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

Calculating Full Months Between Dates in SQL

In SQL, the DATEDIFF function is commonly used to calculate the difference between two dates. However, this function returns the difference in months, not the number of full months. To determine the number of full months between two dates, a more specific calculation is required.

UDF Approach

One approach is to create a User-Defined Function (UDF) that calculates the number of full months between two dates. The function, named FullMonthsSeparation, is written as follows:

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

Example Usage

To use the FullMonthsSeparation function, simply call it with the two dates as parameters. For example:

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

This function will return the number of full months between the two dates. It accounts for the possibility that the start date may have a larger day of the month than the end date, in which case one month is subtracted from the result.

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