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Assigning categories to values 2


Assigning categories to values 2

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Hugo Kornelis
Hugo Kornelis
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Dave62 (5/15/2013)
A minor typo in the explanation.
"... As soon as one evaluates to True, the corresponding ELSE is returned. ..."

ELSE should be THEN in that sentence.


and
hany.helmy (5/15/2013)
In the Explanation: "A CASE expression will always evaluate the WHEN clauses from top to bottom. As soon as one evaluates to True, the corresponding [ELSE] is returned."

I think it should be the corresponding [THEN] not else, correct me if I am wrong.


You are both absolutely right. Sorry for that mistake. Unfortunately, I am unable to change the explanation. Sad


Hugo Kornelis, SQL Server MVP
Visit my SQL Server blog: http://sqlblog.com/blogs/hugo_kornelis
Revenant
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It was so simple that I suspected a trick. :-)

Thanks, Hugo!
johnf_amic
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I suspected a question like this would be coming soon, so I practiced yesterday! Cool
Christian Buettner-167247
Christian Buettner-167247
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I was also looking for the trickSmile

Regarding the aggregates & case evaluation:
the CASE evaluation order is guaranteed for the SELECT phase. I think it helps to understand it when one thinks about the aggregation as a type of "subquery".

Example: The following query throws an error because the aggregate "MIN(1/0)" cannot be calculated.
DECLARE @i int = 1;
SELECT CASE WHEN @i = 1 THEN 1 ELSE MIN(1/0) END Agg
FROM sys.objects



This query is logically the same as the following query:
DECLARE  @i int = 1
SELECT CASE WHEN @i = 1 THEN 1 ELSE AggSub END Agg
FROM (SELECT MIN(1/0) AggSub
FROM sys.objects
) S


I think that in this version, one can easier see when the error actually happens --> before the case evaluation starts. In the first version, the location of the aggregate function can mislead people to assume that this aggregate is only calculated during the evaluation of the CASE statement. But it is not. It is already performed before the CASE evaluation starts.

And to provide an example with aggregates that shows that the evaluation order is honored, consider this one:
DECLARE  @i int = 1
SELECT CASE WHEN @i = 1 THEN 1 ELSE MIN(1) / 0 END Agg
FROM sys.objects


In this example, there is no error in the aggregation phase, as MIN(1) produces no error.
And there is no error in the SELECT phase, because the CASE evaluation stops right after the first WHEN (which is before the ELSE part which would cause an error).

And for completeness, here is the version in Subquery format:
DECLARE  @i int = 1
SELECT CASE WHEN @i = 1 THEN 1 ELSE AggSub / 0 END Agg
FROM (SELECT MIN(1) AggSub
FROM sys.objects
) S



Best Regards,

Chris Büttner
Narud
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Very positive knowledge acquired.

Thanks Hugo!
Ford Fairlane
Ford Fairlane
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Good hump day QotD thanks

Hope this helps...

Ford Fairlane
Rock and Roll Detective





Anipaul
Anipaul
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Thanks Hugo..learned something new...



Bangla
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Nice one Hugo..learned something....
Miles Neale
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I thought about this one too long but still got there. Nice!

Not all gray hairs are Dinosaurs!
Koen Verbeeck
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Another great question, thanks Hugo.


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