求一条SELECT语句，有趣，常用，望高手解答
有一张表
t_test

id   int4,   主健
user_id   int4,  
visit_date   timestamp

要求输出id,   user_id,visit_date   ，但是user_id相同的记录并且visit_date里同一天的数据只输出当天时间最晚的一条,
而且，   排序是按   user_id   升序，   visit_date   降序

insert   into   t_test   values(1,   13,   timestamp   '2007-4-12   09:30:21 ');
insert   into   t_test   values(2,   13,   timestamp   '2007-4-12   12:31:32 ');
insert   into   t_test   values(3,   13,   timestamp   '2007-4-12   23:11:00 ');
insert   into   t_test   values(4,   13,   timestamp   '2007-5-23   07:41:42 ');
insert   into   t_test   values(5,   15,   timestamp   '2007-4-12   23:51:52 ');


正确输出应该是
id user_id visit_date
4 13 2007-05-23   07:41:42
3 13 2007-04-12   23:11:00
5 15 2007-04-12   23:51:52

谢谢

------解决方案--------------------
select
t.*
from
t_test t
where
t.visit_date=(select max(visit_date) from t_test where user_id=t.user_id and datediff(day,visit_date,t.visit_date)=0)
------解决方案--------------------

create table #t(id int,[user_id] int,visit_date datetime)

insert into #t values(1, 13, '2007-4-12 09:30:21 ');
insert into #t values(2, 13, '2007-4-12 12:31:32 ');
insert into #t values(3, 13, '2007-4-12 23:11:00 ');
insert into #t values(4, 13, '2007-5-23 07:41:42 ');
insert into #t values(5, 15, '2007-4-12 23:51:52 ');


select *
from #t AS a
where id =
(
select top 1 id
from #t
where datediff(d,visit_date,a.visit_date)=0
and [user_id]=a.user_id
order by visit_date desc)
order by [user_id] ,visit_date desc

drop table #t