling commit or rollback explicitly while
AutoCommit is on would be ineffective because the changes would
have already been commited.
Changing AutoCommit from off to on will trigger a commit.
For databases which don't support a specific auto-commit mode, the
driver has to commit each statement automatically using an explicit
COMMIT after it completes successfully (and roll it back using an
explicit ROLLBACK if it fails). The error information reported to the
application will correspond to the statement which was executed, unless
it succeeded and the commit or rollback failed.
* Databases in which a transaction must be explicitly started
For these databases, the intention is to have them act like databases in which a transaction is always active (as described above).
To do this, the driver will automatically begin an explicit transaction
when AutoCommit is turned off, or after a commit or
rollback (or when the application issues the next database
operation after one of those events).
In this way, the application does not have to treat these databases as a special case.
See commit, disconnect and Transactions for other important notes about transactions.
Driver (handle)
$dbh->{Driver}->{Name}
Name (string)dbi:DriverName:...'' string used to connect to the database,
but with the leading ``dbi:DriverName:'' removed.
Statement (string, read-only)RaiseError is enabled and the exception handler checks $@
and sees that a 'prepare' method call failed.
RowCacheSize (integer)SELECT statements.
If a row cache is not implemented, then setting RowCacheSize is ignored
and getting the value returns undef.
Some RowCacheSize values have special meaning, as follows:
0 - Automatically determine a reasonable cache size for each C<SELECT> 1 - Disable the local row cache >1 - Cache this many rows <0 - Cache as many rows that will fit into this much memory for each C<SELECT>.
Note that large cache sizes may require a very large amount of memory (cached rows * maximum size of row). Also, a large cache will cause a longer delay not only for the first fetch, but also whenever the cache needs refilling.
See also the RowsInCache statement handle attribute.
Username (string)
This section lists the methods and attributes associated with DBI statement handles.
The DBI defines the following methods for use on DBI statement handles:
bind_param$rc = $sth->bind_param($p_num, $bind_value) or die $sth->errstr; $rv = $sth->bind_param($p_num, $bind_value, \%attr) or ... $rv = $sth->bind_param($p_num, $bind_value, $bind_type) or ...
The bind_param method takes a copy of $bind_value and associates it
(binds it) with a placeholder, identified by $p_num, embedded in
the prepared statement. Placeholders are indicated with question
mark character (?). For example:
$dbh->{RaiseError} = 1; # save having to check each method call
$sth = $dbh->prepare("SELECT name, age FROM people WHERE name LIKE ?");
$sth->bind_param(1, "John%"); # placeholders are numbered from 1
$sth->execute;
DBI::dump_results($sth);
See Placeholders and Bind Values for more information.
Data Types for Placeholders
The \%attr parameter can be used to hint at the data type the
placeholder should have. Typically, the driver is only interested in
knowing if the placeholder should be bound as a number or a string.
$sth->bind_param(1, $value, { TYPE => SQL_INTEGER });
As a short-cut for the common case, the data type can be passed
directly, in place of the \%attr hash reference. This example is
equivalent to the one above:
$sth->bind_param(1, $value, SQL_INTEGER);
The TYPE value indicates the standard (non-driver-specific) type for
this parameter. To specify the driver-specific type, the driver may
support a driver-specific attribute, such as { ora_type => 97 }.
The SQL_INTEGER and other related constants can be imported using
use DBI qw(:sql_types);
See DBI Constants for more information.
The data type for a placeholder cannot be changed after the first
bind_param call. In fact the whole \%attr parameter is 'sticky'
in the sense that a driver only needs to consider the \%attr parameter
for the first call, for a given $sth and parameter. After that the driver
may ignore the \%attr parameter for that placeholder.
Perl only has string and number scalar data types. All database types that aren't numbers are bound as strings and must be in a format the database will understand.
As an alternative to specifying the data type in the bind_param call,
you can let the driver pass the value as the default type (VARCHAR).
You can then use an SQL function to convert the type within the statement.
For example:
INSERT INTO price(code, price) VALUES (?, CONVERT(MONEY,?))
The CONVERT function used here is just an example. The actual function
and syntax will vary between different databases and is non-portable.
See also Placeholders and Bind Values for more information.
bind_param_inout$rc = $sth->bind_param_inout($p_num, \$bind_value, $max_len) or die $sth->errstr; $rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, \%attr) or ... $rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, $bind_type) or ...
This method acts like bind_param, but also enables values to be
updated by the statement. The statement is typically
a call to a stored procedure. The $bind_value must be passed as a
reference to the actual value to be used.
Note that unlike bind_param, the $bind_value variable is not
copied when bind_param_inout is called. Instead, the value in the
variable is read at the time execute is called.
The additional $max_len parameter specifies the minimum amount of
memory to allocate to $bind_value for the new value. If the value
returned from the database is too
big to fit, then the execution should fail. If unsure what value to use,
pick a generous length, i.e., a length larger than the longest value that would ever be
returned. The only cost of using a larger value than needed is wasted memory.
It is expected that few drivers will support this method. The only driver currently known to do so is DBD::Oracle (DBD::ODBC may support it in a future release). Therefore it should not be used for database independent applications.
Undefined values or undef are used to indicate null values.
See also Placeholders and Bind Values for more information.
bind_param_array$rc = $sth->bind_param_array($p_num, $array_ref_or_value) $rc = $sth->bind_param_array($p_num, $array_ref_or_value, \%attr) $rc = $sth->bind_param_array($p_num, $array_ref_or_value, $bind_type)
The bind_param_array method is used to bind an array of values
to a placeholder embedded in the prepared statement which is to be executed
with execute_array. For example:
$dbh->{RaiseError} = 1; # save having to check each method call
$sth = $dbh->prepare("INSERT INTO staff (first_name, last_name, dept) VALUES(?, ?, ?)");
$sth->bind_param_array(1, [ 'John', 'Mary', 'Tim' ]);
$sth->bind_param_array(2, [ 'Booth', 'Todd', 'Robinson' ]);
$sth->bind_param_array(3, "SALES"); # scalar will be reused for each row
$sth->execute_array( { ArrayTupleStatus => \my @tuple_status } );
The %attr ($bind_type) argument is the same as defined for bind_param.
Refer to bind_param for general details on using placeholders.
(Note that bind_param_array() can not be used to expand a
placeholder into a list of values for a statement like ``SELECT foo
WHERE bar IN (?)''. A placeholder can only ever represent one value
per execution.)
Each array bound to the statement must have the same number of elements. Some drivers may define a method attribute to relax this safety check.
Scalar values, including undef, may also be bound by
bind_param_array. In which case the same value will be used for each
execute call. Driver-specific implementations may behave
differently, e.g., when binding to a stored procedure call, some
databases may permit mixing scalars and arrays as arguments.
The default implementation provided by DBI (for drivers that have not implemented array binding) is to iteratively call execute for each parameter tuple provided in the bound arrays. Drivers may provide more optimized implementations using whatever bulk operation support the database API provides. The default driver behaviour should match the default DBI behaviour, but always consult your driver documentation as there may be driver specific issues to consider.
Note that the default implementation currently only supports non-data
returning statements (insert, update, but not select). Also,
bind_param_array and bind_param cannot be mixed in the same
statement execution, and bind_param_array must be used with
execute_array; using bind_param_array will have no effect
for execute.
The bind_param_array method was added in DBI 1.22.
execute$rv = $sth->execute or die $sth->errstr; $rv = $sth->execute(@bind_values) or die $sth->errstr;
Perform whatever processing is necessary to execute the prepared
statement. An undef is returned if an error occurs. A successful
execute always returns true regardless of the number of rows affected,
even if it's zero (see below). It is always important to check the
return status of execute (and most other DBI methods) for errors
if you're not using RaiseError.
For a non-SELECT statement, execute returns the number of rows
affected, if known. If no rows were affected, then execute returns
``0E0'', which Perl will treat as 0 but will regard as true. Note that it
is not an error for no rows to be affected by a statement. If the
number of rows affected is not known, then execute returns -1.
For SELECT statements, execute simply ``starts'' the query within the
database engine. Use one of the fetch methods to retrieve the data after
calling execute. The execute method does not return the number of
rows that will be returned by the query (because most databases can't
tell in advance), it simply returns a true value.
If any arguments are given, then execute will effectively call
bind_param for each value before executing the statement. Values
bound in this way are usually treated as SQL_VARCHAR types unless
the driver can determine the correct type (which is rare), or unless
bind_param (or bind_param_inout) has already been used to
specify the type.
If execute() is called on a statement handle that's still active
($sth->{Active} is true) then it should effectively call finish()
to tidy up the previous execution results before starting this new
execution.
execute_array$rv = $sth->execute_array(\%attr) or die $sth->errstr; $rv = $sth->execute_array(\%attr, @bind_values) or die $sth->errstr;
Execute the prepared statement once for each parameter tuple (group of values) provided either in the @bind_values, or by prior calls to bind_param_array, or via a reference passed in \%attr.
The execute_array() method returns the number of tuples executed,
or undef if an error occured. Like execute(), a successful
execute_array() always returns true regardless of the number of
tuples executed, even if it's zero. See the ArrayTupleStatus
attribute below for how to determine the execution status for each
tuple.
Bind values for the tuples to be executed may be supplied by an
ArrayTupleFetch attribute, or else in the @bind_values argument,
or else by prior calls to bind_param_array.
The ArrayTupleFetch attribute can be used to specify a reference
to a subroutine that will be called to provide the bind values for
each tuple execution. The subroutine should return an reference to
an array which contains the appropriate number of bind values, or
return an undef if there is no more data to execute.
As a convienience, the ArrayTupleFetch attribute can also be
used to specify a statement handle. In which case the fetchrow_arrayref()
method will be called on the given statement handle in order to
provide the bind values for each tuple execution.
The values specified via bind_param_array() or the @bind_values
parameter may be either scalars, or arrayrefs. If any @bind_values
are given, then execute_array will effectively call bind_param_array
for each value before executing the statement. Values bound in
this way are usually treated as SQL_VARCHAR types unless the
driver can determine the correct type (which is rare), or unless
bind_param, bind_param_inout, bind_param_array, or
bind_param_inout_array has already been used to specify the type.
See bind_param_array for details.
The mandatory ArrayTupleStatus attribute is used to specify a
reference to an array which will receive the execute status of each
executed parameter tuple.
For tuples which are successfully executed, the element at the same ordinal position in the status array is the resulting rowcount. If the execution of a tuple causes an error, then the corresponding status array element will be set to a reference to an array containing the error code and error string set by the failed execution.
If any tuple execution returns an error, execute_array will
return undef. In that case, the application should inspect the
status array to determine which parameter tuples failed.
Some databases may not continue executing tuples beyond the first
failure. In this case the status array will either hold fewer
elements, or the elements beyond the failure will be undef.
If all parameter tuples are successfully executed, execute_array
returns the number tuples executed. If no tuples were executed,
then execute_array() returns ``0E0'', just like execute() does,
which Perl will treat as 0 but will regard as true.
For example:
$sth = $dbh->prepare("INSERT INTO staff (first_name, last_name) VALUES (?, ?)");
my $tuples = $sth->execute_array(
{ ArrayTupleStatus => \my @tuple_status },
\@first_names,
\@last_names,
);
if ($tuples) {
print "Successfully inserted $tuples records\n";
}
else {
for my $tuple (0..@last_names-1) {
my $status = $tuple_status[$tuple];
$status = [0, "Skipped"] unless defined $status;
next unless ref $status;
printf "Failed to insert (%s, %s): %s\n",
$first_names[$tuple], $last_names[$tuple], $status->[1];
}
}
Support for data returning statements, i.e., select, is driver-specific and subject to change. At present, the default implementation provided by DBI only supports non-data returning statements.
Transaction semantics when using array binding are driver and
database specific. If AutoCommit is on, the default DBI
implementation will cause each parameter tuple to be inidividually
committed (or rolled back in the event of an error). If AutoCommit
is off, the application is responsible for explicitly committing
the entire set of bound parameter tuples. Note that different
drivers and databases may have different behaviours when some
parameter tuples cause failures. In some cases, the driver or
database may automatically rollback the effect of all prior parameter
tuples that succeeded in the transaction; other drivers or databases
may retain the effect of prior successfully executed parameter
tuples. Be sure to check your driver and database for its specific
behaviour.
Note that, in general, performance will usually be better with
AutoCommit turned off, and using explicit commit after each
execute_array call.
The execute_array method was added in DBI 1.22, and ArrayTupleFetch
was added in 1.36.
execute_for_fetch$rc = $sth->execute_for_fetch($fetch_tuple_sub); $rc = $sth->execute_for_fetch($fetch_tuple_sub, \@tuple_status);
The execute_for_fetch() method is used to perform bulk operations
and is most often used via the execute_array() method, not directly.
The fetch subroutine, referenced by $fetch_tuple_sub, is expected to return a reference to an array (known as a 'tuple') or undef.
The execute_for_fetch() method calls $fetch_tuple_sub, without any
parameters, until it returns a false value. Each tuple returned is
used to provide bind values for an $sth->execute(@$tuple) call.
The number of tuples executed is returned, regardless of the success or failure of those executions. Use tuple_status to check.
If \@tuple_status is passed then the execute_for_fetch method uses
it to return status information. The tuple_status array holds one
element per tuple. If the corresponding execute() did not fail then
the element holds the return value from execute(), which is typically
a row count. If the execute() did fail then the element holds a
reference to an array containing ($sth->err, $sth->errstr, $sth->state).
Although each tuple returned by $fetch_tuple_sub is effectively used
to call $sth->execute(@$tuple_array_ref) the exact timing may vary.
Drivers are free to accumulate sets of tuples to pass to the
database server in bulk group operations for more efficient execution.
However, the $fetch_tuple_sub is specifically allowed to return
the same array reference each time.
