SQL Server CTE -Find top parentID forEach childID?

Question

I have a table which contains hierarchy data - something like:

childID  |  parentID
____________________
  1      |     5
  5      |     9
  9      |     20         


        

Answer 1

With cteherarchy as 
(
Select ChileId,Name,ParentId from tblHerarchy
where ParentId is null 
union ALL
Select h.ChileId,h.Name,h.ParentId  from cte
inner join tblHerarchy h on h.ParentId=cte.ChileId
) 
Select * from cteherarchy 

Answer 2

Can't you do something like this?

;WITH cte AS (....)
SELECT
    * 
FROM 
    cte
CROSS APPLY 
    dbo.myTable tbl ON cte.XXX = tbl.XXX

Put the CROSS APPLY after the CTE definition - into the one SQL statement that refers back to the CTE. Wouldn't that work??

OR: - flip around your logic - do a "top-down" CTE, that picks the top-level nodes first, and then iterates through the hiearchy. This way, you can easily determine the "top-level father" in the first part of the recursive CTE - something like this:

;WITH ChildParent AS
(
    SELECT
        ID,
        ParentID = ISNULL(ParentID, -1),
        SomeName, 
        PLevel = 1,   -- defines level, 1 = TOP, 2 = immediate child nodes etc.
        TopLevelFather = ID  -- define "top-level" parent node
    FROM dbo.[Agent_Agents] 
    WHERE ParentID IS NULL

    UNION ALL

    SELECT
        a.ID,
        ParentID = ISNULL(a.ParentID, -1),
        a.SomeName, 
        PLevel = cp.PLevel + 1,
        cp.TopLevelFather   -- keep selecting the same value for all child nodes
    FROM dbo.[Agent_Agents] a
    INNER JOIN ChildParent cp ON r.ParentID = cp.ID
)
SELECT  
   ID,
   ParentID,
   SomeName,
   PLevel,
   TopLevelFather   
FROM ChildParent

This would give you nodes something like this (based on your sample data, slightly extended):

ID  ParentID  SomeName      PLevel  TopLevelFather
20    -1      Top#20           1          20
 4    -1      TOP#4            1           4
 8    -1      TOP#8            1           8
 7     8      ChildID = 7      2           8
 3     7      ChildID = 3      3           8
 2     4      ChildID = 2      2           4
 9    20      ChildID = 9      2          20
 5     9      ChildID = 5      3          20
 1     5      ChildID = 1      4          20

Now if you select a particular child node from this CTE output, you'll always get all the infos you need - including the "level" of the child, and its top-level parent node.

Answer 3

I have not yet the time to look further into your question and am not sure whether or not i've understood your problem, but couldn't you use this svf to get the top father's id?

CREATE FUNCTION [dbo].[getTopParent] (
    @ChildID INT
)

RETURNS int
AS
BEGIN
    DECLARE @result int;
    DECLARE @ParentID int;

    SET @ParentID=(
        SELECT ParentID FROM ChildParent
        WHERE ChildID = @ChildID 
    )

    IF(@ParentID IS NULL)
        SET @result = @ChildID 
    ELSE
        SET @result = [dbo].[getTopParent](@ParentID)

    RETURN @result    
END

Then you should be able to find each top parent in this way:

SELECT ChildID
    ,  [dbo].[getTopParent](ChildID) AS TopParentID
FROM ChildParent