Datetime precision

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Datetime precision

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crussell-931424

Posted Monday, November 30, 2009 7:23 AM

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Obviously I can't read. I didn't see that .100 hanging out there!!!

Post #826190

Steve Jones - SSC Editor

Posted Monday, November 30, 2009 8:23 AM

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the dateadd is not commented out, just the note.

SQL is only precise to .0, .3, and .7ms, therefore you can't have .1 as the answer. It could only be one of those 3

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Post #826237

SQLBOT

Posted Monday, November 30, 2009 8:37 AM

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jim.taylor (11/30/2009)

I'm confused, if DateTime is only precise to 3.33 ms, why then is the answer just 1 ms off? Why not 3 ms?

SQL Server rounded down to 100 ms from the 101ms that was added.
if you add 102ms it rounds up to 103 ms.

I posted this question because the inaccuracy caught me off guard a couple months ago when I kept getting unexpected results out of some code.

After suspecting sheer lunacy to be the root cause, I finally got the notion that maybe the SQL Server clock was dodgy. I wrote the query that you see in the question to test that theory and the BOL confirmed the rest.

To avoid the issue, use datetime2 which is more precise, but still has the same problem when you get down to 100 nanoseconds.

My issue was resolved by ignoring milliseconds since I didn't need to be that precise, and accuracy was more important than precision.

Thanks All!

~Craig

Craig Outcalt

Tips for new DBAs: http://www.sqlservercentral.com/articles/Career/64632
My other articles: http://www.sqlservercentral.com/Authors/Articles/Craig_Outcalt/560258

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Michael Poppers

Posted Monday, November 30, 2009 8:38 AM

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jim.taylor (11/30/2009)

...if DateTime is only precise to 3.33 ms, why then is the answer just 1 ms off? Why not 3 ms?

When the SELECT output's ms aspect can be one of 100 (from 99.9) or 103 (from 103.23), 100 is closer Smile

.

I chose the wrong answer because I was thinking of roundoff in 3ms increments instead of in 3.33ms increments (although I should have realized that the choice should then have been between 99 and 102 Smile

), so thank you to the questioner!

Post #826255

Dan Guzman - Not the MVP

Posted Monday, November 30, 2009 9:38 AM

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Based on all the comments here, I got completely lucky.

I assumed it was a trick question, and figured Midnight was one of those mystery values between today and yesterday, and that 1 millisecond was 'consumed' in the math. I'm clearly thinking way to hard about these QotDs.

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Tom Garth

Posted Wednesday, December 02, 2009 8:38 AM

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peter.house (11/30/2009)

While looking at the answer choices, my brain kept saying "Yesterday Midnight + 100ms"

For me, Midnight today is tonight and the SQL answer is this morning at 100ms after midnight or 100ms after the beginning of day.

The time element for midnight is 0, not 24. If it were 24 then the answer would have been 101 ms after midnight (tomorrow morning). Hehe

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Bhavesh_Patel

Posted Thursday, December 10, 2009 10:22 PM

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I am confused !

Bhavesh Patel

http://bhaveshgpatel.wordpress.com/

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