Question

I’m new to CTE’s in T-SQL but am loving them. However, I cannot get the logic right for this particular stored procedure I’m writing.

Given Table A with the columns:

Id Name Inherits ...

Where the column Inherits stores an int that is an id to another row in this same table.

And Table B with the columns:

Id Name AId ...

Where AId is a foreign key to a row in Table A.

How could one use a CTE to start from an arbitrary row (x) in A, collect all rows in B where AId = x.Id, and then recurse upwards in A by setting x to the row pointed to by x.Inherits. This should proceed until x.Inherits IS NULL.

So the overall effect is I want to return the related B rows for the starting A Id, and then all inherited B rows that we discover by examining the Inherits column of A recursively.

My thinking is to set up the CTE to recurse to the ‘root’ of A from an arbitrary row, and inside each recursive call have a JOIN to B. This is as far as I’ve got:

WITH c 
AS
(

    SELECT A.Inherits,A.Id, B.Name, 1 AS Depth
    FROM tbl_A A
    INNER JOIN tbl_B B ON B.AId = A.Id
    WHERE A.Id = @ArbitraryStartingAId

    UNION ALL

    SELECT T.Inherits,T.Id, c.Name, c.Depth + 1 AS 'Level'
    FROM tbl_A T        
    INNER JOIN c ON T.Id = c.Inherits

)
SELECT *
FROM c 

Which produces:

1        4  b_val   1
11       1  b_val   2
NULL    11  b_val   3

Where b_val is taken from a Table B row (y) where @ArbitraryStartingAId = y.AId. The recursion on A is working, but not pulling in the correct corresponding B data upon each iteration.

If someone could help rewrite this to yield the results I need then that would be great.

Many thanks

Answer 1

Supposing we have

CREATE TABLE A ( Id int, Name nvarchar(10), Inherits int );
CREATE TABLE B ( Id int, Name nvarchar(10), AId int );

And some data:

INSERT A VALUES ( 1, 'one', 2 );
INSERT A VALUES ( 2, 'two', 3 );
INSERT A VALUES ( 3, 'three', null );
INSERT A VALUES ( 4, 'four', 3 );

INSERT B VALUES ( 1, 'B one', 1 );
INSERT B VALUES ( 2, 'B two', 2 );
INSERT B VALUES ( 3, 'B three', 3 );
INSERT B VALUES ( 4, 'B four', 4 );

We can build a CTE on A, and then join to B:

WITH cteA AS 
(
    -- Anchor
    SELECT Id, Name, Inherits FROM A
    WHERE Id = 1
    UNION 
    -- Recursive
    SELECT A.Id, A.Name, A.Inherits FROM A
    INNER JOIN cteA ON A.Id = cteA.Inherits
)
SELECT * FROM cteA INNER JOIN B ON cteA.Id = B.AId;

to get (with the anchor at 1):

Id          Name       Inherits    Id          Name       AId
----------- ---------- ----------- ----------- ---------- -----------
1           one        2           1           B one      1
2           two        3           2           B two      2
3           three      NULL        3           B three    3