select bh,date, datediff(ms,date+' ' +max(time), date + ' ' + min(time))from tb  group by bh,date

------解决方案--------------------
SQL code
create table #tt (bh varchar(10),[date] varchar(20),[time] varchar(20))

insert into #tt 
select '1','20070809','08:00:00'
union all
select '1','20070809','09:00:00'
union all
select '1','20070809','17:00:00'
union all
select '1','20070810','07:00:00'
union all
select '2','20070809','08:00:00'
union all
select '3','20070810','08:00:00'


select bh,[date],datediff(minute,min([time]),max([time]))  '相差（分）' from #tt group by bh,[date] order by [date]