How to get anchor details when using CTE? - sql

Question

I have a table with 2 columns (id, childId) with the following data:
1, 2
2, 4
3, 5
4, 6
5, null
6, null
I have the following CTE that gets the records and it works fine.
DECLARE #id TABLE (id int, childId int);
INSERT INTO #id SELECT 1, 2;
INSERT INTO #id SELECT 2, 4;
INSERT INTO #id SELECT 3, 5;
INSERT INTO #id SELECT 4, 6;
INSERT INTO #id SELECT 5, null;
INSERT INTO #id SELECT 6, null;
WITH cte AS
(
SELECT id, childId
FROM #id
WHERE id = 1
UNION ALL
SELECT b.id, b.childId
FROM #id b
INNER JOIN cte
ON b.id = cte.childId
)
SELECT * FROM cte
However, I would like to add the anchor details so that the results will look like:
1, 2, null
2, 4, 1
4, 6, 1
6, null, 1
So that that 3rd column is the main anchor record.
Is this possible?

Answer 1

Just add a thrid column in CTE.
declare #mytable table ( id int, childId int );
insert #mytable
( id, childId )
values ( 1, 2 ),
( 2, 4 ),
( 3, 5 ),
( 4, 6 ),
( 5, null ),
( 6, null );
declare #id table ( id int );
insert into #id
select 1;
insert into #id
select 3;
with cte
as ( select id ,
childId ,
id root
from #mytable
where id in ( select id
from #id )
union all
select b.id ,
b.childId ,
cte.root
from #mytable b
inner join cte on b.id = cte.childId
)
select *
from cte;

Answer 2

WITH cte AS
(
SELECT id, childId, id AS anchorId
FROM mytable
WHERE
id IN (SELECT id FROM #id)
UNION ALL
SELECT b.id, b.childId, cte.anchorId
FROM mytable b
INNER JOIN cte
ON b.id = cte.childId
)
SELECT
id
, childId
, case
WHEN id = anchorId THEN NULL
ELSE anchorId
END as anchorId
FROM cte
With test code provided:
DECLARE #id TABLE (id int, childId int);
INSERT INTO #id SELECT 1, 2;
INSERT INTO #id SELECT 2, 4;
INSERT INTO #id SELECT 3, 5;
INSERT INTO #id SELECT 4, 6;
INSERT INTO #id SELECT 5, null;
INSERT INTO #id SELECT 6, null;
WITH cte AS
(
SELECT id, childId, id AS anchorId
FROM #id
WHERE id IN (1,3)
UNION ALL
SELECT b.id, b.childId, cte.anchorId
FROM #id b
INNER JOIN cte
ON b.id = cte.childId
)
SELECT
id
, childId
, CASE
WHEN id = anchorId THEN NULL
ELSE anchorId
END AS anchorId
FROM cte