CTE query that fails to find ancestors

I have a table foo

with the content

So the records of the table foo can graphically be represented with this graph.

When I run a procedure that should return ancestors using @starting_id=9

(represented with a circle in the graph), I get this result

• 9

instead of

• 9

• 3 and

• 1

Why?

CREATE PROCEDURE [dbo].[GetParents]
    @starting_id int
AS
BEGIN
    WITH chainIDsUpwards AS
    (
        SELECT id, parent_id FROM foo WHERE id = @starting_id

        UNION ALL

        SELECT foo.id, foo.parent_id FROM foo
        JOIN chainIDsUpwards p ON p.id = foo.parent_id
    )

    SELECT id FROM chainIDsUpwards
END

Fiddle at https://dbfiddle.uk/?rdbms=sqlserver_2019&fiddle=81be3d86dc7581eb60bc7af4c09077e4

I believe you made a mistake in your join predicate. Try:

WITH chainIDsUpwards AS
    (
        SELECT id, parent_id FROM foo WHERE id = @starting_id

        UNION ALL

        SELECT foo.id, foo.parent_id FROM foo
        JOIN chainIDsUpwards p ON p.parent_id = foo.id
    )

    SELECT id FROM chainIDsUpwards

It is a common mistake, and I often do it myself. I find it easiest to get it right if I start with the CTE:

SELECT foo.id, foo.parent_id 
    FROM chainIDsUpwards p
    JOIN foo p

and then think of the CTE as a linked list where I need to find the next element:

ON p.parent_id = foo.id

StackExchange Database Administrators Q#274233, answer score: 6