sub order_numbers {
    my $class = shift;

    my $customers = $class->_table;
    my $orders    = Order->_table;

    my %cust  = map { $_ => $class->_column($_) }
        qw{ first_name last_name customer_id };
    my %order = map { $_ => Order->_column($_) }
        qw{ order_num cust_id customer_id };

    my $sql = <<"    END_SQL";
    SELECT $customers.$cust{last_name}, $customers.$cust{first_name}
           $orders.$order{order_num}
    FROM   $customers
    LEFT OUTER JOIN $orders 
        ON $customers.$cust{customer_id} = $orders.$order{customer_id}
    END_SQL
    return $class->_fetch_records($sql);
}