How can I compare time in SQL Server?

Question

I'm trying to compare time in a datetime field in a SQL query, but I don't know if it's right. I don't want to compare the date part, just the time part.

I'm doing this:

SELECT timeEvent 
FROM tbEvents 
WHERE convert(datetime, startHour, 8) >= convert(datetime, @startHour, 8)

Is it correct?

I'm asking this because I need to know if 08:00:00 is less or greater than 07:30:00 and I don't want to compare the date, just the time part.

Thanks!

Answer 1

Your compare will work, but it will be slow because the dates are converted to a string for each row. To efficiently compare two time parts, try:

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

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 datetime
set @mydate = '2009-04-30 19:47:16.123'

Let's convert it to a float:

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

Now take the part after the comma character, i.e. the time:

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

Convert it back to a datetime:

declare @mytime datetime
set @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.