原帖是在php版看到的http://topic.csdn.net/u/20120225/20/2c8064d1-af07-47bc-a6a3-e28fbcd10a6c.html?77352，我想来想去觉得不可能实现:
原帖的table: t (`id`, `username`, `title`) SQL是： SELECT * FROM `t` WHERE `title` LIKE 'example%'
这样会有漏洞，可能有时候一个用户的各种帖子会出现很多次(SEO)，如何杜绝这种情况?
例如每个用户最多取2条帖子，不用考虑分页的问题，SQL语句该如何来写？？
原帖的table: t (`id`, `username`, `title`) SQL是： SELECT * FROM `t` WHERE `title` LIKE 'example%'
这样会有漏洞，可能有时候一个用户的各种帖子会出现很多次(SEO)，如何杜绝这种情况?
例如每个用户最多取2条帖子，不用考虑分页的问题，SQL语句该如何来写？？
[征集]分组取最大N条记录方法征集，及散分....
drop table #tab
GO
create table #tab (
[id] [char](10),[age] [int],[rdate] [datetime])insert into #tab(id,age,rdate) values('a' , 1 , '2006-01-01')
insert into #tab(id,age,rdate) values('a' , 2 , '2006-01-02')
insert into #tab(id,age,rdate) values('a' , 3 , '2006-01-03')
insert into #tab(id,age,rdate) values('a' , 4 , '2006-01-04')
insert into #tab(id,age,rdate) values('a' , 5 , '2006-01-05')
insert into #tab(id,age,rdate) values('a' , 6 , '2006-01-06')
insert into #tab(id,age,rdate) values('b' , 1 , '2006-02-01')
insert into #tab(id,age,rdate) values('b' , 2 , '2006-02-02')
insert into #tab(id,age,rdate) values('b' , 3 , '2006-02-03')
insert into #tab(id,age,rdate) values('b' , 4 , '2006-02-04')
insert into #tab(id,age,rdate) values('c' , 1 , '2006-03-01')
insert into #tab(id,age,rdate) values('c' , 2 , '2006-03-02')
insert into #tab(id,age,rdate) values('c' , 3 , '2006-03-03')
insert into #tab(id,age,rdate) values('d' , 1 , '2006-04-01')
insert into #tab(id,age,rdate) values('d' , 2 , '2006-04-02')
insert into #tab(id,age,rdate) values('e' , 1 , '2006-05-01')--按时间rdate升序取前三条
select * from #tab t
where rdate in
(
select top 3 rdate from #tab where id=t.id order by rdate
)id age rdate
---------- ----------- ------------------------------------------------------
a 1 2006-01-01 00:00:00.000
a 2 2006-01-02 00:00:00.000
a 3 2006-01-03 00:00:00.000
b 1 2006-02-01 00:00:00.000
b 2 2006-02-02 00:00:00.000
b 3 2006-02-03 00:00:00.000
c 1 2006-03-01 00:00:00.000
c 2 2006-03-02 00:00:00.000
c 3 2006-03-03 00:00:00.000
d 1 2006-04-01 00:00:00.000
d 2 2006-04-02 00:00:00.000
e 1 2006-05-01 00:00:00.000(所影响的行数为 12 行)--按时间rdate降序取前三条
select * from #tab t
where rdate in
(
select top 3 rdate from #tab where id=t.id order by rdate desc
)
order by id , rdate descid age rdate
---------- ----------- ------------------------------------------------------
a 6 2006-01-06 00:00:00.000
a 5 2006-01-05 00:00:00.000
a 4 2006-01-04 00:00:00.000
b 4 2006-02-04 00:00:00.000
b 3 2006-02-03 00:00:00.000
b 2 2006-02-02 00:00:00.000
c 3 2006-03-03 00:00:00.000
c 2 2006-03-02 00:00:00.000
c 1 2006-03-01 00:00:00.000
d 2 2006-04-02 00:00:00.000
d 1 2006-04-01 00:00:00.000
e 1 2006-05-01 00:00:00.000
