oracle-分组并查询最大值||每组的前几条数据

查询sql 

select * from 
(select t.*, row_number() over(partition by t.em order by t.id desc ) rn
from test.com t )
where rn <=1

数据中原数据：

NAME	ID	SAL	EM
yang	1.00	1000.00	1.00
liu	2.00	2000.00	1.00
wang	3.00	4900.00	2.00
zhi	4.00	5666.00	2.00
sha	5.00	6777.00	3.00
deng	6.00	833.00	3.00

 查询结果：

NAME	ID	SAL	EM	RN
liu	2.00	2000.00	1.00	1.00
zhi	4.00	5666.00	2.00	1.00
deng	6.00	833.00	3.00	1.00