Modifying a column in a table

Question

I have the following table test

iD Name    ParentId   GroupID
-----------------------------
1            1        Null
2            1        Null
3         


        

Answer 1

; WITH CTE 
AS
 (
 SELECT iDName, ParentId, GroupID,
        DENSE_RANK() OVER (ORDER BY ParentId ASC) RN
  FROM Test
 )
UPDATE CTE 
SET GroupID = RN

Working SQL FIDDLE