Question

Could pls explain why I’ not able to get valid result ( with precision to miliseconds ) after doing this (adding 7hrs to date):

select getdate() as dt into #t1
select dt, dt+(7.*60/1440) as dt_new from #t1

I got:

dt                      dt_new
2010-10-25 04:56:33.640 2010-10-25 11:56:33.580

As you can see there is difference in 60ms for dt_new – WHY?
AFAIR I WAS able to do this sort of things in Oracle 10g – even with minutes:
date + 5/1440 -to produce 5-min periods of datetime.

UPD:

My mistake – the CORRECT way of doing this is:

select getdate(), getdate()+(convert(float, 5)/1440) as dt_new 

I suppose there is wrong implicit convertion choosen by Ms Sql server.

UPD2:
Thanks for replies, but my point was “arifmetic” method – because there are cases when calculating dates without dateadd really improves Query Plan – by eliminating Table scan that is the case with Dateadd and other functions.
Anyway it works – thanks!

Answer 1

You could also use the inbuilt functions, such as

select dt, DATEADD(hour, 7, dt) as dt_new from #t1

My guess as to the imprecision in using + is that its converting from datetime to integer, then back again.