sql分组取最大一条

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]<t.[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)
*/

　　select * from test where b in (select max(id) from test group by a)
　　适用于所有数据库：
　　
　　select t1.a,t1.b,t1.c
　　from test t1
　　inner join
　　(seelct a,max(b) as b from test group by a) t2
　　on t1.a=t2.a and t1.b=t2.b
　　
　　适用于所有数据库：
　　
　　select a,b,c
　　from(
　　select a,b,c
　　,row_number()over(partition by a order by b desc) rn
　　from test
　　)
　　where rn=1