创建插入后删除的触发器

Question

I want to create a trigger that delete any new row I insert if the value is more than a number, for example 8.

I tried this:

DELIMITER |
CREATE TRIGGER mytrigger AFTER INSERT ON mytab
    FOR EACH ROW 
    BEGIN
        DELETE FROM mytab WHERE myparameter > 8;
    END
|

DELIMITER ;

I tried to:

INSERT INTO mytab VALUES (9);

and this is the message error:

#1442 - Can't update table 'mytab' in stored function/trigger because it is already used by statement which invoked this stored function/trigger. 

also if I put:

INSERT INTO mytab VALUES (4);

it runs the same error...

Thanks!

Answer 1

Don't delete! Just produce an error on the insert:

DELIMITER |
CREATE TRIGGER mytrigger BEFORE INSERT ON mytab
    FOR EACH ROW 
    BEGIN
        IF new.myparameter > 8 THEN
             signal sqlstate '45000' set message_text = 'Parameter value can be no larger than 8';
        END IF;
    END
|

DELIMITER ;