mysql - MySQL 触发器中的WITH (...) IF ... IN ...

Question

给定简单的表(我使用的是 MySQL Server 8.0.17)

CREATE TABLE folders (
  id varchar(3) NOT NULL,
  parent varchar(3) DEFAULT NULL,
  PRIMARY KEY (id),
  KEY fk_folders_parent_idx (parent),
  CONSTRAINT fk_folders_parent FOREIGN KEY (parent) REFERENCES folders(id) ON DELETE RESTRICT ON UPDATE RESTRICT
);

我想防止循环引用。我通过创建更新触发器来尝试此操作(因为在逐个插入项目时不可能创建循环依赖项)。

触发代码如下所示(我定位自己 here 和 here ):

DELIMITER $$

CREATE TRIGGER `test`.`folders_BEFORE_UPDATE` BEFORE UPDATE ON `folders` FOR EACH ROW
BEGIN
    WITH RECURSIVE children (id) AS 
    (
        SELECT id FROM folders WHERE parent = NEW.id
        UNION ALL
        SELECT f.id FROM folders f INNER JOIN children ON f.parent = children.id
    ) 
    IF NEW.parent IN children THEN
        signal sqlstate '45000' set message_text = 'My Error Message'
    END IF
END$$

我收到非常有用的错误消息:

Error Code: 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 'IF NEW.parent IN children THEN         signal sqlstate '45000' set message_text ' at line 9

我相信这是因为我无法使用WITH Children (...) IF NEW.parent IN Children

我尝试WITH Children (...) IF EXISTS (SELECT id FROM Children WHERE id = NEW.parent)，但得到了相同的响应。

MySQL Workbench 告诉我，第 4 行中的 BEGIN 没有 END。不过我认为这不是问题所在。

Answer 1

我的 MySQL 变得有点生疏和过时，因为我现在的大多数任务都是使用 MS SQL，但尝试这样的操作:

DECLARE found INT;

WITH RECURSIVE children (id) AS 
(
   SELECT id FROM folders WHERE parent = NEW.id
   UNION ALL
   SELECT f.id FROM folders f INNER JOIN children ON f.parent = children.id
) 
SELECT id INTO found
FROM children 
WHERE id = NEW.parent
LIMIT 1
;

IF NEW.parent = found THEN
   signal sqlstate '45000' set message_text = 'My Error Message'
END IF

<小时/>

编辑(下面是Le 'nton的最终解决方案):

DELIMITER $$

DROP TRIGGER IF EXISTS `test`.`folders_BEFORE_UPDATE`$$

CREATE TRIGGER `test`.`folders_BEFORE_UPDATE` BEFORE UPDATE ON `folders` FOR EACH ROW
BEGIN
    DECLARE result varchar(3);

    WITH RECURSIVE children (id) AS 
    (
        SELECT id FROM folders WHERE parent = NEW.id
        UNION ALL
        SELECT f.id FROM folders f INNER JOIN children ON f.parent = children.id
    ) SELECT id INTO result FROM children WHERE id = NEW.parent LIMIT 1;

    IF NEW.parent = result then
        signal sqlstate '45000' set message_text = 'Circular dependency detected!';
    END IF;
END$$