How can I compare time in SQL Server?

To efficiently compare two time parts, try:

declare @first datetimeset @first = '2009-04-30 19:47:16.123'
declare @second datetimeset @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

Long explanation: a date in SQL server is stored as a floating point number. The digits before the decimal point represent the date. The digits after the decimal point represent the time.

So here's an example date:

declare @mydate datetimeset @mydate = '2009-04-30 19:47:16.123'

Let's convert it to a float:

declare @myfloat floatset @myfloat = cast(@mydate as float)
select @myfloat-- Shows 39931,8244921682

Now take the part after the digit, i.e. the time:

set @myfloat = @myfloat - floor(@myfloat) select @myfloat-- Shows 0,824492168212601

Convert it back to a datetime:

declare @mytime datetimeset @mytime = convert(datetime,@myfloat)
select @mytime-- Shows 1900-01-01 19:47:16.123

The 1900-01-01 is just the "zero" date; you can display the time part with convert, specifying for example format 108, which is just the time:

select convert(varchar(32),@mytime,108)
-- Shows 19:47:16

Conversions between datetime and float are pretty fast, because they're basically stored in the same way.