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:为了修改当前记录的值，不使用UPDATE语句，使用NEW.columname

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