mysql>
mysql> DELIMITER //
mysql> CREATE FUNCTION myProc (cost DECIMAL(10,2))
 -> RETURNS DECIMAL(10,2)
->
 -> SQL SECURITY DEFINER
->
 -> tax: BEGIN
 -> DECLARE order_tax DECIMAL(10,2);
 -> SET order_tax = cost * .05;
 -> RETURN order_tax;
 -> END
 -> //
Query OK, 0 rows affected (0.00 sec)

mysql> DELIMITER ;
mysql>
mysql> select myProc(123.45);
+----------------+
| myProc(123.45) |
+----------------+
| 6.17 |
+----------------+
1 row in set, 1 warning (0.00 sec)

mysql>
mysql> drop function myProc;
Query OK, 0 rows affected (0.00 sec)

mysql>