收藏：均匀的按某一字段分类取到等数的记录

http://community.youkuaiyun.com/Expert/topic/3752/3752955.xml?temp=.3692591
创建记录
--create table t(id numeric,class varchar(50))

--delete t
declare @i numeric
set @i=0
while @i<5000
begin
set @i=@i+1
insert into t(id,class) values(@i,'a')
end
while @i<10000
begin
set @i=@i+1
insert into t(id,class) values(@i,'b')
end
while @i<15000
begin
set @i=@i+1
insert into t(id,class) values(@i,'c')
end
用时0:00:06

取六条记录
方法一
select   a.*  from   t a
where   a.id in (select top 2 id from t where class = a.class order by id)
用时0:02:10


方法二
select * from t a
where (select count(1) from t where class=a.class and id<=a.id) between 1 and 2
用时0:00:30