For example:
my $sel = $dbh1->prepare("select foo, bar from table1");
$sel->execute;
my $ins = $dbh2->prepare("insert into table2 (foo, bar) values (?,?)");
$ins->execute;
my $fetch_tuple_sub = sub { $sel->fetchrow_arrayref };
my @tuple_status;
$rc = $ins->execute_for_fetch($fetch_tuple_sub, \@tuple_status);
my @errors = grep { ref $_ } @tuple_status;
The execute_for_fetch method was added in DBI 1.38.
fetchrow_arrayref$ary_ref = $sth->fetchrow_arrayref; $ary_ref = $sth->fetch; # alias
Fetches the next row of data and returns a reference to an array
holding the field values. Null fields are returned as undef
values in the array.
This is the fastest way to fetch data, particularly if used with
$sth->bind_columns.
If there are no more rows or if an error occurs, then fetchrow_arrayref
returns an undef. You should check $sth->err afterwards (or use the
RaiseError attribute) to discover if the undef returned was due to an
error.
Note that the same array reference is returned for each fetch, so don't store the reference and then use it after a later fetch. Also, the elements of the array are also reused for each row, so take care if you want to take a reference to an element. See also bind_columns.
fetchrow_array@ary = $sth->fetchrow_array;
An alternative to fetchrow_arrayref. Fetches the next row of data
and returns it as a list containing the field values. Null fields
are returned as undef values in the list.
If there are no more rows or if an error occurs, then fetchrow_array
returns an empty list. You should check $sth->err afterwards (or use
the RaiseError attribute) to discover if the empty list returned was
due to an error.
If called in a scalar context for a statement handle that has more
than one column, it is undefined whether the driver will return
the value of the first column or the last. So don't do that.
Also, in a scalar context, an undef is returned if there are no
more rows or if an error occurred. That undef can't be distinguished
from an undef returned because the first field value was NULL.
For these reasons you should exercise some caution if you use
fetchrow_array in a scalar context.
fetchrow_hashref$hash_ref = $sth->fetchrow_hashref; $hash_ref = $sth->fetchrow_hashref($name);
An alternative to fetchrow_arrayref. Fetches the next row of data
and returns it as a reference to a hash containing field name and field
value pairs. Null fields are returned as undef values in the hash.
If there are no more rows or if an error occurs, then fetchrow_hashref
returns an undef. You should check $sth->err afterwards (or use the
RaiseError attribute) to discover if the undef returned was due to an
error.
The optional $name parameter specifies the name of the statement handle
attribute. For historical reasons it defaults to ``NAME'', however using either
``NAME_lc'' or ``NAME_uc'' is recomended for portability.
The keys of the hash are the same names returned by $sth->{$name}. If
more than one field has the same name, there will only be one entry in
the returned hash for those fields.
Because of the extra work fetchrow_hashref and Perl have to perform, it
is not as efficient as fetchrow_arrayref or fetchrow_array.
Currently, a new hash reference is returned for each row. This will change in the future to return the same hash ref each time, so don't rely on the current behaviour.
fetchall_arrayref$tbl_ary_ref = $sth->fetchall_arrayref; $tbl_ary_ref = $sth->fetchall_arrayref( $slice ); $tbl_ary_ref = $sth->fetchall_arrayref( $slice, $max_rows );
The fetchall_arrayref method can be used to fetch all the data to be
returned from a prepared and executed statement handle. It returns a
reference to an array that contains one reference per row.
If there are no rows to return, fetchall_arrayref returns a reference
to an empty array. If an error occurs, fetchall_arrayref returns the
data fetched thus far, which may be none. You should check $sth->err
afterwards (or use the RaiseError attribute) to discover if the data is
complete or was truncated due to an error.
If $slice is an array reference, fetchall_arrayref uses fetchrow_arrayref
to fetch each row as an array ref. If the $slice array is not empty
then it is used as a slice to select individual columns by perl array
index number (starting at 0, unlike column and parameter numbers which
start at 1).
With no parameters, or if $slice is undefined, fetchall_arrayref
acts as if passed an empty array ref.
If $slice is a hash reference, fetchall_arrayref uses fetchrow_hashref
to fetch each row as a hash reference. If the $slice hash is empty then
fetchrow_hashref() is simply called in a tight loop and the keys in the hashes
have whatever name lettercase is returned by default from fetchrow_hashref.
(See FetchHashKeyName attribute.) If the $slice hash is not
empty, then it is used as a slice to select individual columns by
name. The values of the hash should be set to 1. The key names
of the returned hashes match the letter case of the names in the
parameter hash, regardless of the FetchHashKeyName attribute.
For example, to fetch just the first column of every row:
$tbl_ary_ref = $sth->fetchall_arrayref([0]);
To fetch the second to last and last column of every row:
$tbl_ary_ref = $sth->fetchall_arrayref([-2,-1]);
To fetch all fields of every row as a hash ref:
$tbl_ary_ref = $sth->fetchall_arrayref({});
To fetch only the fields called ``foo'' and ``bar'' of every row as a hash ref (with keys named ``foo'' and ``BAR''):
$tbl_ary_ref = $sth->fetchall_arrayref({ foo=>1, BAR=>1 });
The first two examples return a reference to an array of array refs. The third and forth return a reference to an array of hash refs.
If $max_rows is defined and greater than or equal to zero then it
is used to limit the number of rows fetched before returning.
fetchall_arrayref() can then be called again to fetch more rows.
This is especially useful when you need the better performance of
fetchall_arrayref() but don't have enough memory to fetch and return
all the rows in one go. Here's an example:
my $rows = []; # cache for batches of rows
while( my $row = ( shift(@$rows) || # get row from cache, or reload cache:
shift(@{$rows=$sth->fetchall_arrayref(undef,10_000)||[]) )
) {
...
}
That is the fastest way to fetch and process lots of rows using the DBI.
fetchall_hashref$hash_ref = $sth->fetchall_hashref($key_field);
The fetchall_hashref method can be used to fetch all the data to be
returned from a prepared and executed statement handle. It returns a
reference to a hash that contains, at most, one entry per row.
If there are no rows to return, fetchall_hashref returns a reference
to an empty hash. If an error occurs, fetchall_hashref returns the
data fetched thus far, which may be none. You should check
$sth->err afterwards (or use the RaiseError attribute) to
discover if the data is complete or was truncated due to an error.
The $key_field parameter provides the name of the field that holds the value to be used for the key for the returned hash. For example:
$dbh->{FetchHashKeyName} = 'NAME_lc';
$sth = $dbh->prepare("SELECT FOO, BAR, ID, NAME, BAZ FROM TABLE");
$hash_ref = $sth->fetchall_hashref('id');
print "Name for id 42 is $hash_ref->{42}->{name}\n";
The $key_field parameter can also be specified as an integer column number (counting from 1). If $key_field doesn't match any column in the statement, as a name first then as a number, then an error is returned.
This method is normally used only where the key field value for each row is unique. If multiple rows are returned with the same value for the key field then later rows overwrite earlier ones.
finish$rc = $sth->finish;
Indicate that no more data will be fetched from this statement handle
before it is either executed again or destroyed. The finish method
is rarely needed, and frequently overused, but can sometimes be
helpful in a few very specific situations to allow the server to free
up resources (such as sort buffers).
