Get begin of ISO week from a date

fu_get_ISOWeekBegin.sql

IF  EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[fu_get_ISOWeekBegin]') AND type in (N'FN', N'IF', N'TF', N'FS', N'FT'))
DROP FUNCTION [dbo].[fu_get_ISOWeekBegin]
GO




-- PRE:  Valid year and week no#
-- POST: First day of ISO week no#
CREATE FUNCTION [dbo].[fu_get_ISOWeekBegin](@year int, @ISO_WEEK int) 
	RETURNS datetime 
AS 
BEGIN 
    DECLARE @FirstISOweekOfYearDay datetime
    DECLARE @WeekBegin datetime
    
    SELECT @FirstISOweekOfYearDay = CAST(CAST(@year AS char(4)) AS datetime)
    SELECT @FirstISOweekOfYearDay = DATEADD(ww, DATEDIFF(ww, 0, @FirstISOweekOfYearDay), 0)
    IF DATEPART(isowk, @FirstISOweekOfYearDay) > 1 
		SET @FirstISOweekOfYearDay = @FirstISOweekOfYearDay + 7
    
    SET @WeekBegin = DATEADD(dd, @ISO_WEEK * 7 - 7, @FirstISOweekOfYearDay)
    RETURN @WeekBegin
END


GO