sub order_numbers {
    # assume, for the sake of argument, that order numbers
    # are optional
    my $class = shift;
    my $sql = <<'    END_SQL';
    SELECT c.last_name, c.first_name. o.order_num
    FROM   customers c
    LEFT OUTER JOIN orders o ON c.customer_id = o.cu_id
    END_SQL
    return $class->_fetch_records($sql);
}