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).

Answer 2

The day of the week? Why don't you just use DATEPART?

DATEPART(weekday, YEAR_DATE)

Answer 3

Can't you just select it with something like:

SELECT DATENAME(dw, GETDATE());

Answer 4

The proposed solution has one problem - it returns 0 for Saturdays. Assuming that we're looking for something compatible with DATEPART(WEEKDAY) this is an issue.

Nothing a simple CASE statement won't fix, though.

Answer 5

Taken from Deterministic scalar function to get week of year for a date

;
with 
Dates(DateValue) as 
(
    select cast('2000-01-01' as date)
    union all 
    select dateadd(day, 1, DateValue) from Dates where DateValue < '2050-01-01'
)
select 
    year(DateValue) * 10000 + month(DateValue) * 100 + day(DateValue) as DateKey, DateValue,        
    datediff(day, dateadd(week, datediff(week, 0, DateValue), 0), DateValue) + 2 as DayOfWeek,
    datediff(week, dateadd(month, datediff(month, 0, DateValue), 0), DateValue) + 1 as WeekOfMonth,
    datediff(week, dateadd(year, datediff(year, 0, DateValue), 0), DateValue) + 1 as WeekOfYear
    from Dates option (maxrecursion 0)

Answer 6

Make a function, and have @dbdate varchar(8) as your input variable.

Have it return the following:

RETURN (DATEDIFF(dd, -1, convert(datetime, @dbdate, 112)) % 7)+1;

The value 112 is the sql style YYYYMMDD.

This is deterministic because the datediff does not receive a string input, if it were to receive a string it would no longer work because it internally converts it to a datetime object. Which is not deterministic.