Programming the Perl DBI

Binding Parameters to Statements

One topic we have mentioned in our discussion of the preparation of statement handles is bind values. You may also have come across the phrases placeholders , parameters , and binding . What are these things?

A bind value is a value that can be bound to a placeholder declared within an SQL statement. This is similar to creating an on-the-fly SQL statement such as:

$sth = $dbh->prepare( "
            SELECT name, location
            FROM megaliths
            WHERE name = " . $dbh->quote( $siteName ) . "
        " );

but instead of interpolating the generated value into the SQL statement, you specify a placeholder and then bind the generated value to that. For example:

$sth = $dbh->prepare( "
            SELECT name, location
            FROM megaliths
            WHERE name = ?
        " );
$sth->bind_param( 1, $siteName );

The bind_ param( ) method is the call that actually associates the supplied value with the given placeholder. The underlying database will correctly parse the placeholder and reserve a space for it, which is “filled in” when bind_ param( ) is called. It is important to remember that bind_ param( ) must be called before execute( ); otherwise, the missing value will not have been filled in and the statement execution will fail.

It’s equally simple to specify multiple bind values within one statement, since bind_ param( ) takes the index, starting from 1, of the parameter to bind the given value to. For example:

$sth = $dbh->prepare( " SELECT name, location FROM megaliths WHERE name = ? AND mapref = ? AND type LIKE ? " ...