When all the data has been fetched from a SELECT statement, the
driver should automatically call finish for you. So you should
not normally need to call it explicitly except when you know
that you've not fetched all the data from a statement handle.
The most common example is when you only want to fetch one row,
but in that case the selectrow_* methods are usually better anyway.
Adding calls to finish after each fetch loop is a common mistake,
don't do it, it can mask genuine problems like uncaught fetch errors.
Consider a query like:
SELECT foo FROM table WHERE bar=? ORDER BY foo
where you want to select just the first (smallest) ``foo'' value from a
very large table. When executed, the database server will have to use
temporary buffer space to store the sorted rows. If, after executing
the handle and selecting one row, the handle won't be re-executed for
some time and won't be destroyed, the finish method can be used to tell
the server that the buffer space can be freed.
Calling finish resets the Active attribute for the statement. It
may also make some statement handle attributes (such as NAME and TYPE)
unavailable if they have not already been accessed (and thus cached).
The finish method does not affect the transaction status of the
database connection. It has nothing to do with transactions. It's mostly an
internal ``housekeeping'' method that is rarely needed.
See also disconnect and the Active attribute.
The finish method should have been called cancel_select.
rows$rv = $sth->rows;
Returns the number of rows affected by the last row affecting command, or -1 if the number of rows is not known or not available.
Generally, you can only rely on a row count after a non-SELECT
execute (for some specific operations like UPDATE and DELETE), or
after fetching all the rows of a SELECT statement.
For SELECT statements, it is generally not possible to know how many
rows will be returned except by fetching them all. Some drivers will
return the number of rows the application has fetched so far, but
others may return -1 until all rows have been fetched. So use of the
rows method or $DBI::rows with SELECT statements is not
recommended.
One alternative method to get a row count for a SELECT is to execute a
``SELECT COUNT(*) FROM ...'' SQL statement with the same ``...'' as your
query and then fetch the row count from that.
bind_col$rc = $sth->bind_col($column_number, \$var_to_bind); $rc = $sth->bind_col($column_number, \$var_to_bind, \%attr );
Binds an output column (field) of a SELECT statement to a Perl variable.
See bind_columns below for an example. Note that column numbers count
up from 1.
Whenever a row is fetched from the database, the corresponding Perl variable is automatically updated. There is no need to fetch and assign the values manually. The binding is performed at a very low level using Perl aliasing so there is no extra copying taking place. This makes using bound variables very efficient.
For maximum portability between drivers, bind_col should be called after
execute. This restriction may be removed in a later version of the DBI.
You do not need to bind output columns in order to fetch data, but it can be useful for some applications which need either maximum performance or greater clarity of code. The bind_param method performs a similar but opposite function for input variables.
bind_columns$rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);
Calls bind_col for each column of the SELECT statement.
The bind_columns method will die if the number of references does not
match the number of fields.
For maximum portability between drivers, bind_columns should be called
after execute.
For example:
$dbh->{RaiseError} = 1; # do this, or check every call for errors
$sth = $dbh->prepare(q{ SELECT region, sales FROM sales_by_region });
$sth->execute;
my ($region, $sales);
# Bind Perl variables to columns: $rv = $sth->bind_columns(\$region, \$sales);
# you can also use Perl's \(...) syntax (see perlref docs): # $sth->bind_columns(\($region, $sales));
# Column binding is the most efficient way to fetch data
while ($sth->fetch) {
print "$region: $sales\n";
}
For compatibility with old scripts, the first parameter will be
ignored if it is undef or a hash reference.
Here's a more fancy example that binds columns to the values inside a hash (thanks to H.Merijn Brand):
$sth->execute;
my %row;
$sth->bind_columns( \( @row{ @{$sth->{NAME_lc} } } ));
while ($sth->fetch) {
print "$row{region}: $row{sales}\n";
}
dump_results$rows = $sth->dump_results($maxlen, $lsep, $fsep, $fh);
Fetches all the rows from $sth, calls DBI::neat_list for each row, and
prints the results to $fh (defaults to STDOUT) separated by $lsep
(default "\n"). $fsep defaults to ", " and $maxlen defaults to 35.
This method is designed as a handy utility for prototyping and testing queries. Since it uses neat_list to format and edit the string for reading by humans, it is not recomended for data transfer applications.
This section describes attributes specific to statement handles. Most of these attributes are read-only.
Changes to these statement handle attributes do not affect any other existing or future statement handles.
Attempting to set or get the value of an unknown attribute is fatal, except for private driver specific attributes (which all have names starting with a lowercase letter).
Example:
... = $h->{NUM_OF_FIELDS}; # get/read
Note that some drivers cannot provide valid values for some or all of
these attributes until after $sth->execute has been called.
See also finish to learn more about the effect it may have on some attributes.
NUM_OF_FIELDS (integer, read-only)DELETE and CREATE
set NUM_OF_FIELDS to 0.
NUM_OF_PARAMS (integer, read-only)NAME (array-ref, read-only)
print "First column name: $sth->{NAME}->[0]\n";
NAME_lc (array-ref, read-only)NAME_uc (array-ref, read-only)NAME_hash (hash-ref, read-only)NAME_lc_hash (hash-ref, read-only)NAME_uc_hash (hash-ref, read-only)NAME_hash, NAME_lc_hash, and NAME_uc_hash attributes
return column name information as a reference to a hash.
The keys of the hash are the names of the columns. The letter case of
the keys corresponds to the letter case returned by the NAME,
NAME_lc, and NAME_uc attributes respectively (as described above).
The value of each hash entry is the perl index number of the corresponding column (counting from 0). For example:
$sth = $dbh->prepare("select Id, Name from table");
$sth->execute;
@row = $sth->fetchrow_array;
print "Name $row[ $sth->{NAME_lc_hash}{name} ]\n";
TYPE (array-ref, read-only)The values correspond to the international standards (ANSI X3.135 and ISO/IEC 9075) which, in general terms, means ODBC. Driver-specific types that don't exactly match standard types should generally return the same values as an ODBC driver supplied by the makers of the database. That might include private type numbers in ranges the vendor has officially registered with the ISO working group:
ftp://sqlstandards.org/SC32/SQL_Registry/
Where there's no vendor-supplied ODBC driver to be compatible with, the DBI driver can use type numbers in the range that is now officially reserved for use by the DBI: -9999 to -9000.
All possible values for TYPE should have at least one entry in the
output of the type_info_all method (see type_info_all).
PRECISION (array-ref, read-only)SCALE (array-ref, read-only)undef) values indicate columns where scale is not applicable.
NULLABLE (array-ref, read-only)0
(or an empty string) = no, 1 = yes, 2 = unknown.
print "First column may return NULL\n" if $sth->{NULLABLE}->[0];
CursorName (string, read-only)"where current of ..." SQL syntax, then it returns undef.
Database (dbh, read-only)ParamValues (hash ref, read-only)See ShowErrorStatement for an example of how this is used.
If the driver supports ParamValues but no values have been bound
yet then the driver should return a hash with placeholders names
in the keys but all the values undef, but some drivers may return
a ref to an empty hash.
