Modification todate dimension in SQL Server - sql

Question

I need a suggestion around one of the columns that I'm creating in the Date dimension in SQL Server, basically rolling weeks..
I have a table dimDate in my datawarehouse.
I want to create a column in the dimdate table which will have week number in any year and each week should have 7 days.
For eg: In year 2015 there are 53 weeks but the 53rd week has only 5 days (because the week starts on Sunday in SQL Server I guess).
I want to include 2 more days from 2016 (1st and 2nd Jan in 2016) to complete the 53rd week with 7 days and also the the 1st week in 2016 should start on 3rd of Jan 2016, so on and so forth.
If there are any suggestions that will be great to start with.

Answer 1

Assuming that you already have weeks populated (but not extended into the next year), and making some assumptions about columns names
This query finds the last week in a year (which would almost always always be 53 but don't count on it:) and the date that it ends on
SELECT YearNo, MAX(Week) As Week, MAX(DateKey) As DateKey
FROM dimDate
GROUP BY YearNo
This query finds all weeks that are shorter than 7 days, and how many extra days are required to make them 7 days.
SELECT
YearNo,
Week,
7-COUNT(DISTINCT DateKey) As ExtraDaysRequired
FROM dimDate
GROUP BY YearNo, Week
HAVING COUNT(DISTINCT DateKey) < 7
This might always be the last week of the year but lets not make assumptions.
Lets combine these to find all final weeks that have less than 7 days, as well as add the number of days required:
SELECT
Under7Days.YearNo, Under7Days.Week, Under7Days.ExtraDaysRequired,
FinalWeeks.DateKey StartDate,
DATEADD(d,Under7Days.ExtraDaysRequired,FinalWeeks.DateKey) EndDate
FROM
(
SELECT YearNo, MAX(Week) As Week, MAX(DateKey) As DateKey
FROM dimDate
GROUP BY YearNo
) As FinalWeeks
INNER JOIN
(
SELECT YearNo, Week, 7-COUNT(DISTINCT DateKey) As ExtraDaysRequired
FROM dimDate
GROUP BY YearNo, Week
HAVING COUNT(DISTINCT DateKey) < 7
) As Under7Days
ON FinalWeeks.Week = Under7Days.Week
AND FinalWeeks.YearNo = Under7Days.YearNo
So we have a query that identifies the start date and end date and week number that it needs to be updated to. So now we run an update:
UPDATE TGT
SET Week = SRC.Week
FROM dimDate TGT
INNER JOIN
(
SELECT
Under7Days.YearNo, Under7Days.Week, Under7Days.ExtraDaysRequired,
FinalWeeks.DateKey StartDate,
DATEADD(d,Under7Days.ExtraDaysRequired,FinalWeeks.DateKey) EndDate
FROM
(
SELECT YearNo, MAX(Week) As Week, MAX(DateKey) As DateKey
FROM dimDate
GROUP BY YearNo
) As FinalWeeks
INNER JOIN
(
SELECT YearNo, Week, 7-COUNT(DISTINCT DateKey) As ExtraDaysRequired
FROM dimDate
GROUP BY YearNo, Week
HAVING COUNT(DISTINCT DateKey) < 7
) As Under7Days
ON FinalWeeks.Week = Under7Days.Week
AND FinalWeeks.YearNo = Under7Days.YearNo
) SRC
ON TGT.DateID BETWEEN SRC.StartDate AND SRC.EndDate
Looks complicated? There's half a dozen ways to write the same thing but this approach is step-by-step. You could probably write a windowing function to do the same thing but I leave that as an exercise for someone else.