Left Join Null Column

gistfile1.txt

What Comes
-------------------------------------------------
id |  type      |  pet_name

1  |  aardvark  |  NULL
2  |  dog       |  Charlie
3  |  dog       |  Rover
4  |  cat       |  Tabby
5  |  cat       |  Sandy
6  |  cat       |  MeowMeow

Required
-------------------------------------------------

id |  type      |  petname

1  |  aardvark  |  NULL
2  |  dog       |  Charlie
3  |  NULL      |  Rover
4  |  cat       |  Tabby
5  |  NULL      |  MeowMeow
6  |  NULL      |  Sandy


SELECT id,
       IF(type=@prev,NULL,@prev:=type) AS type,
       petname
  FROM (
            select pt.*, pn.pet_name 
              from PET_TYPES pt
         left join PET_NAMES pn on pt.id=pn.type_id
       ) AS list,
  (SELECT @prev:='') AS init
 ORDER BY list.type, list.id