Question

I’m having troubles with an insert trigger that is supposed to fix a column in a table:

id        - auto increment int
thread_id - int [NULL]

What I want to achieve is to set the thread_id to id if it’s inserted as NULL. I have failed because:

• using before insert trigger still does not have the new value for id and mysteriously sets thread_id to 0
• using after insert - update trigger throws reasonable exception can't update table because it is already used by the statement.
• you can not add additional auto increment field

What is the solution to this problem?

Answer 1

DELIMITER $$

CREATE TRIGGER mytrigger BEFORE INSERT ON yourtable 
FOR EACH ROW BEGIN

SET NEW.thread_id = IF(ISNULL(NEW.thread_id), 0, NEW.thread_id);

END;

$$

Edit: in order to modify values of the current record, you don’t use UPDATE statement, you access them by using NEW.columname