MSSQL - Getting last 6 weeks returns last 8 weeks - sql

Question

I am having a problem with week numbers. The customers week starts on a Tuesday, so ends on a Monday. So I have done:
Set DateFirst 2
When I then use
DateAdd(ww,#WeeksToShow, Date)
It occasionally gives me 8 weeks of information. I think it is because it goes over to the previous year, but I am not sure how to fix it.
If I do:
(DatePart(dy,Date) / 7) - #WeeksToShow
Then it works better, but obviously doesn't work going through to previous years as it just goes to minus figures.
Edit:
My currently SQL (If it helps at all without any data)
Set DateFirst 2
Select
DATEPART(yyyy,SessionDate) as YearNo,
DATEPART(ww,SessionDate) as WeekNo,
DATEADD(DAY, 1 - DATEPART(WEEKDAY, SessionDate + SessionTime), CAST(SessionDate +SessionTime AS DATE)) [WeekStart],
DATEADD(DAY, 7 - DATEPART(WEEKDAY, SessionDate + SessionTime), CAST(SessionDate + SessionTime AS DATE)) [WeekEnd],
DateName(dw,DATEADD(DAY, 7 - DATEPART(WEEKDAY, SessionDate + SessionTime), CAST(SessionDate + SessionTime AS DATE))) as WeekEndName,
Case when #ConsolidateSites = 1 then 0 else SiteNo end as SiteNo,
Case when #ConsolidateSites = 1 then 'All' else CfgSites.Name end as SiteName,
GroupNo,
GroupName,
DeptNo,
DeptName,
SDeptNo,
SDeptName,
PluNo,
PluDescription,
SUM(Qty) as SalesQty,
SUM(Value) as SalesValue
From
PluSalesExtended
Left Join
CfgSites on PluSalesExtended.SiteNo = CfgSites.No
Where
Exists (Select Descendant from DescendantSites where Parent in (#SiteNo) and Descendant = PluSalesExtended.SiteNo)
AND (DATEPART(WW,SessionDate + SessionTime) !=DATEPART(WW,GETDATE()))
AND SessionDate + SessionTime between DATEADD(ww,#NumberOfWeeks * -1,#StartingDate) and #StartingDate
AND TermNo = 0
AND PluEntryType <> 4
Group by
DATEPART(yyyy,SessionDate),
DATEPART(ww,SessionDate),
DATEADD(DAY, 1 - DATEPART(WEEKDAY, SessionDate + SessionTime), CAST(SessionDate +SessionTime AS DATE)),
DATEADD(DAY, 7 - DATEPART(WEEKDAY, SessionDate + SessionTime), CAST(SessionDate + SessionTime AS DATE)),
Case when #ConsolidateSites = 1 then 0 else SiteNo end,
Case when #ConsolidateSites = 1 then 'All' else CfgSites.Name end,
GroupNo,
GroupName,
DeptNo,
DeptName,
SDeptNo,
SDeptName,
PluNo,
PluDescription
order by WeekEnd

