In SQL Server 2008 the isoweek can be found with this:
SELECT datepart(iso_week, getdate())
Before SQL Server 2008 there were no built-in function to find isoweek.
I have been searching high and low for a good syntax to find a userdefined iso_week for SQL Server 2005. I found quite a few solutions. Didn't like any of the solutions I found, most of them didn't work, and they were way too long.
Since the issue is very old, I would expect this issue is depleted and the best solution has been found. I was unable to find a good method though.
I wrote a solution, which I am going to post later. But before I do, I want to make absolutely sure that no one else can match the solution I wrote.
I am hoping to earn the self-learner badge. I urge people to find the best answers out there for this ancient question.
I am going to post my answer after giving people a chance of finding a good solution.
If you want to get a day from a date in a table, use the SQL Server DAY() function. This function takes only one argument – the date. This can be a date or date and time data type.
If you need to extract the ISO week number from a date in SQL Server, you can use the iso_week argument when calling the DATEPART() function. You can alternatively use the isowk or isoww arguments to do the same thing. By “ISO week”, I'm referring to the ISO 8601 date and time standard.
MySQL WEEKDAY() Function The WEEKDAY() function returns the weekday number for a given date. Note: 0 = Monday, 1 = Tuesday, 2 = Wednesday, 3 = Thursday, 4 = Friday, 5 = Saturday, 6 = Sunday.
There is a link here for other earlier attempts http://www.sqlteam.com/forums/topic.asp?TOPIC_ID=60510
This is the OLD code for the function
CREATE function f_isoweek(@date datetime)
RETURNS INT
as
BEGIN
DECLARE @rv int
SELECT @rv = datediff(ww, dateadd(ww, datediff(d, 0, dateadd(yy, datediff(yy, 0, day4),3))/7,-4),day4)
FROM (SELECT dateadd(ww, datediff(day, 0, @date)/7, 3) day4) a
RETURN @rv
END
After combining @AndriyM 's brilliant answer with my own, we are down to 1 line. This is the NEW code.
CREATE function f_isoweek(@date datetime)
RETURNS INT
as
BEGIN
RETURN (datepart(DY, datediff(d, 0, @date) / 7 * 7 + 3)+6) / 7
-- replaced code for yet another improvement.
--RETURN (datepart(DY, dateadd(ww, datediff(d, 0, @date) / 7, 3))+6) / 7
END
Explanation for the old code (not going to explain the new code. It is fragments from my code and AndriyM's code):
Finding weekday 4 of the chosen date
dateadd(week, datediff(day, 0, @date)/7, 3)
Finding isoyear - year of weekday 4 of a week is always the same year as the isoyear of that week
datediff(yy, 0, day4)
When adding 3 days to the first day of the isoyear a random day of the first isoweek of the isoyear is found
dateadd(yy, datediff(yy, 0, day4),3)
finding relative week of the first isoweek of the isoyear
datediff(d, 0, dateadd(yy, datediff(yy, 0, day4),3))/7
Finding the monday minus 4 days of the first isoweek results in thursday of the week BEFORE the first day of the first isoweek of the isoyear
dateadd(ww, datediff(d, 0, dateadd(yy, datediff(yy, 0, day4),3))/7,-4)
Knowing first thursday of the week before the first isoweek and first thursday of the chosen week, makes it is quite easy to calculate the week, it doesn't matter which setting datefirst has since the weekdays of both dates are thursdays.
datediff(ww, dateadd(ww, datediff(d, 0, dateadd(yy, datediff(yy, 0, day4),3))/7,-4),day4)
Here's an approach that is similar to yours in that it also relies on this week's Thursday. But in the end it uses the date differently.
Get the date of this (ISO) week's Thursday.
Your own solution uses the hard coded date of a known Thursday. Alternatively, this week's Thursday could be found with the help of @@DATEFIRST
:
SELECT Th = DATEADD(DAY, 3 - (DATEPART(WEEKDAY, @date) + @@DATEFIRST - 2) % 7, @date)
(I wasn't struggling too much for the right formula because it was already known to me.)
Get the Thursday's day of year:
SELECT DY = DATEPART(DAYOFYEAR, Th)
Use the number to find out the week like this:
SELECT ISOWeek = (DY - 1) / 7 + 1
Here are the above calculations in a single statement:
SELECT ISOWeek = (DATEPART(DAYOFYEAR, Th) - 1) / 7 + 1
FROM (
SELECT Th = DATEADD(DAY, 3 - (DATEPART(WEEKDAY, @date) + @@DATEFIRST - 2) % 7, @date)
) s;
Wow! Very good topic and solution to avoid using "set datefirst 1". I just want to add something. If like me, you also want to return the year with the ISO week, like "2015-01" as being "Year 2015, Week 01", it might be useful for reporting purpose. Since the year from ISO week can be different from the actual year of the date! Here is how I did in combination to your code.
DECLARE @Date AS DATETIME
SET @Date = '2014-12-31'
SELECT
CAST(CASE WHEN MONTH(@Date) = 1 AND Q.ISOweek > 50 THEN YEAR(@Date) - 1
WHEN MONTH(@Date) = 12 AND Q.ISOweek < 3 THEN YEAR(@Date) + 1
ELSE YEAR(@Date)
END
AS VARCHAR(4))
+ '-'
+ RIGHT('00' + CAST(Q.ISOweek AS NVARCHAR(2)), 2) AS ISOweek
FROM (SELECT (datepart(DY, datediff(d, 0, @Date) / 7 * 7 + 3) + 6) / 7 AS ISOweek) Q
Will return "2015-01".
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