(所影响的行数为 12 行)
--上面包含了总数不到3个的记录(即id为d,e的数据)，如果要取消它们，以升序为例(降序同理)
select * from #tab m
where rdate in
(
select top 3 rdate from
(
select * from #tab t
where id in
(
select id from #tab group by id having(count(*)) >= 3
)
) n
where m.id = n.id order by rdate
)id age rdate
---------- ----------- ------------------------------------------------------
a 1 2006-01-01 00:00:00.000
a 2 2006-01-02 00:00:00.000
a 3 2006-01-03 00:00:00.000
b 1 2006-02-01 00:00:00.000
b 2 2006-02-02 00:00:00.000
b 3 2006-02-03 00:00:00.000
c 1 2006-03-01 00:00:00.000
c 2 2006-03-02 00:00:00.000
c 3 2006-03-03 00:00:00.000
(所影响的行数为 9 行)--在上面的例中我们发现rdate都是不相同的，如果相同怎么办？
--例如id=a，第三条，第四条rdate相同，都为2006-01-03。
id age rdate
---------- ----------- ------------------------------------------------------
a 1 2006-01-01 00:00:00.000
a 2 2006-01-02 00:00:00.000
a 3 2006-01-03 00:00:00.000
a 4 2006-01-03 00:00:00.000
a 5 2006-01-05 00:00:00.000
a 6 2006-01-06 00:00:00.000
b 1 2006-02-01 00:00:00.000
b 2 2006-02-02 00:00:00.000
b 3 2006-02-03 00:00:00.000
b 4 2006-02-04 00:00:00.000
c 1 2006-03-01 00:00:00.000
c 2 2006-03-02 00:00:00.000
c 3 2006-03-03 00:00:00.000
d 1 2006-04-01 00:00:00.000
d 2 2006-04-02 00:00:00.000
e 1 2006-05-01 00:00:00.000--如果想把第三、四都取出来，使用上面的语句即可。如果只取一条(只取第三不取第四)则要使用临时表了。
if object_id('tempdb..#tab') is not null
drop table #tab
GO
if object_id('tempdb..#temp') is not null
drop table #temp
GO
create table #tab (
[id] [char](10),[age] [int],[rdate] [datetime])insert into #tab(id,age,rdate) values('a' , 1 , '2006-01-01')
insert into #tab(id,age,rdate) values('a' , 2 , '2006-01-02')
insert into #tab(id,age,rdate) values('a' , 3 , '2006-01-03')
insert into #tab(id,age,rdate) values('a' , 4 , '2006-01-03')
insert into #tab(id,age,rdate) values('a' , 5 , '2006-01-05')
insert into #tab(id,age,rdate) values('a' , 6 , '2006-01-06')
insert into #tab(id,age,rdate) values('b' , 1 , '2006-02-01')
insert into #tab(id,age,rdate) values('b' , 2 , '2006-02-02')
insert into #tab(id,age,rdate) values('b' , 3 , '2006-02-03')
insert into #tab(id,age,rdate) values('b' , 4 , '2006-02-04')
insert into #tab(id,age,rdate) values('c' , 1 , '2006-03-01')
insert into #tab(id,age,rdate) values('c' , 2 , '2006-03-02')
insert into #tab(id,age,rdate) values('c' , 3 , '2006-03-03')
insert into #tab(id,age,rdate) values('d' , 1 , '2006-04-01')
insert into #tab(id,age,rdate) values('d' , 2 , '2006-04-02')
insert into #tab(id,age,rdate) values('e' , 1 , '2006-05-01')--按时间rdate升序取前三条(其他方法同上)
select id1=identity(int,1,1),* into #temp from #tab order by id , rdate /*(降序用rdate desc)*/
select * from #temp t
where id1 in
(
select top 3 id1 from #temp where id=t.id order by id1
)