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，我正在尽最大努力尝试找到灵魂，但我失败了；<

Answer 1

您需要在这一行中添加单词 SET：

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

此外，正如评论者指出的那样，BEFORE INSERT 触发器中没有 OLD 值。