一个关于时间的sql语句

在别的论坛看到这么一个关于计算时间的sqlserver的题...

我有一些时间段，怎么能计算出去重复的总时间，比如

a b

9:00:00 10:00:00

8:00:00 9:30:00

11:00:00 12:00:00

结果应该是3小时，如果把时间段查出来在外面计算太慢了，能不能直接在SQL里面算好

--代码如下

with tb(a,b)as(

select '2001-01-01 6:00:00','2001-01-01 7:00:00' union

select '2001-01-01 9:00:00','2001-01-01 10:00:00' union

select '2001-01-01 8:00:00','2001-01-01 9:30:00' union

select '2001-01-01 8:00:00','2001-01-01 10:30:00' union

select '2001-01-01 11:00:00','2001-01-01 12:00:00'),tc as(

select CONVERT(datetime,a)a,CONVERT(datetime,b)b,ROW_NUMBER() over(order by convert(datetime,a),convert(datetime,b))num from tb

),td as(

select * from tc where num=1

union all

select case when tc.a between td.a and td.b then td.a else tc.a end

,case when tc.a between td.a and td.b then case when tc.b>td.b then tc.b else td.b end else tc.b end,tc.num

from td,tc where td.num=tc.num-1)

select a,max(b)b from td

group by a