带触发器的MySQL列更新

Question

I have this reservation table that has RESERVATION_ID , ROOM_NUM, Date_Start , Date_End and cost columns . what I want to do is insert all the columns except cost and fill the cost automatically.

delimiter //
CREATE TRIGGER upd_check before INSERT ON reservation
    FOR EACH ROW
    BEGIN
        IF NEW.RESERVATION_COST = NULL THEN
            SET NEW.RESERVATION_COST = 'timestampdiff(day, NEW.RESERVATION_STARTD, NEW.RESERVATION_ENDD)*70';
        END IF;
    END;//
delimiter ;

I wrote this trigger to do it, but whenever I press Apply to insert everything nothing is inserted to the RESERVATION_COST column.

why?

Answer 1

I would put this in a comment if I had enough reputation, but anyways. Triggers cannot act on the same table which activated them. Seems limiting, but we've had the same issue.

This link doesn't explicitly say this but it does say "in the associated table". Not the same: https://dev.mysql.com/doc/refman/5.0/en/triggers.html

Answer 2

You can't compare a value to NULL , you need to check if it IS NULL instead.

eg

 IF NEW.RESERVATION_COST IS NULL THEN