Answer 1

There are two issues here, the first is that I suspect you are defining 8 weeks of data as having 8 different values for DATEPART(WEEK, in which case you can replicate the root cause of the issue by looking at what ISO would define as the first week of 2015:
SET DATEFIRST 2;
SELECT Date, Week = DATEPART(WEEK, Date)
FROM (VALUES
('20141229'), ('20141230'), ('20141231'), ('20150101'),
('20150102'), ('20150103'), ('20150104')
) d (Date);
Which gives:
Date Week
-----------------
2014-12-29 52
2014-12-30 53
2014-12-31 53
2015-01-01 1
2015-01-02 1
2015-01-03 1
2015-01-04 1
So although you only have 7 days, you have 3 different week numbers. The problem is that DATEPART(WEEK is quite a simplistic function, and will simply return the number of week boundaries passed since the first day of the year, a better function would be ISO_WEEK since this takes into account year boundaries nicely:
SET DATEFIRST 2;
SELECT Date, Week = DATEPART(ISO_WEEK, Date)
FROM (VALUES
('20141229'), ('20141230'), ('20141231'), ('20150101'),
('20150102'), ('20150103'), ('20150104')
) d (Date);
Which gives:
Date Week
-----------------
2014-12-29 1
2014-12-30 1
2014-12-31 1
2015-01-01 1
2015-01-02 1
2015-01-03 1
2015-01-04 1
The problem is, that this does not take into account that the week starts on Tuesday, since the ISO week runs Monday to Sunday, you could adapt your usage slightly to get the week number of the day before:
SET DATEFIRST 2;
SELECT Date, Week = DATEPART(ISO_WEEK, DATEADD(DAY, -1, Date))
FROM (VALUES
('20141229'), ('20141230'), ('20141231'), ('20150101'),
('20150102'), ('20150103'), ('20150104')
) d (Date);
Which would give:
Date Week
-----------------
2014-12-29 52
2014-12-30 1
2014-12-31 1
2015-01-01 1
2015-01-02 1
2015-01-03 1
2015-01-04 1
So Monday the 29th December is now recognized as the previous week. The problem is that there is no ISO_YEAR built in function, so you will need to define your own. This is a fairly trivial function, even so I almost never create scalar functions because they perform terribly, instead I use an inline table valued function, so for this I would use:
CREATE FUNCTION dbo.ISOYear (#Date DATETIME)
RETURNS TABLE
AS
RETURN
( SELECT IsoYear = DATEPART(YEAR, #Date) +
CASE
-- Special cases: Jan 1-3 may belong to the previous year
WHEN (DATEPART(MONTH, #Date) = 1 AND DATEPART(ISO_WEEK, #Date) > 50) THEN -1
-- Special case: Dec 29-31 may belong to the next year
WHEN (DATEPART(MONTH, #Date) = 12 AND DATEPART(ISO_WEEK, #Date) < 45) THEN 1
ELSE 0
END
);
Which just requires a subquery to be used, but the extra typing is worth it in terms of performance:
SET DATEFIRST 2;
SELECT Date,
Week = DATEPART(ISO_WEEK, DATEADD(DAY, -1, Date)),
Year = (SELECT ISOYear FROM dbo.ISOYear(DATEADD(DAY, -1, Date)))
FROM (VALUES
('20141229'), ('20141230'), ('20141231'), ('20150101'),
('20150102'), ('20150103'), ('20150104')
) d (Date);
Or you can use CROSS APPLY:
SET DATEFIRST 2;
SELECT Date,
Week = DATEPART(ISO_WEEK, DATEADD(DAY, -1, Date)),
Year = y.ISOYear
FROM (VALUES
('20141229'), ('20141230'), ('20141231'), ('20150101'),
('20150102'), ('20150103'), ('20150104')
) d (Date)
CROSS APPLY dbo.ISOYear(d.Date) y;
Which gives:
Date Week Year
---------------------------
2014-12-29 52 2014
2014-12-30 1 2015
2014-12-31 1 2015
2015-01-01 1 2015
2015-01-02 1 2015
2015-01-03 1 2015
2015-01-04 1 2015
Even with this method, by simply getting a date 6 weeks ago you sill still end up with 7 weeks if the date you are using is not a Tuesday, because you will have 5 full weeks, and a part week at the start and a part week at the end, this is the second issue. So you need to make sure your start date is a Tuesday. The following will get you Tuesday of 7 weeks ago:
SELECT CAST(DATEADD(DAY, 1 - DATEPART(WEEKDAY, GETDATE()), DATEADD(WEEK, -6, GETDATE())) AS DATE);
The logic of this is explained better in this answer, the following is the part that will get the start of the week (based on your datefirst settings):
SELECT DATEADD(DAY, 1 - DATEPART(WEEKDAY, GETDATE()), GETDATE());
Then all I have done is substitute the second GETDATE() with DATEADD(WEEK, -6, GETDATE()) so that it is getting the start of the week 6 weeks ago, then there is just a cast to date to remove the time element from it.

Answer 2

This will get you current week + 5 previous weeks starting tuesday:
WHERE dateadd(week, datediff(d, 0, getdate()-1)/7 - 4, 1) <= yourdatecolumn
This will show examples:
DECLARE #wks int = 6 -- Weeks To Show
SELECT
dateadd(week, datediff(d, 0, getdate()-1)/7 - 4, 1) tuesday5weeksago,
dateadd(week, datediff(d, 0, getdate()-1)/7 - 5, 1) tuesday6weeksago,
dateadd(week, datediff(d, 0, getdate()-1)/7 - 6, 1) tuesday7weeksago,
dateadd(week, datediff(d, 0, getdate()-1)/7 - #wks + 1, 1) tuesdaydynamicweeksago
Result:
tuesday5weeksago tuesday6weeksago tuesday7weeksago tuesdaydynamicweeksago
2015-01-27 2015-01-20 2015-01-13 2015-01-20