fiscal year sql server - sql-server

Question

I want to have a function that calculates the fiscal year. The fiscal year must begin on the first Monday in March. Thank you!
example:
CREATE FUNCTION dbo.fnc_FiscalYear( #AsOf DATETIME )
RETURNS INT
AS BEGIN
DECLARE #Answer INT
SET DATEFIRST 1
IF ( MONTH(#AsOf) < 3 )
or MONTH(#AsOf=3) and datename(weekday, #AsOf) = 'Monday' and datepart(day, #AsOf)>=1 and datepart(day, #AsOf)<=7;
SET #Answer = YEAR(#AsOf) - 1
ELSE SET #Answer = YEAR(#AsOf)
RETURN #Answer
END
GO
but it's not working

Answer 1

It looks as though there are a number of syntax errors with your script. Try this instead. I've removed the SETs and returned at the point of the if statements. Also, note the grouping of the if statements.
CREATE FUNCTION dbo.fnc_FiscalYear( #AsOf DATETIME )
RETURNS INT
AS BEGIN
DECLARE #Answer INT
IF (( MONTH(#AsOf) < 3 )
OR (MONTH(#AsOf) = 3
AND DATENAME(weekday, #AsOf) = 'Monday'
AND datepart(day, #AsOf) >= 1
AND datepart(day, #AsOf)<=7))
RETURN (YEAR(#AsOf) - 1)
RETURN YEAR(#AsOf)
END
GO

Answer 2

The logic for this is tricky -- especially for the first week of March:
CREATE FUNCTION dbo.fnc_FiscalYear (
#AsOf DATETIME
)
RETURNS INT AS
BEGIN
RETURN( CASE WHEN MONTH(#AsOf) < 3 THEN YEAR(#AsOf) - 1
WHEN MONTH(#AsOf) > 3 THEN YEAR(#AsOf)
WHEN DAY(#AsOf) >= 7 THEN YEAR(#AsOf)
WHEN DATENAME(#AsOf) = 'Monday' OR
DATENAME(#AsOf) = 'Tuesday' AND DAY(#AsOf) >= 2 OR
DATENAME(#AsOf) = 'Wednesday' AND DAY(#AsOf) >= 3 OR
DATENAME(#AsOf) = 'Thursday' AND DAY(#AsOf) >= 4 OR
DATENAME(#AsOf) = 'Friday' AND DAY(#AsOf) >= 5 OR
DATENAME(#AsOf) = 'Saturday' AND DAY(#AsOf) >= 26
THEN YEAR(#AsOf)
ELSE YEAR(#AsOf) - 1
END);
END ;
GO

Answer 3

( ##DateFirst + DatePart( weekday, SampleDate ) - 1 ) % 7 + 1 will always return an integer from 0 to 6 with 0 corresponding to Sunday regardless of the setting of DateFirst or Language and without string manipulation.
Tweaking the expression to ( ##DateFirst + DatePart( weekday, SampleDate ) - 2 ) % 7 shifts the start of the cycle from Sunday to Monday and the range to 0 to 6 which, in turn, simplifies the following code:
create function dbo.FiscalYear( #Date as Date )
returns Int
begin
return Year( #Date ) - case
when Month( #Date ) <= 2 then 1 -- January and February are always part of the prior year.
-- In March it is the prior year only in the first week and if the calculated
-- DoW is less than the day of the month.
when Month( #Date ) = 3 and Day( #Date ) < 7 and
( ##DateFirst + DatePart( weekday, #Date ) - 2 ) % 7 >= Day( #Date ) then 1
else 0 end;
end;
Test the function with sample data:
with
-- Generate sample dates for 20 years.
YearOffsets as (
select 0 as YearOffset
union all
select YearOffset + 1
from YearOffsets
where YearOffset < 20 ),
SampleDates as (
select Cast( DateAdd( year, YearOffset, '2000-02-28' ) as Date ) as SampleDate, 1 as Counter
from YearOffsets
union all
select DateAdd( day, 1, SampleDate ), Counter + 1
from SampleDates
where Counter < 10 )
-- Calculate the Fiscal Year for each sample date.
select SampleDate, DateName( weekday, SampleDate ) as WeekDay,
( ##DateFirst + DatePart( weekday, SampleDate ) - 2 ) % 7 as DayOfWeek, -- 0 = Monday.
dbo.FiscalYear( SampleDate ) as FiscalYear
from SampleDates
order by SampleDate;