It is possible that the values in the hash returned by ParamValues
are not exactly the same as those passed to bind_param() or execute().
The driver may have modified the values in some way based on the
TYPE the value was bound with. For example a floating point value
bound as an SQL_INTEGER type may be returned as an integer.
It is also possible that the keys in the hash returned by ParamValues
are not exactly the same as those implied by the prepared statement.
For example, DBD::Oracle translates '?' placeholders into ':pN'
where N is a sequence number starting at 1.
The ParamValues attribute was added in DBI 1.28.
Statement (string, read-only)RowsInCache (integer, read-only)SELECT statements, then
this attribute holds the number of un-fetched rows in the cache. If the
driver doesn't, then it returns undef. Note that some drivers pre-fetch
rows on execute, whereas others wait till the first fetch.
See also the RowCacheSize database handle attribute.
install_method
DBD::Foo::db->install_method($method_name, \%attr);
Installs the driver-private method named by $method_name into the
DBI method dispatcher so it can be called directly, avoiding the
need to use the func() method.
It is called as a static method on the driver class to which the
method belongs. The method name must begin with the corresponding
registered driver-private prefix. For example, for DBD::Oracle
$method_name must being with 'ora_', and for DBD::AnyData it
must begin with 'ad_'.
The attributes can be used to provide fine control over how the DBI dispatcher handles the dispatching of the method. However, at this point, it's undocumented and very liable to change. (Volunteers to polish up and document the interface are very welcome to get in touch via dbi-dev@perl.org)
Methods installed using install_method default to the standard error handling behaviour for DBI methods: clearing err and errstr before calling the method, and checking for errors to trigger RaiseError etc. on return. This differs from the default behaviour of func().
Note for driver authors: The DBD::Foo::xx->install_method call won't
work until the class-hierarchy has been setup. Normally the DBI
looks after that just after the driver is loaded. This means
install_method() can't be called at the time the driver is loaded
unless the class-hierarchy is set up first. The way to do that is
to call the setup_driver() method:
DBI->setup_driver('DBD::Foo');
before using install_method().
An application can retrieve metadata information from the DBMS by issuing
appropriate queries on the views of the Information Schema. Unfortunately,
INFORMATION_SCHEMA views are seldom supported by the DBMS.
Special methods (catalog methods) are available to return result sets
for a small but important portion of that metadata:
column_info foreign_key_info primary_key_info table_info
All catalog methods accept arguments in order to restrict the result sets.
Passing undef to an optional argument does not constrain the search for
that argument.
However, an empty string ('') is treated as a regular search criteria
and will only match an empty value.
Note: SQL/CLI and ODBC differ in the handling of empty strings. An empty string will not restrict the result set in SQL/CLI.
Most arguments in the catalog methods accept only ordinary values, e.g.
the arguments of primary_key_info().
Such arguments are treated as a literal string, i.e. the case is significant
and quote characters are taken literally.
Some arguments in the catalog methods accept search patterns (strings
containing '_' and/or '%'), e.g. the $table argument of column_info().
Passing '%' is equivalent to leaving the argument undef.
Caveat: The underscore ('_') is valid and often used in SQL identifiers. Passing such a value to a search pattern argument may return more rows than expected! To include pattern characters as literals, they must be preceded by an escape character which can be achieved with
$esc = $dbh->get_info( 14 ); # SQL_SEARCH_PATTERN_ESCAPE $search_pattern =~ s/([_%])/$esc$1/g;
The ODBC and SQL/CLI specifications define a way to change the default
behaviour described above: All arguments (except list value arguments)
are treated as identifier if the SQL_ATTR_METADATA_ID attribute is
set to SQL_TRUE.
Quoted identifiers are very similar to ordinary values, i.e. their
body (the string within the quotes) is interpreted literally.
Unquoted identifiers are compared in UPPERCASE.
The DBI (currently) does not support the SQL_ATTR_METADATA_ID attribute,
i.e. it behaves like an ODBC driver where SQL_ATTR_METADATA_ID is set to
SQL_FALSE.
Transactions are a fundamental part of any robust database system. They protect against errors and database corruption by ensuring that sets of related changes to the database take place in atomic (indivisible, all-or-nothing) units.
This section applies to databases that support transactions and where
AutoCommit is off. See AutoCommit for details of using AutoCommit
with various types of databases.
The recommended way to implement robust transactions in Perl
applications is to use RaiseError and eval { ... }
(which is very fast, unlike eval "..."). For example:
$dbh->{AutoCommit} = 0; # enable transactions, if possible
$dbh->{RaiseError} = 1;
eval {
foo(...) # do lots of work here
bar(...) # including inserts
baz(...) # and updates
$dbh->commit; # commit the changes if we get this far
};
if ($@) {
warn "Transaction aborted because $@";
# now rollback to undo the incomplete changes
# but do it in an eval{} as it may also fail
eval { $dbh->rollback };
# add other application on-error-clean-up code here
}
If the RaiseError attribute is not set, then DBI calls would need to be
manually checked for errors, typically like this:
$h->method(@args) or die $h->errstr;
With RaiseError set, the DBI will automatically die if any DBI method
call on that handle (or a child handle) fails, so you don't have to
test the return value of each method call. See RaiseError for more
details.
A major advantage of the eval approach is that the transaction will be
properly rolled back if any code (not just DBI calls) in the inner
application dies for any reason. The major advantage of using the
$h->{RaiseError} attribute is that all DBI calls will be checked
automatically. Both techniques are strongly recommended.
After calling commit or rollback many drivers will not let you
fetch from a previously active SELECT statement handle that's a child
of the same database handle. A typical way round this is to connect the
the database twice and use one connection for SELECT statements.
See AutoCommit and disconnect for other important information about transactions.
Many databases support ``blob'' (binary large objects), ``long'', or similar datatypes for holding very long strings or large amounts of binary data in a single field. Some databases support variable length long values over 2,000,000,000 bytes in length.
Since values of that size can't usually be held in memory, and because
databases can't usually know in advance the length of the longest long
that will be returned from a SELECT statement (unlike other data
types), some special handling is required.
In this situation, the value of the $h->{LongReadLen} attribute is used
to determine how much buffer space to allocate when fetching such
fields. The $h->{LongTruncOk} attribute is used to determine how to
behave if a fetched value can't fit into the buffer.
When trying to insert long or binary values, placeholders should be used
since there are often limits on the maximum size of an INSERT
statement and the quote method generally can't cope with binary
data. See Placeholders and Bind Values.
Here's a complete example program to select and fetch some data:
my $data_source = "dbi::DriverName:db_name";
my $dbh = DBI->connect($data_source, $user, $password)
or die "Can't connect to $data_source: $DBI::errstr";
my $sth = $dbh->prepare( q{
SELECT name, phone
FROM mytelbook
}) or die "Can't prepare statement: $DBI::errstr";
my $rc = $sth->execute
or die "Can't execute statement: $DBI::errstr";
print "Query will return $sth->{NUM_OF_FIELDS} fields.\n\n";
print "Field names: @{ $sth->{NAME} }\n";
while (($name, $phone) = $sth->fetchrow_array) {
print "$name: $phone\n";
}
# check for problems which may have terminated the fetch early
die $sth->errstr if $sth->err;
$dbh->disconnect;
Here's a complete example program to insert some data from a file.
