Question

我有两个表，分别是FORUM和COMMENTS

论坛主键：论坛

注释外键：FORUMID

，并且用户要删除论坛中的任何行时，必须执行触发器并首先删除该论坛中的评论

CREATE OR REPLACE trigger trg_delete_comments
BEFORE DELETE ON forum
FOR EACH ROW
DECLARE 
   v_forumID varchar(14);
   v_user    varchar(20);
BEGIN
   select forumID into v_forumID
   from forum
   where forumID =:OLD.forumID;

   DELETE COMMENTS
   WHERE FORUMID = v_forumID;
END;
/

当我尝试执行此触发器时，出现错误“ ORA-04091：表FORUM正在变异，触发器/函数可能看不到它”。有什么办法解决吗？谢谢

Answer 1

您所描述的是可以通过外键约束轻松提供的功能。您可以使用on delete cascade子句配置约束，以便在删除父记录时也删除子记录。

考虑the following example：

create table forums (
    forum_id int primary key,
    user_id int
);

create table comments (
    comment_id int primary key,
    forum_id int,
    constraint fk_comments_forum 
        foreign key (forum_id) 
        references forums(forum_id)
        on delete cascade
);

select * from forums;

FORUM_ID | USER_ID
-------: | ------:
       1 |       1
       2 |       2

select * from comments;

COMMENT_ID | FORUM_ID
---------: | -------:
         1 |        1
         2 |        1
         3 |        2

delete from forums where forum_id = 1;
1 rows affected

select * from comments;

COMMENT_ID | FORUM_ID
---------: | -------:
         3 |        2

Answer 2

您可以将其转换为AFTER STATEMENT触发器，然后可以删除行-

CREATE OR REPLACE trigger trg_delete_comments
AFTER DELETE ON forum
BEGIN

   DELETE COMMENTS
   WHERE FORUMID IN (select forumID
                     from forum
                     where forumID = :OLD.forumID);
END;
/

Answer 3

我不明白为什么您要为此使用两个查询。逻辑似乎是：

BEGIN
   DELETE COMMENTS
   WHERE FORUMID = :OLD.forumID;
END;

Answer 4

我认为您可能在这里重新发明了轮子。您只需将外键定义为on delete cascade，然后让数据库为您完成繁重的工作即可：

ALTER TABLE comments
ADD CONSTRAINT fk_forum
FOREIGN KEY (forumid)
REFERENCES forum(forumid)
ON DELETE CASCADE

触发从另一个表中删除行

4 个答案: