Calculating number of full months between two dates in SQL

I need to calculate the number of FULL month in SQL, i.e.

• 2009-04-16 to 2009-05-15 => 0 full month
• 2009-04-16 to 2009-05-16 => 1 full month
• 2009-04-16 to 2009-06-16 => 2 full months

I tried to use DATEDIFF, i.e.

SELECT DATEDIFF(MONTH, '2009-04-16', '2009-05-15')

but instead of giving me full months between the two date, it gives me the difference of the month part, i.e.

1

anyone know how to calculate the number of full months in SQL Server?

The original post had some bugs... so I re-wrote and packaged it as a UDF.

CREATE FUNCTION FullMonthsSeparation 
(
    @DateA DATETIME,
    @DateB DATETIME
)
RETURNS INT
AS
BEGIN
    DECLARE @Result INT

    DECLARE @DateX DATETIME
    DECLARE @DateY DATETIME

    IF(@DateA < @DateB)
    BEGIN
        SET @DateX = @DateA
        SET @DateY = @DateB
    END
    ELSE
    BEGIN
        SET @DateX = @DateB
        SET @DateY = @DateA
    END

    SET @Result = (
                    SELECT 
                    CASE 
                        WHEN DATEPART(DAY, @DateX) > DATEPART(DAY, @DateY)
                        THEN DATEDIFF(MONTH, @DateX, @DateY) - 1
                        ELSE DATEDIFF(MONTH, @DateX, @DateY)
                    END
                    )

    RETURN @Result
END
GO

SELECT dbo.FullMonthsSeparation('2009-04-16', '2009-05-15') as MonthSep -- =0
SELECT dbo.FullMonthsSeparation('2009-04-16', '2009-05-16') as MonthSep -- =1
SELECT dbo.FullMonthsSeparation('2009-04-16', '2009-06-16') as MonthSep -- =2

What's your definition of a month? Technically a month can be 28,29,30 or 31 days depending on the month and leap years.

It seems you're considering a month to be 30 days since in your example you disregarded that May has 31 days, so why not just do the following?

SELECT DATEDIFF(DAY, '2009-04-16', '2009-05-15')/30
    , DATEDIFF(DAY, '2009-04-16', '2009-05-16')/30
    , DATEDIFF(DAY, '2009-04-16', '2009-06-16')/30

select case when DATEPART(D,End_dATE) >=DATEPART(D,sTAR_dATE) 
THEN ( case when DATEPART(M,End_dATE) = DATEPART(M,sTAR_dATE) AND DATEPART(YYYY,End_dATE) = DATEPART(YYYY,sTAR_dATE) 
        THEN 0 ELSE DATEDIFF(M,sTAR_dATE,End_dATE)END )
ELSE DATEDIFF(M,sTAR_dATE,End_dATE)-1 END

This is for ORACLE only and not for SQL-Server:

months_between(to_date ('2009/05/15', 'yyyy/mm/dd'), 
               to_date ('2009/04/16', 'yyyy/mm/dd'))

And for full month:

round(months_between(to_date ('2009/05/15', 'yyyy/mm/dd'), 
                     to_date ('2009/04/16', 'yyyy/mm/dd')))

Can be used in Oracle 8i and above.

The dateadd function can be used to offset to the beginning of the month. If the endDate has a day part less than startDate, it will get pushed to the previous month, thus datediff will give the correct number of months.

DATEDIFF(MONTH, DATEADD(DAY,-DAY(startDate)+1,startDate),DATEADD(DAY,-DAY(startDate)+1,endDate))