coalesce and nullif

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coalesce and nullif

LP-181697

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June 22, 2010 at 9:19 pm

#220338

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

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Oleg Netchaev SSCertifiable Points: 5276 More actions · Answer 1

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

hemang.patel Old Hand Points: 364 More actions · Answer 2

Thanks for explain precedence logic. Now I understand it perfectly

Hugo Kornelis SSC Guru Points: 64790 More actions · Answer 3

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/Data Platform MVP (2006-2016)
Visit my SQL Server blog: https://sqlserverfast.com/blog/
SQL Server Execution Plan Reference: https://sqlserverfast.com/epr/

vk-kirov SSCertifiable Points: 7686 More actions · Answer 4

Nice question.

OFF

I would like thanks my coorker...

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

Arsi Salo Ten Centuries Points: 1313 More actions · Answer 5

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...

ronmoses@gmail.com SSCarpal Tunnel Points: 4480 More actions · Answer 6

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! 😀

-----
a haiku...

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

LP-181697 SSC Eights! Points: 966 More actions · Answer 7

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

WayneS SSC Guru Points: 95460 More actions · Answer 8

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
Author - SQL Server T-SQL Recipes

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
Performance Problems
Common date/time routines
Understanding and Using APPLY Part 1 & Part 2

forjonathanwilson SSC Enthusiast Points: 139 More actions · Answer 9

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

Hugo Kornelis SSC Guru Points: 64790 More actions · Answer 10

forjonathanwilson (6/23/2010)
null date can be cast as null string, but not the other way, interesting.:hehe:

This is not correct. A null string can be cast as date (result will be NULL). The problem starts when you try to case a non-null string as date - in that case, the string must evaluate to a valid date. And obviously, the string 'Test' does not qualify there.

Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
Visit my SQL Server blog: https://sqlserverfast.com/blog/
SQL Server Execution Plan Reference: https://sqlserverfast.com/epr/

forjonathanwilson SSC Enthusiast Points: 139 More actions · Answer 11

Hugo Kornelis (6/23/2010)
forjonathanwilson (6/23/2010)
null date can be cast as null string, but not the other way, interesting.:hehe:
This is not correct. A null string can be cast as date (result will be NULL). The problem starts when you try to case a non-null string as date - in that case, the string must evaluate to a valid date. And obviously, the string 'Test' does not qualify there.

thanks for clarifying that for me. I also checked this myself after I made my irroneous assumption:

declare @C varchar(10), @c1 varchar(10),

@d datetime, @d1 datetime

select @C = 'test', @c1 = null, @d = '1/1/10', @d1 = null

select coalesce(nullif(@c,@c),nullif(@d,@d))

select coalesce(nullif(@d,@d),nullif(@c,@c))

where the results are compared null datetime to null string by coalesce, which completes just fine.

UMG Developer SSChampion Points: 13482 More actions · Answer 12

Interesting question, thanks.

Thanks Oleg for the explanation of why it happens, I think most people don't already know that, so it is very helpful.

ColdCoffee SSC-Dedicated Points: 39972 More actions · Answer 13

Oleg and Hugo , thanks for the nice explanations and links.. nice question depicting the precedence of data-types... niceee.

Anam Verma SSCertifiable Points: 5365 More actions · Answer 14

thanks for the good QOTD and explanation behind it.