Finding Top level parent of each row of a table [SQL Server 2008]

Question

I have following two tables

Table Person

Id   Name
   1    A
   2    B
   3    C
   4    D
   5    E

Table RelationHierarchy

Answer 1

You could try to use a loop. As you will get many levels of recursion with your approach:

declare @child int = 0
declare @parent int = 1 --child to search
while @child <> @parent 
BEGIN
set @child = @parent
select  @parent = Parentid from @parentchild  where ChildID = @child
END
select @parent

Answer 2

I have also updated the answer in the original question, but never-mind, here is a copy also:

;WITH RCTE AS
(
    SELECT  ParentId, ChildId, 1 AS Lvl FROM RelationHierarchy 

    UNION ALL

    SELECT rh.ParentId, rc.ChildId, Lvl+1 AS Lvl 
    FROM dbo.RelationHierarchy rh
    INNER JOIN RCTE rc ON rh.ChildId = rc.ParentId
)
,CTE_RN AS 
(
    SELECT *, ROW_NUMBER() OVER (PARTITION BY r.ChildID ORDER BY r.Lvl DESC) RN
    FROM RCTE r

)
SELECT pc.Id AS ChildID, pc.Name AS ChildName, r.ParentId, pp.Name AS ParentName
FROM dbo.Person pc 
LEFT JOIN CTE_RN r ON pc.id = r.CHildId AND  RN =1
LEFT JOIN dbo.Person pp ON pp.id = r.ParentId

SQLFiddle DEMO

Note that the slight difference is in recursive part of CTE. ChildID is now rewritten each time from the anchor part. Also addition is ROW_NUMBER() function (and new CTE) to get the top level for each child at the end.

EDIT - Version2

After finding a performance issues with first query, here is an improved version. Going from top-to-bottom, instead of other way around - eliminating creating of extra rows in CTE, should be much faster on high number of recursions:

;WITH RCTE AS
(
    SELECT  ParentId, CHildId, 1 AS Lvl FROM RelationHierarchy r1
    WHERE NOT EXISTS (SELECT * FROM RelationHierarchy r2 WHERE r2.CHildId = r1.ParentId)

    UNION ALL

    SELECT rc.ParentId, rh.CHildId, Lvl+1 AS Lvl 
    FROM dbo.RelationHierarchy rh
    INNER JOIN RCTE rc ON rc.CHildId = rh.ParentId
)
SELECT pc.Id AS ChildID, pc.Name AS ChildName, r.ParentId, pp.Name AS ParentName
FROM dbo.Person pc 
LEFT JOIN RCTE r ON pc.id = r.CHildId
LEFT JOIN dbo.Person pp ON pp.id = r.ParentId 

SQLFiddle DEMO

Answer 3

Another way to do this in a loop if you wanted to work with sets is:

SELECT *
    INTO #parentchild
from RelationHierarchy 


WHILE EXISTS
(select 1 from  #parentchild A inner join #parentchild B on A.ChildID = B.ParentID Where A.ParentID <> B.ParentID )
BEGIN
update B set B.ParentID = A.ParentID
from #parentchild A inner join #parentchild B on A.ChildID = B.ParentID
END

select * from #parentchild