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 ;

它返回一个错误:

#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 

这是我第一次使用触发器，我尽我所能尝试寻找灵魂，但我失败了；<

Answer 1

你需要在这一行添加单词SET:

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

此外，正如评论者所指出的，BEFORE INSERT 触发器中没有 OLD 值。