【SQL进阶】分组后在分组内排序、每个分组中取前N条

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SELECT p_type,p_name,p_view,row_num 
from ( SELECT p_type,p_name,p_view,IF(@pre=p_type,@rownum:=@rownum+1,@rownum:=1) as row_num,@pre:=p_type
from ( SELECT p_type,p_name,p_view from products order by p_type, p_view desc ) a,
(select @pre:='',@rownum:=0 ) b
) c
where c.row_num <= 2