MySQL CREATE TRIGGER，语法错误。我做错了什么？

Question

DELIMITER || 
CREATE TRIGGER `monthly_insert` BEFORE INSERT ON `history_monthly` 
FOR EACH ROW 
BEGIN 
    NEW.`uid` = CONCAT(OLD.`year`, OLD.`month`, OLD.`charactersId`); 
END; 
|| 
DELIMITER ; 

并返回一个错误：

#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '.`uid` = CONCAT(OLD.`year`, OLD.`month`, OLD.`charactersId`); 
END' at line 4 

这是我第一次用触发器和我尽我所能，试图找到soultion但我失败了; <

Answer 1

您需要添加这一行字SET：

SET NEW.`uid` = CONCAT(OLD.`year`, OLD.`month`, OLD.`charactersId`); 

此外，作为评论者指出的那样，有一个BEFORE INSERT触发没有OLD值。