(This example uses RaiseError to avoid needing to check each call).
my $dbh = DBI->connect("dbi:DriverName:db_name", $user, $password, {
RaiseError => 1, AutoCommit => 0
});
my $sth = $dbh->prepare( q{
INSERT INTO table (name, phone) VALUES (?, ?)
});
open FH, "<phone.csv" or die "Unable to open phone.csv: $!";
while (<FH>) {
chomp;
my ($name, $phone) = split /,/;
$sth->execute($name, $phone);
}
close FH;
$dbh->commit; $dbh->disconnect;
Here's how to convert fetched NULLs (undefined values) into empty strings:
while($row = $sth->fetchrow_arrayref) {
# this is a fast and simple way to deal with nulls:
foreach (@$row) { $_ = '' unless defined }
print "@$row\n";
}
The q{...} style quoting used in these examples avoids clashing with
quotes that may be used in the SQL statement. Use the double-quote like
qq{...} operator if you want to interpolate variables into the string.
See Quote and Quote-like Operators in the perlop manpage for more details.
Perl 5.7 and later support a new threading model called iThreads. (The old ``5.005 style'' threads are not supported by the DBI.)
In the iThreads model each thread has it's own copy of the perl interpreter. When a new thread is created the original perl interpreter is 'cloned' to create a new copy for the new thread.
If the DBI and drivers are loaded and handles created before the thread is created then it will get a cloned copy of the DBI, the drivers and the handles.
However, the internal pointer data within the handles will refer to the DBI and drivers in the original interpreter. Using those handles in the new interpreter thread is not safe, so the DBI detects this and croaks on any method call using handles that don't belong to the current thread (except for DESTROY).
Because of this (possibly temporary) restriction, newly created threads must make their own connctions to the database. Handles can't be shared across threads.
But BEWARE, some underlying database APIs (the code the DBD driver uses to talk to the database, often supplied by the database vendor) are not thread safe. If it's not thread safe, then allowing more than one thread to enter the code at the same time may cause subtle/serious problems. In some cases allowing more than one thread to enter the code, even if not at the same time, can cause problems. You have been warned.
Using DBI with perl threads is not yet recommended for production environments.
The first thing to say is that signal handling in Perl is currently not safe. There is always a small risk of Perl crashing and/or core dumping when, or after, handling a signal. (The risk was reduced with 5.004_04 but is still present.)
The two most common uses of signals in relation to the DBI are for
canceling operations when the user types Ctrl-C (interrupt), and for
implementing a timeout using alarm() and $SIG{ALRM}.
To assist in implementing these operations, the DBI provides a cancel
method for statement handles. The cancel method should abort the current
operation and is designed to be called from a signal handler.
However, it must be stressed that: a) few drivers implement this at
the moment (the DBI provides a default method that just returns undef);
and b) even if implemented, there is still a possibility that the statement
handle, and possibly the parent database handle, will not be usable
afterwards.
If cancel returns true, then it has successfully
invoked the database engine's own cancel function. If it returns false,
then cancel failed. If it returns undef, then the database
engine does not have cancel implemented.
DBI can be subclassed and extended just like any other object oriented module. Before we talk about how to do that, it's important to be clear about how the DBI classes and how they work together.
By default $dbh = DBI->connect(...) returns a $dbh blessed
into the DBI::db class. And the $dbh->prepare method
returns an $sth blessed into the DBI::st class (actually it
simply changes the last four characters of the calling handle class
to be ::st).
The leading 'DBI' is known as the 'root class' and the extra
'::db' or '::st' are the 'handle type suffixes'. If you want
to subclass the DBI you'll need to put your overriding methods into
the appropriate classes. For example, if you want to use a root class
of MySubDBI and override the do(), prepare() and execute() methods,
then your do() and prepare() methods should be in the MySubDBI::db
class and the execute() method should be in the MySubDBI::st class.
To setup the inheritance hierarchy the @ISA variable in MySubDBI::db
should include DBI::db and the @ISA variable in MySubDBI::st
should include DBI::st. The MySubDBI root class itself isn't
currently used for anything visible and so, apart from setting @ISA
to include DBI, it should be left empty.
So, having put your overriding methods into the right classes, and setup the inheritance hierarchy, how do you get the DBI to use them? You have two choices, either a static method call using the name of your subclass:
$dbh = MySubDBI->connect(...);
or specifying a RootClass attribute:
$dbh = DBI->connect(..., { RootClass => 'MySubDBI' });
The only difference between the two is that using an explicit RootClass attribute will make the DBI automatically attempt to load a module by that name if the class doesn't exist.
If both forms are used then the attribute takes precedence.
When subclassing is being used then, after a successful new connect, the DBI->connect method automatically calls:
$dbh->connected($dsn, $user, $pass, \%attr);
The default method does nothing. The call is made just to simplify any post-connection setup that your subclass may want to perform. If your subclass supplies a connected method, it should be part of the MySubDBI::db package.
Here's a brief example of a DBI subclass. A more thorough example can be found in t/subclass.t in the DBI distribution.
package MySubDBI;
use strict;
use DBI; use vars qw(@ISA); @ISA = qw(DBI);
package MySubDBI::db; use vars qw(@ISA); @ISA = qw(DBI::db);
sub prepare {
my ($dbh, @args) = @_;
my $sth = $dbh->SUPER::prepare(@args)
or return;
$sth->{private_mysubdbi_info} = { foo => 'bar' };
return $sth;
}
package MySubDBI::st; use vars qw(@ISA); @ISA = qw(DBI::st);
sub fetch {
my ($sth, @args) = @_;
my $row = $sth->SUPER::fetch(@args)
or return;
do_something_magical_with_row_data($row)
or return $sth->set_err(1234, "The magic failed", undef, "fetch");
return $row;
}
When calling a SUPER::method that returns a handle, be careful to check the return value before trying to do other things with it in your overridden method. This is especially important if you want to set a hash attribute on the handle, as Perl's autovivification will bite you by (in)conveniently creating an unblessed hashref, which your method will then return with usually baffling results later on. It's best to check right after the call and return undef immediately on error, just like DBI would and just like the example above.
If your method needs to record an error it should call the set_err()
method with the error code and error string, as shown in the example
above. The error code and error string will be recorded in the
handle and available via $h->err and $DBI::errstr etc.
The set_err() method always returns an undef or empty list as
approriate. Since your method should nearly always return an undef
or empty list as soon as an error is detected it's handy to simply
return what set_err() returns, as shown in the example above.
If the handle has RaiseError, PrintError, or HandleError
etc. set then the set_err() method will honour them. This means
that if RaiseError is set then set_err() won't return in the
normal way but will 'throw an exception' that can be caught with
an eval block.
You can stash private data into DBI handles
via $h->{private_..._*}. See the entry under ATTRIBUTES COMMON TO ALL HANDLES for info and important caveats.
