Question

I am creating a room reservation in php.

I have two tables:

table RESERVE

• reserve_id
• room_id
• room_reserve_qty
• checkout

table ROOM

• room_id
• room_qty

when the “checkout” date reached the date today,that row in the reserve table will be deleted.Actually I already have the event for that :

DROP EVENT `auto_delete_chckout`;
CREATE DEFINER=`root`@`localhost` 
EVENT `auto_delete_chckout` 
ON SCHEDULE EVERY 1 MINUTE STARTS '2013-01-26 13:09:15' 
ON COMPLETION NOT PRESERVE ENABLE DO 
DELETE FROM reserve WHERE checkout <= CURDATE()

so my question is :

If a reservation is being deleted , its room_reserve_qty must be added in the room_qty in room table.

Answer 1

First of all you probably don’t need to fire your event every minute, since you check only for a date part. You might want to do that once a day.

Secondly I would put updating room_qty logic in a trigger. That way no matter how you delete your reservation, your room_qty will be correct.

Your trigger might look like this

DELIMITER $$

CREATE TRIGGER reserve_before_delete BEFORE DELETE ON reserve
    FOR EACH ROW BEGIN
        UPDATE room
           SET room_qty = room_qty + OLD.room_reserve_qty 
         WHERE room_id = OLD.room_id;
    END;
$$

DELIMITER ;

Your DELETE code stays intact when you use a trigger.