Get ISO-Week and year string from date

fu_get_ISOWeek.sql

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



-- PRE: Valid datetime
-- POST: ISO week number + '/' + year, NULL on NULL
CREATE FUNCTION [dbo].[fu_get_ISOWeek](@dtDate as DATETIME) 
	RETURNS nvarchar(7) WITH RETURNS NULL ON NULL INPUT 
AS 
BEGIN 
	DECLARE @intISOWeekNumber int 
	,@intYear int 
	
	SET @intISOWeekNumber = DATEPART(ISO_WEEK, @dtDate) 
	
	IF (DATEPART(mm, @dtDate) = 1 AND @intISOWeekNumber > 50) 
		SET @intYear = DATEPART(yyyy, @dtDate)- 1 
	ELSE IF (DATEPART(mm, @dtDate) = 12 AND @intISOWeekNumber = 1) 
		SET @intYear = DATEPART(yyyy, @dtDate) + 1 
	ELSE 
		SET @intYear = DATEPART(yyyy, @dtDate) 
		
	RETURN (CAST(@intISOWeekNumber AS nvarchar) + '/' + CAST(@intYear AS nvarchar)) 
END


GO