In addition to the trace method, you can enable the same trace
information by setting the DBI_TRACE environment variable before
starting Perl.
On Unix-like systems using a Bourne-like shell, you can do this easily on the command line:
DBI_TRACE=2 perl your_test_script.pl
If DBI_TRACE is set to a non-numeric value, then it is assumed to
be a file name and the trace level will be set to 2 with all trace
output appended to that file. If the name begins with a number
followed by an equal sign (=), then the number and the equal sign are
stripped off from the name, and the number is used to set the trace
level. For example:
DBI_TRACE=1=dbitrace.log perl your_test_script.pl
See also the trace method.
It can sometimes be handy to compare trace files from two different
runs of the same script. However using a tool like diff doesn't work
well because the trace file is full of object addresses that may
differ each run. Here's a handy little command to strip those out:
perl -pe 's/\b0x[\da-f]{6,}/0xNNNN/gi; s/\b[\da-f]{6,}/<long number>/gi'
The DBI module recognizes a number of environment variables, but most of them should not be used most of the time. It is better to be explicit about what you are doing to avoid the need for environment variables, especially in a web serving system where web servers are stingy about which environment variables are available.
The DBI_DSN environment variable is used by DBI->connect if you do not specify a data source when you issue the connect. It should have a format such as ``dbi:Driver:databasename''.
The DBI_DRIVER environment variable is used to fill in the database driver name in DBI->connect if the data source string starts ``dbi::'' (thereby omitting the driver). If DBI_DSN omits the driver name, DBI_DRIVER can fill the gap.
The DBI_AUTOPROXY environment variable takes a string value that starts ``dbi:Proxy:'' and is typically followed by ``hostname=...;port=...''. It is used to alter the behaviour of DBI->connect. For full details, see DBI::Proxy documentation.
The DBI_USER environment variable takes a string value that is used as the user name if the DBI->connect call is given undef (as distinct from an empty string) as the username argument. Be wary of the security implications of using this.
The DBI_PASS environment variable takes a string value that is used as the password if the DBI->connect call is given undef (as distinct from an empty string) as the password argument. Be extra wary of the security implications of using this.
The DBI_DBNAME environment variable takes a string value that is used only when the obsolescent style of DBI->connect (with driver name as fourth parameter) is used, and when no value is provided for the first (database name) argument.
The DBI_TRACE environment variable takes an integer value that specifies the trace level for DBI at startup. Can also be used to direct trace output to a file. See DEBUGGING for more information.
An old variable that should no longer be used; equivalent to DBI_TRACE.
The DBI_PROFILE environment variable can be used to enable profiling of DBI method calls. See <DBI::Profile> for more information.
The DBI_PUREPERL environment variable can be used to enable the use of DBI::PurePerl. See <DBI::PurePerl> for more information.
$dbh handle you're using to call prepare is probably undefined because
the preceding connect failed. You should always check the return status of
DBI methods, or use the RaiseError attribute.
$sth handle you're using to call execute is probably undefined because
the preceeding prepare failed. You should always check the return status of
DBI methods, or use the RaiseError attribute.
(Some rare platforms require ``static linking''. On those platforms, there may be an old DBI or DBD driver version actually embedded in the Perl executable being used.)
A pure-perl emulation of the DBI is included in the distribution for people using pure-perl drivers who, for whatever reason, can't install the compiled DBI. See the DBI::PurePerl manpage.
Refer to the documentation for the DBD driver that you are using.
Refer to the SQL Language Reference Manual for the database engine that you are using.
More detailed information about the semantics of certain DBI methods that are based on ODBC and SQL/CLI standards is available on-line via microsoft.com, for ODBC, and www.jtc1sc32.org for the SQL/CLI standard:
DBI method ODBC function SQL/CLI Working Draft ---------- ------------- --------------------- column_info SQLColumns Page 124 foreign_key_info SQLForeignKeys Page 163 get_info SQLGetInfo Page 214 primary_key_info SQLPrimaryKeys Page 254 table_info SQLTables Page 294 type_info SQLGetTypeInfo Page 239
For example, for ODBC information on SQLColumns you'd visit:
http://msdn.microsoft.com/library/en-us/odbc/htm/odbcsqlcolumns.asp
If that URL ceases to work then use the MSDN search facility at:
http://search.microsoft.com/us/dev/
and search for SQLColumns returns using the exact phrase option.
The link you want will probably just be called SQLColumns and will
be part of the Data Access SDK.
And for SQL/CLI standard information on SQLColumns you'd read page 124 of the (very large) SQL/CLI Working Draft available from:
http://www.jtc1sc32.org/sc32/jtc1sc32.nsf/Attachments/7E3B41486BD99C3488256B410064C877/$FILE/32N0744T.PDF
A hyperlinked, browsable version of the BNF syntax for SQL92 (plus Oracle 7 SQL and PL/SQL) is available here:
http://cui.unige.ch/db-research/Enseignement/analyseinfo/SQL92/BNFindex.html
A BNF syntax for SQL3 is available here:
http://www.sqlstandards.org/SC32/WG3/Progression_Documents/Informal_working_drafts/iso-9075-2-1999.bnf
The following links provide further useful information about SQL. Some of these are rather dated now but may still be useful.
http://www.jcc.com/SQLPages/jccs_sql.htm http://www.contrib.andrew.cmu.edu/~shadow/sql.html http://www.altavista.com/query?q=sql+tutorial
Programming the Perl DBI, by Alligator Descartes and Tim Bunce.
Programming Perl 3rd Ed. by Larry Wall, Tom Christiansen & Jon Orwant.
Learning Perl by Randal Schwartz.
Dr Dobb's Journal, November 1996.
The Perl Journal, April 1997.
Index of DBI related modules available from CPAN:
http://search.cpan.org/search?mode=module&;query=DBIx%3A%3A http://search.cpan.org/search?mode=doc&;query=DBI
For a good comparison of RDBMS-OO mappers and some OO-RDBMS mappers (including Class::DBI, Alzabo, and DBIx::RecordSet in the former category and Tangram and SPOPS in the latter) see the Perl Object-Oriented Persistence project pages at:
http://poop.sourceforge.net
A similar page for Java toolkits can be found at:
http://c2.com/cgi-bin/wiki?ObjectRelationalToolComparison
perl(1), perlmod(1), perlbook(1)
The dbi-users mailing list is the primary means of communication among users of the DBI and its related modules. For details send email to:
dbi-users-help@perl.org
There are typically between 700 and 900 messages per month. You have to subscribe in order to be able to post. However you can opt for a 'post-only' subscription.
