create table Test
(ID int identity (1,1),UserName varchar(10),Times datetime)

insert into Test values('张三','2012-01-02 09:21:21')
insert into Test values('张三','2012-01-02 23:23:23')
insert into Test values('张四','2012-01-03 09:21:21')

select UserName,Min(Times) from Test group by UserName

------解决方案--------------------
select id,name,[datetime] from
(select *,row_number() over(partition by name order by [datetime]) as rownum 
from tab) as tb1
where rownum<2