Finding Top Level Parent ID

Question

In one table I have two columns like below

ID  ParentID
1   0x0
2   1
3   2
9   0x0
5   9
6   5
25  0x0
30  25

How to get top level parent ID

Answer 1

You can use a recursive CTE:

WITH CTE AS
(
    SELECT *, 1 [Level]
    FROM dbo.YourTable
    UNION ALL
    SELECT A.ID, B.[Parent ID], [Level] + 1
    FROM CTE A
    INNER JOIN dbo.YourTable B
        ON A.[Parent ID] = B.Id
), CTE2 AS
(
    SELECT *, RN = ROW_NUMBER() OVER(PARTITION BY ID ORDER BY [Level] DESC)
    FROM CTE
    WHERE [Parent ID] <> 0
)
SELECT ID, [Parent ID] [Top Level Parent Id]
FROM CTE2
WHERE RN = 1
OPTION (MAXRECURSION 0)
;

Here is a link with a demo.

The results are:

╔════╦═════════════════════╗
║ ID ║ Top Level Parent Id ║
╠════╬═════════════════════╣
║  2 ║                   1 ║
║  3 ║                   1 ║
║  5 ║                   9 ║
║  6 ║                   9 ║
║ 30 ║                  25 ║
╚════╩═════════════════════╝