How to Accurately Calculate the Number of Full Months Between Two Dates in SQL Server?

Calculating the Number of Full Months Between Two Dates Using a SQL Server Function

Calculating the number of full months between two dates can be a challenge in SQL Server. A straightforward approach using DATEDIFF might not provide accurate results, as it only considers the difference in the month parts. Here's an improved solution using a custom UDF (User-Defined Function) named FullMonthsSeparation:

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

This UDF takes two datetime parameters, @DateA and @DateB, and calculates the number of full months between them. It handles the case where the second date is earlier than the first. Here's how to use the UDF:

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

The UDF accurately calculates the number of full months between the given dates, ensuring that the day of the month is accounted for.

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