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Standard Deviation


Standard Deviation

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jkelly
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Comments posted to this topic are about the item Standard Deviation



Koen Verbeeck
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Nice question, really learned something.
The BOL page doesn't even mention this subject.


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sharath.chalamgari
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I think the Explanation is bit confusing.

if you give any numeric value to the function without any from clause it will give NULL.
SELECT STDEV(100)



"The STDEV function uses all of the values to calculate the standard deviation. There must be at least two values in the field or the standard deviation will not be calculated and a NULL is returned."

example in my PackageTable the standard deveation of pa_rates column can find using

select stdev(pa_Rates) From Package



for the above Query i will get some result

but if i cange the Query with some where condition,where i will get only one Row (sample) then the stdevp will results the null value

select stdev(pa_Rates) From Package where pa_PackageID = 1


Koen Verbeeck
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sharath.chalamgari (1/28/2011)

but if i cange the Query with some where condition,where i will get only one Row (sample) then the stdevp will results the null value

select stdev(pa_Rates) From Package where pa_PackageID = 1




The WHERE clause is executed first, so you are still calculating the variance over 1 row, hence the NULL.


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Koen (da-zero) (1/28/2011)
sharath.chalamgari (1/28/2011)

but if i cange the Query with some where condition,where i will get only one Row (sample) then the stdevp will results the null value

select stdev(pa_Rates) From Package where pa_PackageID = 1




The WHERE clause is executed first, so you are still calculating the variance over 1 row, hence the NULL.


Ya that's what i have explained above.
Koen Verbeeck
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sharath.chalamgari (1/28/2011)
Koen (da-zero) (1/28/2011)
sharath.chalamgari (1/28/2011)

but if i cange the Query with some where condition,where i will get only one Row (sample) then the stdevp will results the null value

select stdev(pa_Rates) From Package where pa_PackageID = 1




The WHERE clause is executed first, so you are still calculating the variance over 1 row, hence the NULL.


Ya that's what i have explained above.


Then why is the explanation confusing? I find it pretty clear...


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Thanks for introducing new topic. I had not heard about it before

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mdv 9731
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I do not agree that the answer is right.
Think it is supposed to be 0.

the standard deviation σ (sigma) is the square root of the average value of (X − μ)2.

Which means σ = sqrt(((1-1)^2)/1) = 0

Or in other words std. deviation equals sqrt of (population-avarage) sq / number of population values

Please correct me if im wrong on the formula it has been a while since i used my statistics :-D
Koen Verbeeck
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mdv 9731 (1/28/2011)
I do not agree that the answer is right.
Think it is supposed to be 0.

the standard deviation σ (sigma) is the square root of the average value of (X − μ)2.

Which means σ = sqrt(((1-1)^2)/1) = 0

Or in other words std. deviation equals sqrt of (population-avarage) sq / number of population values

Please correct me if im wrong on the formula it has been a while since i used my statistics :-D


Even if your statistics say it is 0, SQL Server returns NULL, so the answer is still correct.
You can verify it by running the query in SQL Server Management Studio.


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mdv 9731
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I'm not really going to comment on that answer.
Just happy you are not creating any kind of BI og reporting in our company :-)

I might be wrong but i still question the result.
Even if i know that it doesnt make much practical sence to talk deviation on 1 value.
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