sql数据分组（GROUPBY）最大值，第一条，前三条实例

SELECT 课程,
SUM(奖金) AS 奖金
FROM ( SELECT 课程,
学号,
成绩
FROM 考试 AS a
WHERE ( SELECT COUNT(*)
FROM 考试
WHERE 课程 = a.课程
AND 学号 <> a.学号
AND 成绩 > a.成绩
) <3
) AS b
GROUP BY 课程

declare @Tab table
(Num int, Name varchar(2), Time DATETIME)
insert into @tab select 1 ,'a', '2009/05/01'
insert into @tab select 1 ,'a', '2009/05/02'
insert into @tab select 1 ,'a', '2009/05/03'
insert into @tab select 2 ,'b', '2009/05/04'
insert into @tab select 2 ,'b', '2009/05/05'
insert into @tab select 3 ,'c', '2009/05/06'
insert into @tab select 3 ,'c', '2009/05/07'
insert into @tab select 5 ,'e', '2009/05/08'
insert into @tab select 1 ,'a', '2009/05/09'
insert into @tab select 1 ,'a', '2009/05/10'

select * from @Tab t where not exists(select 1 from @Tab where num=t.num and [time]

/*
Num Name Time
----------- ---- -----------------------
1 a 2009-05-01 00:00:00.000
2 b 2009-05-04 00:00:00.000
3 c 2009-05-06 00:00:00.000
5 e 2009-05-08 00:00:00.000

(4 行受影响)
*/