如何避免MySQL中的循环触发器依赖

Question

我在MySQL中使用触发器有一点问题.

假设我们有2个表:

• 表A
• 表B

和2个触发器:

• TriggerA:在TableA上删除时触发并更新TableB
• TriggerB:在TableB上删除时触发,在TableA中删除

问题是,当我删除TableB中的某些行时,TriggerB会触发并删除TableA中的某些元素,然后TriggerA会触发并尝试更新TableB.

它失败,因为TriggerA尝试更新TableB中正在删除的某些行.

我怎样才能避免这种循环依赖？

这两个触发器都没用,所以我不知道我该怎么做才能解决这个问题.

Answer 1

尝试使用变量.

第一次触发:

CREATE TRIGGER trigger1
  BEFORE DELETE
  ON table1
  FOR EACH ROW
BEGIN
  IF @deleting IS NULL THEN
    SET @deleting = 1;
    DELETE FROM table2 WHERE id = OLD.id;
    SET @deleting = NULL;
  END IF;
END

第二次触发:

CREATE TRIGGER trigger2
  BEFORE DELETE
  ON table2
  FOR EACH ROW
BEGIN
  IF @deleting IS NULL THEN
    SET @deleting = 1;
    DELETE FROM table1 WHERE id = OLD.id;
    SET @deleting = NULL;
  END IF;
END

额外的AFTER DELETE触发器:

CREATE TRIGGER trigger3
  AFTER DELETE
  ON table1
  FOR EACH ROW
BEGIN
  SET @deleting = NULL;
END

CREATE TRIGGER trigger4
  AFTER DELETE
  ON table2
  FOR EACH ROW
BEGIN
  SET @deleting = NULL;
END