DBI Database Handle Objects

This section covers the methods and attributes associated with database handles.

Database Handle Methods

The following methods are specified for DBI database handles:

do 
 $rc = $dbh->do($statement) || die $dbh->errstr; $rc = $dbh->do($statement, \%attr) || die $dbh->errstr; $rv = $dbh->do($statement, \%attr, @bind_values) || ...

Prepares and executes 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 or is 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') || 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 the section on "Quote and Quote-Like Operators" in the perlop manpage for more details.

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. If called in a scalar context, it returns the first field of the first row. 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.

In a scalar context, selectrow_array returns the value of the first field. An undef is returned if there are no matching rows or if an error occurred. Since that undef can't be distinguished from an undef returned because the first field value was NULL, calling selectrow_array in a scalar context should be used with caution.

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 any method except fetch fails, and RaiseError is not set, selectall_arrayref will return undef. If fetch fails, and RaiseError is not set, then it will return with whatever data it has fetched thus far.

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.

prepare 
$sth = $dbh->prepare($statement) || die $dbh->errstr; $sth = $dbh->prepare($statement, \%attr) || die $dbh->errstr;

Prepares a single 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 you're 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.

This caching can be useful in some applications, but it can also cause problems and should be used with care. A warning will be generated if the cached $sth being returned is active (i.e., it is a SELECT that may still have data to be fetched). This warning can be suppressed by setting $allow_active to true. The cache can be accessed (and cleared) via the CachedKids attribute.

Here's an example of one possible use of prepare_cached: 

while ( ($field, $value) = each %search_fields ) {
 push @sql, "$field = ?"; push @values, $value;
}
$qualifier = ""; $qualifier = "where ".join(" and ", @sql) if @sql; $sth = $dbh->prepare_cached("SELECT * FROM table $qualifier"); $sth->execute(@values);


commit 
$rc = $dbh->commit || die $dbh->errstr;

Commits (makes 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.

rollback 
$rc = $dbh->rollback || die $dbh->errstr;

Rolls back (undoes) 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.

disconnect 
$rc = $dbh->disconnect || warn $dbh->errstr;

Disconnects the database from the database handle. disconnect is typically used only before exiting the program. The handle is of little use after disconnecting.

The transaction behavior 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 roll back 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 behavior 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, you will get a warning. The statement handles should either be cleared (destroyed) before disconnecting, or the finish method should be called on each one.

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 " 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 use for this method. See the specialized Apache::DBI module for one example usage.

table_info (NEW ) 
$sth = $dbh->table_info;

Warning: This method is experimental and may change.

Returns an active statement handle that can be used to fetch information about tables and views that exist in the database.

The handle has at least the following fields in the order show below. Other fields, after these, may also be present.

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.

For more detailed information about the fields and their meanings, refer to:

http://msdn.microsoft.com/library/psdk/dasdk/odch6wqb.htm

If that URL ceases to work, then use the MSDN search facility at:

http://search.microsoft.com/us/dev/

and search for SQLTables returns using the exact phrase option. The link you want will probably just be called SQLTables and will be part of the Data Access SDK.

tables (NEW ) 
@names = $dbh->tables;

Warning: This method is experimental and may change.

Returns a list of table and view names, possibly including a schema prefix. This list should include all tables that can be used in a SELECT statement without further qualification.

Note that table_info might not return records for all tables. Applications can use any valid table regardless of whether it's returned by tables. See also table_info.

type_info_all (NEW ) 
$type_info_all = $dbh->type_info_all;

Warning: This method is experimental and may change.

Returns a reference to an array that holds information about each datatype 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 a hash of Name => Index pairs. The following items are references to arrays, one per supported datatype variant. The leading hash defines the names and order of the fields within the following list of arrays. 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, }, [ '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 datatype, 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 => ) 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.

type_info (NEW ) 
@type_info = $dbh->type_info($data_type);

Warning: This method is experimental and may change.

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 datatype, 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 datatype 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:

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.

For more detailed information about these fields and their meanings, refer to:

http://msdn.microsoft.com/library/psdk/dasdk/odch6yy7.htm

If that URL ceases to work, then use the MSDN search facility at:

http://search.microsoft.com/us/dev/

and search the MSDN library for SQLGetTypeInfo returns using the exact phrase option. The link you want will probably just be called SQLGetTypeInfo (there may be more than one).

The individual datatypes are currently described here:

http://msdn.microsoft.com/library/psdk/dasdk/odap8fcj.htm

If that URL ceases to work, or to get more general information, use the MSDN search facility as described above, and search for SQL Data Types.

quote 
$sql = $dbh->quote($value); $sql = $dbh->quote($value, $data_type);

Quotes 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 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 behavior 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. There is no need to quote values being used with placeholders and bind values.

Database Handle Attributes

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).

For example: 

$h->{AutoCommit} = ...; # set/write ... = $h->{AutoCommit};
 # get/read


AutoCommit ( boolean)

If true, then database changes cannot be rolled back (undone). If false, then database changes automatically occur within a "transaction," which must either be committed or rolled back using the 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 behavior can set $dbh->{AutoCommit} = (or set AutoCommit to 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 that don't support transactions at all.

Databases in which a transaction is always active.

Databases in which a transaction must be explicitly started ('BEGIN WORK').

Databases that 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 behavior. 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. In other words, 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 should issue a commit in most drivers.

Changing AutoCommit from on to off should have no immediate effect.

For databases that don't support a specific autocommit 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 that 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 earlier).

To do this, the DBI driver will automatically begin a transaction when AutoCommit is turned off (from the default "on" state) and will automatically begin another transaction after a commit or rollback. In this way, the application does not have to treat these databases as a special case.

See disconnect for other important notes about transactions.

Driver ( handle)

Holds the handle of the parent driver. The only recommended use for this attribute is to find the name of the driver using: 

$dbh->{Driver}->{Name}


Name (string)

Holds the "name" of the database. Usually (and recommended to be) the same as the "dbi:DriverName:..." string used to connect to the database, but with the leading dbi:DriverName: removed.

RowCacheSize (integer) (NEW )

A hint to the driver indicating the size of the local row cache that the application would like the driver to use for future 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:

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.