SQL Server で時間値を効率的に比較するにはどうすればよいですか?

declare @first datetime
set @first = '2009-04-30 19:47:16.123'
declare @second datetime
set @second = '2009-04-10 19:47:16.123'

select (cast(@first as float) - floor(cast(@first as float))) -
       (cast(@second as float) - floor(cast(@second as float)))
       as Difference

declare @first datetime
set @first = '08:00:00.000'
declare @second datetime
set @second = '07:30:00.000'

select (cast(@first as float) - floor(cast(@first as float))) -
       (cast(@second as float) - floor(cast(@second as float)))
       as Difference

Difference
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0.5