sql server - Calculating number of full months between two dates in SQL

Question

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?

Answer 1

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