Perl in a Nutshell, 2nd Edition

Database Handles and Statement Handles

DBI methods work on two different types of handles: database handles and statement handles. A database handle is like a filehandle; connect is a DBI class method that opens a connection to a database and returns a database handle object:

$db_handle = DBI->connect(dbi:mSQL:bookdb, undef, undef)
    || die("Connect error: $DBI::errstr");

Statement handles are another thing entirely. DBI makes a distinction between the preparation of SQL statements and their execution by allowing you to preformat a statement into a statement handle. You can prepare a statement with the prepare method, which returns a statement handle. You can then assign a SQL statement to the statement handle via various statement handle methods and execute it with the execute method when you’re done. (You can also prepare and execute in the same command with the do method.)

Changes to the database are written to the database automatically if the AutoCommit attribute is turned on. If AutoCommit is off, use the commit method when you’re ready to write the changes to the database.

AutoCommit is only one of many attributes that can be set for both database and statement handles. For example, if $st_handle is a statement handle, you can set $st_handle->{NULLABLE} to determine if the fields can contain null characters. Table 12-2 is a listing of all the attributes supported by database handles, statement handles, or both.

Table 12-2. Attributes for database and statement handles

Attribute ...