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

Yet another shorter CTE :

WITH cte AS(
      SELECT *, id AS topparent 
      FROM t 
      WHERE parentid IS NULL
  UNION ALL
      SELECT t.*, c.topparent 
      FROM t JOIN cte c ON c.id = t.parentid
      WHERE t.id <> t.parentid
)
SELECT * FROM cte

Tested here : SQL Fiddle

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EDIT : This damn SQLfiddle is unresponsive again. Test DDL Code below

CREATE  table t (id INT, parentid INT);
INSERT INTO T VALUES (1  , NULL );
INSERT INTO T VALUES (2  , 1    );           
INSERT INTO T VALUES (3  , 2    );          
INSERT INTO T VALUES (9  , NULL );          
INSERT INTO T VALUES (5  , 9    );          
INSERT INTO T VALUES (6  , 5    );          
INSERT INTO T VALUES (25 , NULL );          
INSERT INTO T VALUES (30 , 25   );