刪除自引用表級聯

Question

我有一個自引用的註釋表。 我試圖在刪除級聯寫，但它需要一些例外

Introducing FOREIGN KEY constraint 'FK_Comments_Comments' on table 'Comments' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.

，然後嘗試寫一個觸發器，但它採取例外再次

CREATE TRIGGER [dbo].[T_comment_Trigger] 
    ON [dbo].[Comments] 
    FOR DELETE 
AS 
    DELETE FROM Comments 
    WHERE ParentId =(SELECT deleted.id FROM deleted) 

couldn't delete rows that have children

我怎麼可以刪除級聯的做我的自引用表？

Answer 1

假設你保持你的FOREIGN KEY約束，你不能在FOR DELETE觸發器中解決問題。 FOR觸發器（也稱爲AFTER觸發器）激活後活動已發生。並且如果外鍵具有引用，則會阻止行被刪除。在刪除之前發生外鍵檢查。

你需要的是一個INSTEAD OF觸發器。您還需要記住，您當前的觸發器只試圖處理一個「級別」的引用。 （因此，如果連續3引用第2行和第2行引用第1行，並刪除第1行，你只觸發試圖刪除第二行）

所以，像這樣：

CREATE TRIGGER [dbo].[T_comment_Trigger] 
    ON [dbo].[Comments] 
    INSTEAD OF DELETE 
AS 
    ;WITH IDs as (
     select id from deleted 
     union all 
     select c.id 
     from Comments c 
       inner join 
      IDs i 
       on 
       c.ParentID = i.id 
    ) 
    DELETE FROM Comments 
    WHERE id in (select id from IDs); 

如果有其他（非自引用）級聯外鍵約束，它們都必須由此觸發器中的操作替換。在這種情況下，我建議引入表變量來保存最終會被從Comments表中刪除所有的ID列表：

CREATE TRIGGER [dbo].[T_comment_Trigger] 
    ON [dbo].[Comments] 
    INSTEAD OF DELETE 
AS 
    declare @deletions table (ID varchar(7) not null); 
    ;WITH IDs as (
     select id from deleted 
     union all 
     select c.id 
     from Comments c 
       inner join 
      IDs i 
       on 
       c.ParentID = i.id 
    ) 
    insert into @deletions(ID) 
    select ID from IDs 

    DELETE FROM OtherTable 
    WHERE CommentID in (select ID from @deletions) 

    --This delete comes last 
    DELETE FROM Comments 
    WHERE id in (select ID from @deletions);