Composite Primary keys and Foreign key constraint error

Question

Hi,

I have an SQL table defined as below:

CREATE TABLE [TestComposite] (  
    ID int,  
    SiteUrl nvarchar(255),  
    Name nvarchar(max) NOT NULL,  
    ParentID int NULL,  
    PRIMARY KEY (ID, SiteUrl)  
);

Items and folders are stored inside the same table, if an item is inside a folder, the ParentID column is the ID of the folder. And I would like to be able to delete CASCADE items/folders when I delete a folder.

An example may be more explicit:

INSERT INTO [TestComposite] VALUES (1, 'site1', 'Item1', NULL)
INSERT INTO [TestComposite] VALUES (2, 'site1', 'Item2', NULL)
INSERT INTO [TestComposite] VALUES (3, 'site1', 'Folder1', NULL)
INSERT INTO [TestComposite] VALUES (4, 'site1', 'Folder1.Item1', 3)
INSERT INTO [TestComposite] VALUES (5, 'site1', 'Folder1.Item2', 3)
INSERT INTO [TestComposite] VALUES (6, 'site1', 'Folder1.Folder1', 3)
INSERT INTO [TestComposite] VALUES (7, 'site1', 'Folder1.Folder1.Item1', 6)
etc...

So if I delete the item 3 (a folder), I want the items/folders 4, 5, 6 and 7 to be deleted too.

I tried to add a constraint similar to:

ALTER TABLE [TestComposite] 
ADD CONSTRAINT fk_parentid 
FOREIGN KEY (ParentID, SiteUrl) 
REFERENCES [TestComposite] (ID, SiteUrl) ON DELETE CASCADE;

But it gives me this error:
Introducing FOREIGN KEY constraint 'fk_parentid' on table 'TestComposite' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.

I also tried to add a second SiteUrl column named ParentSiteUrl, in case the problem was that a column couln't be part of the same FK/PK, but I have the same error message.

Am i doing something wrong?

Thank you,

Answer 1

I think what you want to do can be achieved by adding a new column called ParentId, and then declare it as foreign key with primary key. That way the problem will be solved and you can still do everything you want

Answer 2

The problem is that you create possibility of recursive cascade - when every deleted by cascade can create any number of subsequent deletes. MS SQL doesn't support it. Try to delete them in your code manually. BTW i don't recommend cascade deletes.

http://support.microsoft.com/kb/321843

Answer 3

If you have SQL Server 2008, use from HierarchyID type for this work.

Answer 4

Create an ON DELETE NO ACTION constraint and use this to delete all records and their children:

WITH    q AS
        (
        SELECT  id, SiteURL
        FROM    TestComposite
        WHERE   id = 3
                AND SiteURL = 'site1'
        UNION ALL
        SELECT  tc.id, tc.SiteURL
        FROM    q
        JOIN    TestComposite tc
        ON      tc.ParentID = q.Id
                AND tc.SiteURL = q.SiteURL
        )
DELETE
FROM    TestComposite
WHERE   EXISTS
        (
        SELECT  id, SiteURL
        INTERSECT
        SELECT  *
        FROM    q
        )