coalesce and nullif

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coalesce and nullif

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LP-181697

Posted Tuesday, June 22, 2010 9:19 PM

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Comments posted to this topic are about the item coalesce and nullif

Post #941465

Oleg Netchaev

Posted Tuesday, June 22, 2010 9:39 PM

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This is a very good question, thank you. Presence of nullif does not change anything though, so it is difficult to figure out why it is there. What I mean is that if the question is reduced to declaring and setting @c and @d only and the statements in question are then changed to

select coalesce(@c, @d);

select coalesce(@d, @c);

the result will still be the same, the first statement will fail and the second one will run just fine. I guess that this is because of the major difference between the behavior of coalesce with 2 parameters and isnull function which I believe would execute both statements just fine: coalesce always returns the expression of the highest precedence data type regardless of the parameters' order while isnull returns the data type of the first parameter. In other words, coalesce tries to cast first occurrence of the not null as datetime (datetime is of higher precedence). This is why it fails in the first statement, the first not null is @c which is equal to 'test', it tries to cast 'test' as datetime and fails. The second statement has the first not null is @d which is a datetime already, so the second statement executes without error.

Once again, thank you for an excellent question, I really enjoyed it.

Oleg

Post #941470

hemang.patel

Posted Wednesday, June 23, 2010 12:59 AM

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Thanks for explain precedence logic. Now I understand it perfectly

Post #941534

Hugo Kornelis

Posted Wednesday, June 23, 2010 1:10 AM

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Thanks, Oleg, for the explanation of the difference between COALESCE and ISNULL. I was about to add it myself, since it is lacking in the question, but you beat me to it.

And I agree with you that the NULLIF serves no apparent function here. Other than to obfuscate. (I knew that both COALESCE functions would return datetime; I then pondered for an extra minute to make really sure that the NULLIF does not affect the result at all; then gave the wrong answer because my not-yet awake brain thought both expression would try to convert 'Test' to the target data type).

And for those who want to do some further reading:
* Data type precedence: http://msdn.microsoft.com/en-us/library/ms190309.aspx
* NULLIF: http://msdn.microsoft.com/en-us/library/ms177562.aspx
* COALESCE: http://msdn.microsoft.com/en-us/library/ms190349.aspx
* ISNULL (similar to, yet also different from COALESCE): http://msdn.microsoft.com/en-us/library/ms184325.aspx

Hugo Kornelis, SQL Server MVP
Visit my SQL Server blog: http://sqlblog.com/blogs/hugo_kornelis

Post #941539

vk-kirov

Posted Wednesday, June 23, 2010 1:15 AM

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Nice question.

OFF

I would like thanks my coorker...

What does the word 'coorker' mean? Is it a typo and needs to be 'coworker'?

Post #941540

Arsi Salo

Posted Wednesday, June 23, 2010 1:46 AM

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Nice QotD. It would have been more educational with third section

select @c = 'test', @c1 = null, @d = '01/01/10', @d1 = '01/01/10'

,
which brings nullif into fray...

Post #941568

ronmoses

Posted Wednesday, June 23, 2010 5:30 AM

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This is a great question, and I'm proud that I managed to work out the correct answer once I realized the first statement was going to try and cast 'test' as a datetime. And then I clicked the wrong option and lost the points. Yay for me! BigGrin

-----
a haiku...

NULL is not zero
NULL is not an empty string
NULL is the unknown

Post #941657

LP-181697

Posted Wednesday, June 23, 2010 6:30 AM

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vk-kirov (6/23/2010)

Nice question.

OFF

I would like thanks my coorker...

What does the word 'coorker' mean? Is it a typo and needs to be 'coworker'?

It means coworker. It was typo

Post #941701

WayneS

Posted Wednesday, June 23, 2010 7:26 AM

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Oleg, thanks for the explanation. Hugo, thanks for the links, especially for the data type precedence. Between the two of you'll, this is the explanation that the question should have had.

Wayne
Microsoft Certified Master: SQL Server 2008

If you can't explain to another person how the code that you're copying from the internet works, then DON'T USE IT on a production system! After all, you will be the one supporting it!

Links: For better assistance in answering your questions, How to ask a question, Performance Problems, Common date/time routines,
CROSS-TABS and PIVOT tables Part 1 & Part 2, Using APPLY Part 1 & Part 2, Splitting Delimited Strings

Post #941729

forjonathanwilson

Posted Wednesday, June 23, 2010 8:24 AM

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null date can be cast as null string, but not the other way, interesting. Hehe

Post #941799

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