Get Root parent of child in Hierarchical table

Question

I have a table with hierarchical data in it, the structure goes like this:

ID      ParentId
----   ----------
1       NULL
2       1
3       2
4       2
5       3
6       5

If I pass the node Id I would like to get the top most node Id/details by traversing through all its parents in SQL.

I tried CTE, i somehow cannot get the combination correct. However, i got this working as a function but it is so slow that i had to post this question.

In the above example if I pass 6, i would want to have the top most i.e. 1. By traversing through 6 => 5 => 3 => 2 => [1] (result)

Thanks in advance for your help.

Answer 1

DECLARE @id INT = 6
;WITH parent AS
(
    SELECT id, parentId, 1 AS [level] from tbl WHERE id = @id
    UNION ALL 
    SELECT t.id, t.parentId, [level] + 1 FROM parent
    INNER JOIN tbl t ON t.id =  parent.parentid
)
SELECT TOP 1 id FROM parent ORDER BY [level] DESC

@TechDo's answer assumes the lowest ID will be the parent. If you don't want to rely on this then the above query will sort by the depth.

Answer 2

Please try:

declare @id int=6
;WITH parent AS
(
    SELECT id, parentId  from tbl WHERE id = @id
    UNION ALL 
    SELECT t.id, t.parentId FROM parent
    INNER JOIN tbl t ON t.id =  parent.parentid
)

SELECT TOP 1 id FROM  parent
order by id asc