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

--Refactoring my original answer... I've added the option to define the starting point of the calculation if the starting date happens to be a weekend day: start from that weekend day or shift to the nearest weekday depending on the direction of the delta.

DECLARE
    @input DATE = '2019-06-15', -- if null, then returns null
    @delta INT = 1, -- can be positive or negative; null => zero
    @startFromWeekend BIT = 1 -- null => zero

-- input is null, delta is zero/null
IF @input IS NULL OR ISNULL(@delta, 0) = 0
    SELECT @input

-- input is not null and has delta
ELSE
BEGIN
    DECLARE
        @input_dw INT = (DATEPART(DW, @input) + @@DATEFIRST - 1) % 7, -- input day of week
        @weeks    INT = @delta / 5, -- adjust by weeks
        @days     INT = @delta % 5  -- adjust by days

    -- if input is a weekend day, offset it for proper calculation
    -- !!important!!: depends on *your* definition of the starting date to perform calculation from
    DECLARE @offset INT =
        -- start calc from weekend day that is nearest to a weekday depending on delta direction
        -- pos delta: effectively Sunday of the weekend   (actual: prev Friday)
        -- neg delta: effectively Saturday of the weekend (actual: next Monday)
        CASE WHEN ISNULL(@startFromWeekend, 0) = 1
        THEN CASE WHEN @delta > 0
            THEN CASE @input_dw
                WHEN 0 THEN -2
                WHEN 6 THEN -1
                END
            ELSE CASE @input_dw
                WHEN 0 THEN  1
                WHEN 6 THEN  2
                END
            END
        -- start calc from nearest weekday depending on delta direction
        -- pos delta: next Monday from the weekend
        -- neg delta: prev Friday from the weekend
        ELSE CASE WHEN @delta > 0
            THEN CASE @input_dw
                WHEN 0 THEN  1
                WHEN 6 THEN  2
                END
            ELSE CASE @input_dw
                WHEN 0 THEN -2
                WHEN 6 THEN -1
                END
            END
        END

    -- calculate: add weeks, add days, add initial correction offset
    DECLARE @output DATE = DATEADD(DAY, @days + ISNULL(@offset, 0), DATEADD(WEEK, @weeks, @input))

    -- finally, if output is weekend, add final correction offset depending on delta direction
    SELECT
        CASE WHEN (DATEPART(DW, @output) + @@DATEFIRST - 1) % 7 IN (0,6)
        THEN CASE 
            WHEN @delta > 0 THEN DATEADD(DAY,  2, @output)
            WHEN @delta < 0 THEN DATEADD(DAY, -2, @output)
            END
        ELSE @output
        END
END