Add business days to date in SQL without loops

Question

I currently have a function in my SQL database that adds a certain amount of business days to a date, e.g. if you enter a date that is a Thursday and add two days, it will r

Answer 1

I have tested all of the solutions proposed here and none of them work. Here are some test scenarios that broke a lot of the above solutions. (assuming Saturday and Sunday are the days you are excluding):

-Add 0 days to a Saturday - Expected result = Saturday

-Add 0 days to a Sunday - Expected result = Sunday

-Add 1 day to Friday - Expected result = the following Monday

-Add 1 day to Saturday - Expected result = the following Monday

-Add 1 day to Sunday - Expected result = the following Monday

-Add 3 days to Friday - Expected result = the following Wednesday

-Add 5 days to Saturday - Expected result = the following Friday

-Add 5 days to Friday - Expected result = the following Friday

-Subtract 1 day from Monday - Expected result = the previous Friday

-Subtract 1 day from Sunday - Expected result = the previous Friday

-Subtract 1 day from Saturday - Expected result = the previous Friday

-Subtract 3 days from Monday - Expected result = the previous Wednesday

-Subtract 5 days from Saturday - Expected result = the previous Monday

-Subtract 5 days from Monday - Expected result = the previous Monday

Here is what I wrote after reading this entire thread and picking the good pieces of logic:

CREATE FUNCTION [dbo].[BusinessDateAdd]
(
    @FromDate DATE
    ,@DaysToAdd INT
)
RETURNS DATE 
AS 
BEGIN

    --If there are no days to add or subtract, return the day that was passed in
    IF @DaysToAdd = 0 RETURN @FromDate

    DECLARE @Weeks INT
    DECLARE @DMod INT
    DECLARE @FromDateIndex INT

    --number of weeks
    SET @Weeks = @DaysToAdd/5

    --remainder of days
    SET @dmod = @DaysToAdd%5

    --Get the FromDate day of the week, this logic standardizes the @@DateFirst to Sunday = 1
    SET @FromDateIndex = (DATEPART(weekday, @FromDate) + @@DATEFIRST - 1) % 7 + 1

    /*Splitting the addition vs subtraction logic for readability*/

    --Adding business days
    IF @DaysToAdd > 0 
        BEGIN 

            --If the FromDate is on a weekend, move it to the previous Friday
            IF @FromDateIndex IN(1,7) 
                BEGIN
                    SET @FromDate = DATEADD(dd,CASE @FromDateIndex WHEN 1 THEN -2 WHEN 7 THEN -1 END,@FromDate)
                    SET @FromDateIndex = 6
                END

            SET @FromDate = DATEADD(dd, 
                CASE 
                    --If the mod goes through the weekend, add 2 days to account for it
                    WHEN 
                        ((@FromDateIndex = 3 --Tuesday
                        AND @dmod > 3) --Days until Friday
                        OR
                        (@FromDateIndex = 4  --Wednesday
                        AND @dmod > 2)--Days until Friday
                        OR 
                        (@FromDateIndex = 5 --Thursday
                        AND @dmod > 1)--Days until Friday
                        OR 
                        (@FromDateIndex = 6 --Friday
                        AND @dmod > 0))--Days until Friday
                        THEN 
                            @DMod+2 
                    --Otherwise just add the mod
                    ELSE 
                        @DMod 
                END, @FromDate)

        END

    --Subtracting business days
    IF @DaysToAdd < 0 
        BEGIN 

            --If the FromDate is on a weekend, move it to the next Monday
            IF @FromDateIndex IN(1,7) 
                BEGIN
                    SET @FromDate = DATEADD(dd,CASE @FromDateIndex WHEN 1 THEN 1 WHEN 7 THEN 2 END,@FromDate)
                    SET @FromDateIndex = 2
                END

            SET @FromDate = DATEADD(dd, 
                CASE 
                    --If the mod goes through the weekend, subtract 2 days to account for it
                    WHEN 
                        ((@FromDateIndex = 5 --Thursday
                        AND @dmod < -3) --Days until Monday
                        OR
                        (@FromDateIndex = 4  --Wednesday
                        AND @dmod < -2)--Days until Monday
                        OR 
                        (@FromDateIndex = 3 --Tuesday
                        AND @dmod < -1)--Days until Monday
                        OR 
                        (@FromDateIndex = 2 --Monday
                        AND @dmod < 0))--Days until Monday
                        THEN 
                            @DMod-2 
                    --Otherwise just subtract the mod
                    ELSE 
                        @DMod 
                END, @FromDate)

        END

    --Shift the date by the number of weeks
    SET @FromDate = DATEADD(ww,@Weeks,@FromDate)

    RETURN @FromDate

END