ISO Week Of Year Function

This function returns the ISO 8601 week of the year for the date passed. The first week of each year starts on the first Monday on or before January 4 of that year, so that the year begins from December 28 of the prior year through January 4 of the current year.

This code creates the function and demos it for the first day of each ISO week/year from 1990 to 2030.



drop function dbo.F_ISO_WEEK_OF_YEAR
go
create function dbo.F_ISO_WEEK_OF_YEAR
(
@Datedatetime
)
returnsint
as
/*
Function F_ISO_WEEK_OF_YEAR returns the
ISO 8601 week of the year for the date passed.
*/
begin

declare @WeekOfYearint

select
-- Compute week of year as (days since start of year/7)+1
-- Division by 7 gives whole weeks since start of year.
-- Adding 1 starts week number at 1, instead of zero.
@WeekOfYear =
(datediff(dd,
-- Case finds start of year
case
whenNextYrStart <= @date
thenNextYrStart
whenCurrYrStart <= @date
thenCurrYrStart
elsePriorYrStart
end,@date)/7)+1
from
(
select
-- First day of first week of prior year
PriorYrStart =
dateadd(dd,(datediff(dd,-53690,dateadd(yy,-1,aa.Jan4))/7)*7,-53690),
-- First day of first week of current year
CurrYrStart =
dateadd(dd,(datediff(dd,-53690,aa.Jan4)/7)*7,-53690),
-- First day of first week of next year
NextYrStart =
dateadd(dd,(datediff(dd,-53690,dateadd(yy,1,aa.Jan4))/7)*7,-53690)
from
(
select
--Find Jan 4 for the year of the input date
Jan4=
dateadd(dd,3,dateadd(yy,datediff(yy,0,@date),0))
) aa
) a

return @WeekOfYear

end
go





-- Execute function on first day of first week of year from 1990 to 2030
select
DT,
ISO_WEEK_OF_YEAR =
dbo.F_ISO_WEEK_OF_YEAR(a.DT)
from
(
select DT = getdate()union all
select DT = convert(datetime,'1990/01/01') union all
select DT = convert(datetime,'1990/12/31') union all
select DT = convert(datetime,'1991/12/30') union all
select DT = convert(datetime,'1993/01/04') union all
select DT = convert(datetime,'1994/01/03') union all
select DT = convert(datetime,'1995/01/02') union all
select DT = convert(datetime,'1996/01/01') union all
select DT = convert(datetime,'1996/12/30') union all
select DT = convert(datetime,'1997/12/29') union all
select DT = convert(datetime,'1999/01/04') union all
select DT = convert(datetime,'2000/01/03') union all
select DT = convert(datetime,'2001/01/01') union all
select DT = convert(datetime,'2001/12/31') union all
select DT = convert(datetime,'2002/12/30') union all
select DT = convert(datetime,'2003/12/29') union all
select DT = convert(datetime,'2005/01/03') union all
select DT = convert(datetime,'2006/01/02') union all
select DT = convert(datetime,'2007/01/01') union all
select DT = convert(datetime,'2007/12/31') union all
select DT = convert(datetime,'2008/12/29') union all
select DT = convert(datetime,'2010/01/04') union all
select DT = convert(datetime,'2011/01/03') union all
select DT = convert(datetime,'2012/01/02') union all
select DT = convert(datetime,'2012/12/31') union all
select DT = convert(datetime,'2013/12/30') union all
select DT = convert(datetime,'2014/12/29') union all
select DT = convert(datetime,'2016/01/04') union all
select DT = convert(datetime,'2017/01/02') union all
select DT = convert(datetime,'2018/01/01') union all
select DT = convert(datetime,'2018/12/31') union all
select DT = convert(datetime,'2019/12/30') union all
select DT = convert(datetime,'2021/01/04') union all
select DT = convert(datetime,'2022/01/03') union all
select DT = convert(datetime,'2023/01/02') union all
select DT = convert(datetime,'2024/01/01') union all
select DT = convert(datetime,'2024/12/30') union all
select DT = convert(datetime,'2025/12/29') union all
select DT = convert(datetime,'2027/01/04') union all
select DT = convert(datetime,'2028/01/03') union all
select DT = convert(datetime,'2029/01/01') union all
select DT = convert(datetime,'2029/12/31') union all
select DT = convert(datetime,'2030/12/30')
) a








CODO ERGO SUM