Mysql触发器返回1442错误

Question

我有一个带有 order 属性的测试表。当 order 更改它的值时，我需要重新计算表中的所有 order 值。我有以下触发器:

DELIMITER $$

USE `testing`$$

DROP TRIGGER /*!50032 IF EXISTS */ `UpdateOrder`$$

CREATE
    /*!50017 DEFINER = 'root'@'localhost' */
    TRIGGER `UpdateOrder` AFTER UPDATE ON `upd_tst` 
    FOR EACH ROW BEGIN
    IF (OLD.order > NEW.order) THEN
        UPDATE `upd_tst` t SET t.`order` = t.`order` + 1 WHERE t.`order` < OLD.order AND t.`order` >= NEW.order AND t.id <> OLD.id;
    ELSEIF (OLD.order < NEW.order) THEN
        UPDATE `upd_tst` t SET t.`order` = t.`order` - 1 WHERE t.`order` > OLD.order AND t.`order` <= NEW.order AND t.id <> OLD.id;
    END IF;
    END;
$$

DELIMITER ;

当我尝试更新任何项目时，出现下一个错误:

Executed SQL Statement : update `testing`.`upd_tst` set `order`='3' where `id`='3' 
Error Number : 1442 
Error Message: Can't update table 'upd_tst' in stored function/trigger because it is already used by statement which invoked this stored function/trigger. 

我该如何解决这个问题？我只需要使用触发器。

在 postgresql 中，我可以在触发器 WHEN (pg_trigger_depth() = 0) 中使用这个条件。 mysql中有这样的东西吗？

Answer 1

解决方案:使其成为BEFORE UPDATE ON 触发器 - 这样您就可以在触发器 SQL 中设置/更新字段值:

DELIMITER $$
USE `testing`$$
DROP TRIGGER /*!50032 IF EXISTS */ `UpdateOrder`$$
CREATE
    /*!50017 DEFINER = 'root'@'localhost' */
    TRIGGER `UpdateOrder` BEFORE UPDATE ON `upd_tst` 
    FOR EACH ROW BEGIN
    IF (OLD.order > NEW.order) THEN
        SET NEW.order = NEW.order` + 1;
    ELSEIF (OLD.order < NEW.order) THEN
        SET NEW.order = NEW.order - 1;
    END IF;
    END;
$$
DELIMITER ;

---

原因:您无法更新/删除/插入到调用触发器的表:

Within a stored function or trigger, it is not permitted to modify a table that is already being used (for reading or writing) by the statement that invoked the function or trigger.

这样做会产生错误 1442:

Error Code: 1442
Can't update table 'MyTable' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.