sql语句如何按产品类别查询，取出每个类别的前三条记录

select *
from
(select *,row_number() over(partition by classid order by classid) as rownum 
--上面利用row_number()先按classid分类，然后给每类内分别编号1、2、3...
from tb) as tb1
where rownum<4