declare @i int,@a datetime,@b datetime
set @a='2012-03-28 09:00:00'
set @b='2012-03-28 09:50:00'
set @i=datediff(mi,@a,@b)*1.0/25
select @i
select dateadd(mi,number*@i,@a),dateadd(mi,(number+1)*@i,@a) from master..spt_values 
where type='p'
and number<=25

------解决方案--------------------

关键是你不知道怎么等分吧，参考

SQL code
declare @a datetime = '2012-03-01'
declare @b datetime = '2012-03-02 01:00'
declare @minute int = datediff(minute,@a,@b)/25;
with cte as
(
    select group_id=datediff(minute,dt,@a)/@minute, * from tb where dt between @a and @b
)
select * into #tmp from cte t where [key] in (select top 5 [key] from tb where group_id=t.group_id order by newid())

select max(val)-min(val), avg(max(val)-min(val))over() from #tmp group by group_id