65

SELECT (ROW_NUMBER() OVER (ORDER BY maker, num)),
(CASE
WHEN ROW_NUMBER() OVER(PARTITION BY maker ORDER BY A.num) = 1
THEN MAKER
ELSE '' END), type

FROM
(SELECT
DISTINCT maker, type,
CASE
WHEN type='PC' THEN 1
WHEN type='laptop' THEN 2
ELSE 3 END AS num
FROM product) AS A

GROUP BY type, A.num, maker