Calculating a running count of Weeks - sql

Question

I am looking to calculate a running count of the weeks that have occurred since a starting point. The biggest problem here is that the calendar I am working on is not a traditional Gregorian calendar.
The easiest dimension to reference would be something like 'TWEEK' which actually tells you the week of the year that the record falls into.
Example data:
CREATE TABLE #foobar
( DateKey INT
,TWEEK INT
,CumWEEK INT
);
INSERT INTO #foobar (DateKey, TWEEK, CumWEEK)
VALUES(20150630, 1,1),
(20150701,1,1),
(20150702,1,1),
(20150703,1,1),
(20150704,1,1),
(20150705,1,1),
(20150706,1,1),
(20150707,2,2),
(20150708,2,2),
(20150709,2,2),
(20150710,2,2),
(20150711,2,2),
(20150712,2,2),
(20150713,2,2),
(20150714,1,3),
(20150715,1,3),
(20150716,1,3),
(20150717,1,3),
(20150718,1,3),
(20150719,1,3),
(20150720,1,3),
(20150721,2,4),
(20150722,2,4),
(20150723,2,4),
(20150724,2,4),
(20150725,2,4),
(20150726,2,4),
(20150727,2,4)
For sake of ease, I did not go all the way to 52, but you get the point. I am trying to recreate the 'CumWEEK' column. I have a column already that tells me the correct week of the year according to the weird calendar convention ('TWEEK').
I know this will involve some kind of OVER() windowing, but I cannot seem to figure It out.

Answer 1

The windowing function LAG() along with a summation of ORDER BY ROWS BETWEEN "Changes" should get you close enough to work with. The caveat to this is that the ORDER BY ROWS BETWEEN can only take an integer literal.
Year Rollover : I guess you could create another ranking level based on mod 52 to start the count fresh. So 53 would become year 2, week 1, not 53.
SELECT
* ,
SUM(ChangedRow) OVER (ORDER BY DateKey ROWS BETWEEN 99999 PRECEDING AND CURRENT ROW)
FROM
(
SELECT
DateKey,
TWEEK,
ChangedRow=CASE WHEN LAG(TWEEK) OVER (ORDER BY DateKey) <> TWEEK THEN 1 ELSE 0 END
FROM
#foobar F2
)AS DETAIL

Answer 2

Some minutes ago I answered a different question, in a way this is a similar question to
https://stackoverflow.com/a/31303395/5089204
The idea is roughly to create a table of a running number and find the weeks with modulo 7. This you could use as grouping in an OVER clause...
EDIT: Example
CREATE FUNCTION dbo.RunningNumber(#Counter AS INT)
RETURNS TABLE
AS
RETURN
SELECT TOP (#Counter) ROW_NUMBER() OVER(ORDER BY o.object_id) AS RunningNumber
FROM sys.objects AS o; --take any large table here...
GO
SELECT 'test',CAST(numbers.RunningNumber/7 AS INT)
FROM dbo.RunningNumber(100) AS numbers
Dividing by 7 "as INT" offers a quite nice grouping criteria.
Hope this helps...