Troubles isolating target cell in recursive sql query - sql

Question

I have a table, let's say it looks like this:
c | p
=====
|1|3|
|2|1|
|7|5|
c stands for current and p stands for parent
Given a c value of 2 I would return its top most ancestor (which has no parent) this value is 3. Since this is a self referencing table, I figured using CTE would be the best method however I am very new to using it. Nevertheless, I gave it a shot:
WITH Tree(this, parent) AS
( SELECT c ,p
FROM myTable
WHERE c = '2'
UNION ALL
SELECT M.c ,M.p
FROM myTable M
JOIN Tree T ON T.parent = M.c )
SELECT parent
FROM Tree
However this returns:
1
3
I only want 3 though. I have tried putting WHERE T.parent <> M.c but that doesn't entirely make sense. Neadless to say, I am a little confused for how to isolate the grandparent.

Answer 1

DECLARE #Table AS TABLE (Child INT, Parent INT)
INSERT INTO #Table VALUES (1,3),(2,1),(7,5)
;WITH cteRecursive AS (
SELECT
OriginalChild = Child
,Child
,Parent
,Level = 0
FROM
#Table
WHERE
Child = 2
UNION ALL
SELECT
c.OriginalChild
,t.Child
,t.Parent
,Level + 1
FROM
cteRecursive c
INNER JOIN #Table t
ON c.Parent = t.Child
)
SELECT TOP 1 TopAncestor = Parent
FROM
cteRecursive
ORDER BY
Level DESC
Use a recursive cte to Recuse up the tree until you cannot. Keep track of the Level of recursion, then take the last level of recursions parent and you have the top ancestor.
And just because I wrote it I will add in if you wanted to find the top ancestor of every child. The concept is still the same but you would need to introduce a row_number() to find the last level that was recursed.
DECLARE #Table AS TABLE (Child INT, Parent INT)
INSERT INTO #Table VALUES (1,3),(2,1),(7,5),(5,9)
;WITH cteRecursive AS (
SELECT
OriginalChild = Child
,Child
,Parent
,Level = 0
FROM
#Table
UNION ALL
SELECT
c.OriginalChild
,t.Child
,t.Parent
,Level + 1
FROM
cteRecursive c
INNER JOIN #Table t
ON c.Parent = t.Child
)
, cteTopAncestorRowNum AS (
SELECT
*
,TopAncestorRowNum = ROW_NUMBER() OVER (PARTITION BY OriginalChild ORDER BY Level DESC)
FROM
cteRecursive
)
SELECT
Child = OriginalChild
,TopMostAncestor = Parent
FROM
cteTopAncestorRowNum
WHERE
TopAncestorRowNum = 1