get the number of months between start and end dates

Hello.

I have to calculated fields:
,[Contract_NetStartDate] = CASE WHEN ContractStartDate <= @ReportStartDate THEN @ReportStartDate ELSE ContractStartDate END
,[Contract_NetEndDate] = CASE WHEN ContractEndDate >= @ReportEndDate THEN @ReportEndDate ELSE ContractEndDate END

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I need to add a third calculated field:
[Contract_NetNumberOfPeriods]

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The third field is the number of months between [Contract_NetStartDate] and [Contract_NetEndDate]

Is it possible that some one show me how to obtain that number from the two fields?

Thank you much.
metropiaAsked:
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Brian CroweConnect With a Mentor Database AdministratorCommented:
DATEDIFF(MONTH, ContractStartDate, ContractEndDate)

This depends on how you want to calculate the number of months.  In the case above...

DATEDIFF(MONTH, '1/31/2014', '2/1/2014') = 1
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Scott PletcherSenior DBACommented:
Do you want to do the calculation on the original table columns (OR) on the results of the CASE expressions?

What do you mean by "month"?  That it crossed a calendar month, or that it was a minimum number of days for each "month"?
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cshark247Connect With a Mentor Commented:
if you want to use original table columns you can use.

SELECT
             [Contract_NetStartDate] = CASE WHEN ContractStartDate <= @ReportStartDate THEN @ReportStartDate ELSE ContractStartDate END
            ,[Contract_NetEndDate] = CASE WHEN ContractEndDate >= @ReportEndDate THEN @ReportEndDate ELSE ContractEndDate END
             ,Contract_NetNumberOfPeriods =  datediff(m,CASE WHEN ContractStartDate <= @ReportStartDate THEN @ReportStartDate ELSE ContractStartDate END,CASE WHEN ContractEndDate >= @ReportEndDate THEN @ReportEndDate ELSE ContractEndDate END)


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Scott PletcherSenior DBACommented:
I wouldn't repeat the expressions twice, too hard to keep in sync later.
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cshark247Connect With a Mentor Commented:
you are right, it is always good to avoid duplication. Another solution could be to wrap in outer query like this:

SELECT *,
 datediff(m,Contract_NetStartDate , Contract_NetEndDate ) as Contract_NetNumberOfPeriods
 FROM
(
	SELECT 
			 [Contract_NetStartDate] = CASE WHEN ContractStartDate <= @ReportStartDate THEN @ReportStartDate ELSE ContractStartDate END
			,[Contract_NetEndDate] = CASE WHEN ContractEndDate >= @ReportEndDate THEN @ReportEndDate ELSE ContractEndDate END
	
	   
) x

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metropiaAuthor Commented:
Thank you experts.

I have a question, why when I use:

DATEDIFF(m,Contract_NetStartDate , Contract_NetEndDate)

I get negative numbers, but if I switch to:

DATEDIFF(m, Contract_NetEndDate, Contract_NetStartDate)

Then the number of months is positive

??
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awking00Commented:
Because your Contract_NetEndDate must be earlier than your Contract_NetStartDate.
For syntax, see here -
http://www.w3schools.com/sql/func_datediff.asp
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