Question

I have the following table:

if object_id(N'dbo.Node') is null
create table dbo.Node
(
    ID bigint identity primary key,
    ParentID bigint null foreign key references Node(ID) on delete no action,
    DateCreated datetime not null,
    LastUpdated datetime not null,
    [Name] nvarchar(500) not null,
);

Now, because SQL Server complains when I try to set the foreign key as a cascaded delete, I have created a trigger to do the work:

create trigger Node_Delete on Node for delete
as
begin
    delete from Node where ParentID in (select id from deleted)
end

Now here’s a sample data set:

ID                   ParentID             DateCreated             LastUpdated             Name
520                  1                    2010-01-12 02:26:26.890 2010-01-12 02:26:26.890 Test 1
523                  520                  2010-01-12 02:32:44.777 2010-01-12 02:32:44.777 Test 2

Now let’s run this bit of SQL:

delete from Node where ID=520

The node should delete, along with the child node. So why do I get this error?

Msg 547, Level 16, State 0, Line 1
The DELETE statement conflicted with the SAME TABLE REFERENCE constraint "FK__Node__ParentID__117F9D94". The conflict occurred in database "mydb", table "dbo.Node", column 'ParentID'.
The statement has been terminated.

Answer 1

The foreign key blocks the initial delete, and the trigger never fires. Try an INSTEAD OF trigger that deletes from the furthest decedent on up.