Sql-server – CTE query that fails to find ancestors

cterecursivesql servert-sql

I have a table foo

table

with the content

enter image description here

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

enter image description here

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

Best Answer

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
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