Question

我在MySQL中有这个触发器

DROP TRIGGER IF EXISTS BeforeGivingRight;
DELIMITER $$
CREATE TRIGGER BeforeGivingRight
BEFORE INSERT ON right_code 
FOR EACH ROW
BEGIN
    DECLARE num_rows INT NULL;
    SELECT count(right_code) FROM valid_rights WHERE right_code  =
                                            new.user_right.right INTO num_rows;

    IF num_rows = 0
    THEN
        set msg = concat('Error: That right is not allowed!',
                                      cast(new.user_right.right as char));
        signal sqlstate '45000' set message_text = msg;
    END IF;
END $$
DELIMITER ;

它给了我这个错误

#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 'NULL; SELECT
    count(right_code) FROM valid_rights WHERE right_code  = new.user_' at line 5

这里有什么问题？我想要做的是，在我插入一些权利（例如201）之前，使用触发器检查valid_rights是否存在。

编辑（回答）

因此，当您需要一个触发异常的触发器时，您可以使用它：

DROP TRIGGER IF EXISTS BeforeGivingRight;
DELIMITER $$
CREATE TRIGGER BeforeGivingRight
BEFORE INSERT ON user_rights 
FOR EACH ROW
BEGIN
    DECLARE num_rows INT DEFAULT NULL;
    DECLARE msg VARCHAR(32) DEFAULT "";
    SELECT count(valid_rights.right_code) FROM valid_rights WHERE right_code  = new.right INTO num_rows;

    IF num_rows = 0
    THEN
        set msg = concat('Error: That right is not allowed!', cast(new.right as char));
        signal sqlstate '45000' set message_text = msg;
    END IF;
END $$
DELIMITER ;

Answer 1

您错过了关键字 DEFAULT。使用0代替NULL。

DECLARE total_sale INT DEFAULT 0;

<强> See HERE: Variables in Stored Procedures

Answer 2

作为一种解决方法：如果表是InnoDb，那么您可以使用外键来限制不存在的值。只需将外键添加到user_rights表，服务器就会引发错误。

例如 -

ALTER TABLE right_code
  ADD CONSTRAINT FK_right_code FOREIGN KEY (`right`)
    REFERENCES valid_rights(right_code) ON DELETE RESTRICT ON UPDATE RESTRICT;

另外，如果不是，请将字段valid_rights。right_code设为唯一字段。

Answer 3

因此，当你想要一个抛出异常的触发器时，你可以使用它：

DROP TRIGGER IF EXISTS BeforeGivingRight;
DELIMITER $$
CREATE TRIGGER BeforeGivingRight
BEFORE INSERT ON user_rights 
FOR EACH ROW
BEGIN
    DECLARE num_rows INT DEFAULT NULL;
    DECLARE msg VARCHAR(32) DEFAULT "";
    SELECT count(valid_rights.right_code) FROM valid_rights WHERE right_code  = new.right INTO num_rows;

    IF num_rows = 0
    THEN
        set msg = concat('Error: That right is not allowed!', cast(new.right as char));
        signal sqlstate '45000' set message_text = msg;
    END IF;
END $$
DELIMITER ;

简单的TRIGGER抛出错误并停止执行

3 个答案: