关于自引用表的删除级联

Question

我有一个自引用的注释表。 我试图在删除级联上写，但它需要一些例外

在表 'Comments' 上引入 FOREIGN KEY 约束 'FK_Comments_Comments' 可能会导致循环或多个级联路径。指定 ON DELETE NO ACTION 或 ON UPDATE NO ACTION，或修改其他 FOREIGN KEY 约束。

然后尝试编写触发器，但它再次发生异常

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

无法删除有子行的行

如何删除我的自引用表的级联？

Answer 1

假设您保留了 FOREIGN KEY 约束，则无法在 FOR DELETE 触发器中解决问题。 FOR 触发器（也称为AFTER 触发器）在活动发生后触发。如果有引用，外键将防止行被删除。外键检查发生在删除之前。

您需要的是一个INSTEAD OF 触发器。您还需要记住，您当前的触发器仅尝试处理一个“级别”的引用。 （所以，如果第 3 行引用第 2 行，第 2 行引用第 1 行，而您删除第 1 行，则您的触发器仅尝试删除第 2 行）

所以，类似：

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）级联外键约束，它们都必须被此触发器中的操作替换。在这种情况下，我建议引入一个表变量来保存最终将从 Comments 表中删除的所有 ID 的列表：

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

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