SELECT p_type,p_name,p_view,row_num from (

SELECT p_type,p_name,p_view,IF(@bak=p_type,@rownum:=@rownum+1,@rownum:=1) as row_num, @bak:=p_type

FROM

( SELECT p_type,p_name,p_view from products 

order by p_type,p_view desc ) a , ( SELECT @rownum:=0,@bak:=‘‘) b 

) c WHERE c.row_num <=2

IF函数

IF(条件,表达式1，表达式2)

 如果条件成立 则返回表达式1，否则是2

分组后在分组内排序、每个分组中取前N条