具有逻辑错误条件的插入防止触发器

Question

我正在尝试阻止将Card1 and Card2 更新为refer 表上的用户，该用户的资金少于表total_balance.total_money

是否可以将这样的逻辑与触发器相结合？

    DELIMITER $$
        CREATE trigger LessMoneyThanRequirePrevent
        Before update ON refer
        FOR EACH ROW
      BEGIN
           IF (total_balance.total_money<10 WHERE total_balance.username= NEW.refer.username)
              THEN
            SIGNAL SQLSTATE '45000' 
                SET MESSAGE_TEXT = 'Not enough money in your account! Insertion canceled';
              END IF;
         END
        $$
        DELIMITER;

我认为 WHERE 条件有问题？

表格：参考

 ID        username        Card1        Card2  
+-------+-------------+-------------+-----------+
|   1   |  John       |  Value      |    Value  |
+-------+-------------+-------------+-----------+
|   2   |  Alex       |  Null       |    Null   |
+-------+-------------+-------------+-----------+

表格：total_balance

 ID        username     total_money        
+-------+-------------+-------------+
|   1   |  John       |     10.0    |
+-------+-------------+-------------+
|   2   |  Alex       |     7.0     |
+-------+-------------+-------------+

例如： 用户 John 在 total_balance 上有足够的钱，允许更新 Card1 and Card2 但 Alex 不够 不允许

Answer 1

需要更改 IF 条件。以下是触发器：

DELIMITER $$
CREATE trigger LessMoneyThanRequirePrevent
Before update ON refer
FOR EACH ROW
BEGIN
   IF EXISTS(SELECT 1 FROM total_balance b WHERE b.total_money<10 AND b.username= NEW.username)
   THEN
      SIGNAL SQLSTATE '45000' 
        SET MESSAGE_TEXT = 'Not enough money in your account! Insertion canceled';
   END IF;
END
$$