Programming the Perl DBI

Executing Select Statements

The second stage of the data fetch cycle is to inform the database to go ahead and execute the SQL statement that you have prepared. This execution stage will actually tell the database to perform the query and begin to collect the result set of data.

Performing the execution of the SQL statement occurs via a valid statement handle created when the prepare() method successfully completes. For example, execution of an SQL statement can be expressed as simply as:

### Create the statement handle
my $sth = $dbh->prepare( "SELECT id, name FROM megaliths" );

### Execute the statement handle
$sth->execute();

Assuming that all goes well with the execution of your statement, a true value will be returned from the execute() call. Otherwise, a value of undef is returned, signifying that the execution has failed.

As with most DBI methods, if PrintError is enabled, then an error message will be generated via warn() . Alternatively, if RaiseError is enabled, an exception will be generated via die() . However you choose to do it, it is always a good idea to check for errors.^[44]

After execute() returns successfully, the database has not necessarily completed the execution of the SELECT statement; it may have only just started. Imagine that megaliths are very common, and our megaliths table has ten million rows. In response to the execute() earlier, the database may do no more than set a pointer, known as a cursor, to just above the first row of the table.

So, after ...