使用 MariaDB 触发 ON DELETE

Question

我有两个表'topic'和'answer'：

表 1-> 主题

+-----+------------+
+  id +   name     +
+=====+============+
+  17 +  question1 +
+-----+------------+
+  18 +  question2 +
+-----+------------+

表 2 -> 答案

+----+---------+----------+
+ id + idtopic +   val    +
+----+---------+----------+
+  1 +  17     +  anwer1  +
+----+---------+----------+
+  2 +  18     +  answer2 +
+----+---------+----------+

在从表“主题”中删除项目时，我只是想创建一个触发器以从表“答案”中删除项目。 如果尝试了这段代码，但我认为 MariaDB 语法有问题：

CREATE TRIGGER delete_answer AFTER DELETE on topic
FOR EACH ROW
BEGIN
DELETE FROM answer
    WHERE answer.idtopic = topic.id;
END

Answer 1

这是可以使用的版本。

DELIMITER //
CREATE TRIGGER delete_answer AFTER DELETE on topic
FOR EACH ROW
BEGIN
DELETE FROM answer
    WHERE answer.idtopic = OLD.id;
END
//
DELIMITER ;

Answer 2

使用OLD 关键字来引用主题中被删除的当前行。

CREATE TRIGGER delete_answer AFTER DELETE on topic
FOR EACH ROW
BEGIN
DELETE FROM answer
    WHERE answer.idtopic = OLD.id;
END

但是，我想知道您为什么要编写触发器而不是使用具有级联删除的外键。你没有为这个表使用 InnoDB 吗？