mysql分组查询取每组前N条

Question

请大神支招：怎样用分组查询得到每组前4条数，（注：数据量较大，不能使用子查询）

Answer 1

select * from (
select row_number() over(partition by ancester分组列 order by productid排序列) as a,* from Products
) as p where a<4前4个