Deterministic scalar function to get day of week for a date

Question

SQL Server, trying to get day of week via a deterministic UDF.

Im sure this must be possible, but cant figure it out.

UPDATE: SAMPLE CODE..

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Answer 1

There is an already built-in function in sql to do it:

SELECT DATEPART(weekday, '2009-11-11')

EDIT: If you really need deterministic UDF:

CREATE FUNCTION DayOfWeek(@myDate DATETIME ) 
RETURNS int
AS
BEGIN
RETURN DATEPART(weekday, @myDate)
END
GO
SELECT dbo.DayOfWeek('2009-11-11')

EDIT again: this is actually wrong, as DATEPART(weekday) is not deterministic.

UPDATE: DATEPART(weekday) is non-deterministic because it relies on DATEFIRST (source).
You can change it with SET DATEFIRST but you can't call it inside a stored function.

I think the next step is to make your own implementation, using your preferred DATEFIRST inside it (and not considering it at all, using for example Monday as first day).