mysql Por que a variável não está mudando se

SELECT rn, id, category_id, title
  FROM
(
  SELECT *, @a := IF(@c = category_id, @a + 1, 1) rn, @c := category_id
    FROM photos CROSS JOIN (SELECT @c := NULL, @a := 0) i
   ORDER BY category_id
) q
 WHERE rn <= 4;

| RN | ID | CATEGORY_ID |  TITLE |
|----|----|-------------|--------|
|  1 | 43 |           1 | Title1 |
|  1 | 28 |           2 | Title2 |
|  2 | 42 |           2 | Title3 |
|  1 | 11 |           3 | Title4 |
|  1 |  3 |           4 | Title5 |
|  2 | 29 |           4 | Title6 |
|  3 | 33 |           4 | Title7 |

SET @a:=0;
SET @cid:=0;   
SELECT @a, @cid, q.*, @cid:=q.category_id FROM (
    SELECT *
    FROM photos
    ORDER BY category_id
) AS q
WHERE @a := IF(@cid=q.category_id, @a+1, 1)