sql - On delete cascade for self-referencing table

Question

I have a comment table that is self-referencing. I tried to write on delete cascade but it take some exception

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.

and then try to write a trigger but it take exception again

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

how can I do on delete cascade for my self-referencing table?

Answer 1

Assuming you're keeping your FOREIGN KEY constraint in place, you cannot fix the issue in a FOR DELETE trigger. FOR triggers (also known as AFTER triggers) fire after the activity has taken place. And a foreign key will prevent a row from being deleted if it has references. Foreign key checks occur before deletion.

What you need is an INSTEAD OF trigger. You also need to bear in mind that your current trigger only tried to deal with one "level" of referencing. (So, if row 3 references row 2 and row 2 references row 1, and you delete row 1, your trigger only tried to remove row 2)

So, something like:

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);

If there are other (non-self-referencing) cascading foreign key constraints, they all have to be replaced by actions in this trigger. In such a case, I'd recommend introducing a table variable to hold the list of all IDs that will eventually be deleted from the Comments table:

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);