SQL 分组后,获取每组中的最大值对应的数据

select gr,num,dt,(select bys from test where gr=b.gr and dt=b.dt) bys
from 
(
	select gr,count(0) num,max(dt) dt from test group by gr
) b

//如果有重复项，可用如下语句(针对Mysql的limit,Oracle 可用 rownum<2)

select gr,num,dt,(select bys from test where gr=b.gr and dt=b.dt limit 1) bys
from 
(
	select gr,count(0) num,max(dt) dt from test group by gr
) b