Mailing list archives (of variable quality) are held at:
http://www.xray.mpe.mpg.de/mailing-lists/dbi/ http://groups.yahoo.com/group/dbi-users http://www.bitmechanic.com/mail-archives/dbi-users/ http://marc.theaimsgroup.com/?l=perl-dbi&;r=1&w=2 http://www.mail-archive.com/dbi-users%40perl.org/
The DBI ``Home Page'':
http://dbi.perl.org/
Other DBI related links:
http://tegan.deltanet.com/~phlip/DBUIdoc.html http://dc.pm.org/perl_db.html http://wdvl.com/Authoring/DB/Intro/toc.html http://www.hotwired.com/webmonkey/backend/tutorials/tutorial1.html http://bumppo.net/lists/macperl/1999/06/msg00197.html http://gmax.oltrelinux.com/dbirecipes.html
Other database related links:
http://www.jcc.com/sql_stnd.html http://cuiwww.unige.ch/OSG/info/FreeDB/FreeDB.home.html
Security, especially the ``SQL Injection'' attack:
http://www.ngssoftware.com/research/papers.html http://www.ngssoftware.com/papers/advanced_sql_injection.pdf http://www.ngssoftware.com/papers/more_advanced_sql_injection.pdf http://www.esecurityplanet.com/trends/article.php/2243461 http://www.spidynamics.com/papers/SQLInjectionWhitePaper.pdf http://www.webcohort.com/Blindfolded_SQL_Injection.pdf http://online.securityfocus.com/infocus/1644
Commercial and Data Warehouse Links
http://www.dwinfocenter.org http://www.datawarehouse.com http://www.datamining.org http://www.olapcouncil.org http://www.idwa.org http://www.knowledgecenters.org/dwcenter.asp
Recommended Perl Programming Links
http://language.perl.com/style/
Please also read the DBI FAQ which is installed as a DBI::FAQ module.
You can use perldoc to read it by executing the perldoc DBI::FAQ command.
DBI by Tim Bunce. This pod text by Tim Bunce, J. Douglas Dunlop,
Jonathan Leffler and others. Perl by Larry Wall and the
perl5-porters.
The DBI module is Copyright (c) 1994-2003 Tim Bunce. Ireland. All rights reserved.
You may distribute under the terms of either the GNU General Public License or the Artistic License, as specified in the Perl README file.
I would like to acknowledge the valuable contributions of the many people I have worked with on the DBI project, especially in the early years (1992-1994). In no particular order: Kevin Stock, Buzz Moschetti, Kurt Andersen, Ted Lemon, William Hails, Garth Kennedy, Michael Peppler, Neil S. Briscoe, Jeff Urlwin, David J. Hughes, Jeff Stander, Forrest D Whitcher, Larry Wall, Jeff Fried, Roy Johnson, Paul Hudson, Georg Rehfeld, Steve Sizemore, Ron Pool, Jon Meek, Tom Christiansen, Steve Baumgarten, Randal Schwartz, and a whole lot more.
Then, of course, there are the poor souls who have struggled through untold and undocumented obstacles to actually implement DBI drivers. Among their ranks are Jochen Wiedmann, Alligator Descartes, Jonathan Leffler, Jeff Urlwin, Michael Peppler, Henrik Tougaard, Edwin Pratomo, Davide Migliavacca, Jan Pazdziora, Peter Haworth, Edmund Mergl, Steve Williams, Thomas Lowery, and Phlip Plumlee. Without them, the DBI would not be the practical reality it is today. I'm also especially grateful to Alligator Descartes for starting work on the ``Programming the Perl DBI'' book and letting me jump on board.
Much of the DBI and DBD::Oracle was developed while I was Technical Director (CTO) of the Paul Ingram Group (www.ig.co.uk). So I'd especially like to thank Paul for his generosity and vision in supporting this work for many years.
A German translation of this manual (possibly slightly out of date) is available, thanks to O'Reilly, at:
http://www.oreilly.de/catalog/perldbiger/
Some other translations:
http://cronopio.net/perl/ - Spanish http://member.nifty.ne.jp/hippo2000/dbimemo.htm - Japanese
The DBI is free software. IT COMES WITHOUT WARRANTY OF ANY KIND.
Commercial support for Perl and the DBI, DBD::Oracle and Oraperl modules can be arranged via The Perl Clinic. For more details visit:
http://www.perlclinic.com
For direct DBI and DBD::Oracle support, enhancement, and related work I am available for consultancy on standard commercial terms.
References to DBI related training resources. No recommendation implied.
http://www.treepax.co.uk/ http://www.keller.com/dbweb/
See the DBI FAQ for a more comprehensive list of FAQs. Use the
perldoc DBI::FAQ command to read it.
To measure the speed of the DBI and DBD::Oracle code, I modified DBD::Oracle so you can set an attribute that will cause the same row to be fetched from the row cache over and over again (without involving Oracle code but exercising *all* the DBI and DBD::Oracle code in the code path for a fetch).
The results (on my lightly loaded old Sparc 10) fetching 50000 rows using:
1 while $csr->fetch;
| were: | |
| one field: 5300 fetches per cpu second (approx) | |
| ten fields: 4000 fetches per cpu second (approx) |
Obviously results will vary between platforms (newer faster platforms can reach around 50000 fetches per second), but it does give a feel for the maximum performance: fast. By way of comparison, using the code:
1 while @row = $csr->fetchrow_array;
(fetchrow_array is roughly the same as ora_fetch) gives:
one field: 3100 fetches per cpu second (approx)
ten fields: 1000 fetches per cpu second (approx)
Notice the slowdown and the more dramatic impact of extra fields. (The fields were all one char long. The impact would be even bigger for longer strings.)
Changing that slightly to represent actually doing something in Perl with the fetched data:
while(@row = $csr->fetchrow_array) {
$hash{++$i} = [ @row ];
}
gives: ten fields: 500 fetches per cpu second (approx)
That simple addition has *halved* the performance.
I therefore conclude that DBI and DBD::Oracle overheads are small compared with Perl language overheads (and probably database overheads).
So, if you think the DBI or your driver is slow, try replacing your fetch loop with just:
1 while $csr->fetch;
and time that. If that helps then point the finger at your own code. If that doesn't help much then point the finger at the database, the platform, the network etc. But think carefully before pointing it at the DBI or your driver.
(Having said all that, if anyone can show me how to make the DBI or drivers even more efficient, I'm all ears.)
Read the information in the references below. Please do not post CGI related questions to the dbi-users mailing list (or to me).
http://www.perl.com/cgi-bin/pace/pub/doc/FAQs/cgi/perl-cgi-faq.html http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html http://www.boutell.com/faq/ http://www.perl.com/perl/faq/
For information on the Apache httpd server and the mod_perl module see
http://perl.apache.org/
A DBD::ODBC driver module for ODBC is available and works well.
No. The DBI has no knowledge or understanding of dates at all.
Individual drivers (DBD::*) may have some date handling code but are unlikely to have year 2000 related problems within their code. However, your application code which uses the DBI and DBD drivers may have year 2000 related problems if it has not been designed and written well.
See also the ``Does Perl have a year 2000 problem?'' section of the Perl FAQ:
http://www.perl.com/CPAN/doc/FAQs/FAQ/PerlFAQ.html
mod_perl. Establishes a database connection which
remains open for the lifetime of the HTTP daemon. This way the CGI
connect and disconnect for every database access becomes superfluous.
jdbc:dbi://host.domain.etc:999/Driver/DBName.
It seems to be very similar to some commercial products, such as jdbcKona.
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DBI - Database independent interface for Perl |