SET DATEFIRST in FUNCTION

Question

I want to SET DATEFIRST in my function but it is not allowed.

SET DATEFIRST 1

I can add the code in a SP and call the SP from the function

Answer 1

Alternative way is to explicitly specify the first day of week value as parameter and avoid depending on @@DATEFIRST setting. You can use the following formula to achieve that in your function:

(DATEPART(dw, GETDATE()) + @@DATEFIRST + 6 - @WeekStartDay) % 7 + 1

where @WeekStartDay is the first day of the week you want for your system (from 1 to 7 which means from Monday to Sunday).

I have wrapped it into below function so we can reuse it easily:

CREATE FUNCTION [dbo].[GetDayInWeek](@InputDateTime DATETIME, @WeekStartDay INT)
RETURNS INT
AS
BEGIN
    --Note: @WeekStartDay is number from [1 - 7] which is from Monday to Sunday
    RETURN (DATEPART(dw, @InputDateTime) + @@DATEFIRST + 6 - @WeekStartDay) % 7 + 1 
END

Example usage: GetDayInWeek('2019-02-04 00:00:00', 1)

It is equivalent to following (but independent to DATEFIRST setting):

SET DATEFIRST 1
DATEPART(dw, '2019-02-04 00:00:00')