SQL中的嵌套触发器

Question

我有以下表格：回复和消息。 我想模仿Facebook的行为：删除一条消息时，所有相关的回复也会被删除。 我的表如下所示：

REPLIES
messageId                               replyId
6b61d107-dff3-4374-a3a2-75ac7478a2f2    865c873d-0210-482a-b8bd-371c4f07f0cf

MESSAGES
id                                      body
865c873d-0210-482a-b8bd-371c4f07f0cf    this is the reply
6b61d107-dff3-4374-a3a2-75ac7478a2f2    this is the message

我创建了一个第一个触发器，该触发器起作用，该触发器在删除消息时会删除“回复”中的相关行。 现在，我想创建另一个触发器，该触发器将在每次删除“答复”中的一行时删除相关消息。 现在，答复变成了自己的消息，这没有任何意义。 这是第二个触发器：

CREATE TRIGGER TRG_DEL_MESSAGES
ON Replies
FOR DELETE
AS
DELETE FROM Messages WHERE id = (SELECT replyId FROM DELETED)

当我尝试删除某些内容时会带来以下错误：

Maximum stored procedure, function, trigger, or view nesting level exceeded (limit 32).

显然存在无限循环，我认为这是因为DELETED表中填充了第一个触发器（引发了第二个触发器）中的数据。 但实际上我不确定，也希望能有所帮助。 谢谢！

Answer 1

这样的事情对您有用吗？ 只需在Messages上使用单个触发器即可删除递归，该触发器可以预先计算所有相关消息，因此您最多应该获得一个嵌套调用？ 它从消息和回复中删除所有相关消息

CREATE TRIGGER TRG_DEL_REPLIES
ON [Messages]
FOR DELETE
AS
BEGIN
    DECLARE @Related TABLE (MessageId uniqueidentifier)

    --get all related messages so that we don't recurse
    BEGIN
        WITH AllReplies (MessageId)
        AS
        (
            --Anchor
            SELECT  D.MessageId
            FROM    Deleted D
            UNION ALL
            --Recurse
            SELECT  R.ReplyId
            FROM    AllReplies AR
            JOIN    Replies R
                    ON AR.MessageId = R.MessageId
        )
        INSERT  INTO @Related
        SELECT  * 
        FROM    AllReplies
    END

    --delete the replies

    DELETE  R
    FROM    Replies R
    JOIN    @Related REL
            ON R.MessageId = REL.MessageId

    --delete the messages

    DELETE  M
    FROM    [Messages] M
    JOIN    @Related REL
            ON  REL.MessageId = M.MessageId 
    LEFT
    JOIN    DELETED D
            ON REL.MessageId = D.MessageId
    WHERE   D.MessageId IS NULL
END

要将其转换为所需的存储过程，我会这样做，而不是执行多个单独删除的循环：

CREATE PROCEDURE DeleteMessageWithReplies(@messageId uniqueidentifier)
AS
BEGIN
    DECLARE @Related TABLE (MessageId uniqueidentifier)

    --get all related messages
    BEGIN
        WITH AllReplies (MessageId)
        AS
        (
            --Anchor
            SELECT  @messageId
            UNION ALL
            --Recurse
            SELECT  R.ReplyId
            FROM    AllReplies AR
            JOIN    Replies R
                    ON AR.MessageId = R.MessageId
        )
        INSERT  INTO @Related
        SELECT  * 
        FROM    AllReplies
    END

    --delete the replies

    DELETE  R
    FROM    Replies R
    JOIN    @Related REL
            ON R.MessageId = REL.MessageId

    --delete the messages that haven't already been deleted

    DELETE  M
    FROM    [Messages] M
    JOIN    @Related REL
            ON  REL.MessageId = M.MessageId 
END

Answer 2

感谢您的帮助Fergus，非常感谢。 但是，正如@Ben指出的那样，存储过程更容易编写。 这是我刚刚写的，可能可以改进，但至少可以奏效。

EXEC('CREATE PROCEDURE deleteMessageWithReplies(@messageId uniqueidentifier)
AS
BEGIN
    DECLARE @repliesCount int
    SELECT @repliesCount = (SELECT COUNT(*) FROM Replies WHERE messageId=@messageId)
    DECLARE @cpt int
SET @cpt = 0
DELETE FROM Messages WHERE id = @messageId
WHILE @cpt < @repliesCount
BEGIN
    DECLARE @replyId uniqueidentifier
    SELECT @replyId = (SELECT TOP 1 replyId FROM Replies WHERE messageId=@messageId)
    DELETE FROM Replies WHERE replyId = @replyId
    DELETE FROM Messages WHERE id=@replyId
    SET @cpt = @cpt+1
END
END')

Answer 3

我建议按照Fergus Bown的建议在触发器中一次生成所有删除操作，或者将删除逻辑移动到存储过程调用中。 在我们的应用程序中，我们对所有CRUD操作（创建，读取，更新，删除）使用存储过程方法。

不利的一面是，如果菜鸟支持DBA使用SQL删除单个Reply而不删除与之关联的所有其他消息，则它们可能会犯错。 但是，这样的DBA应该知道使用存储过程来代替（或者首先获得SQL）。