Subtract N from weeknum of current year - sql

Question

We have table WeeklyResults that looks like this:
Year WeekNum Value
2021 47 11.0
2021 48 14.0
2021 49 12.0
2021 50 17.0
2021 51 11.8
2021 52 11.3
2021 53 11.1
2022 01 11.5
2022 02 11.5
2022 03 81.5
We have a report with two parameters: Date and WeekNum. The report needs to show the last 6 weeks based on the weeknumber selected.
The issue is that, if user selects Week 2 of 2022, how can I subtract 6 weeks so that I get weeks 50, 51, 52, 53 of 2021 and weeks 1, 2 from 2022?
So, if the user selects 2022 and Week 02, it would show the last 6 weeks based on Weeknum 2 of year 2022 (wk50 to Wk02). If user selects 2021 and 52, it would show wk47-52.

Answer 1

You can use a little < and <= logic along side TOP and ORDER BY to achieve this:
DECLARE #Year int = 2022,
#WeekNum int = 3; --Note, if you are storing WeekNum as a (var)char,
--your leading zeros imply you are, then define the
--variable as a char(2).
SELECT TOP (6)
[Year],
WeekNum,
[Value]
FROM dbo.YourTable
WHERE ([Year] = #Year AND WeekNum <= #WeekNum)
OR [Year] < #Year
ORDER BY [Year] DESC,
WeekNum DESC;

Answer 2

Try this:
DECLARE #Year int = 2022
, #WeekNum varchar(02) = '02'
;
WITH FinalTable AS
(
SELECT TOP 6 *
FROM WeeklyResults
ORDER BY LTRIM(Year) + WeekNum DESC
)
SELECT *
FROM FinalTable
ORDER BY Year, WeekNum

Answer 3

Another option that doesn't involve an ORDER BY. Using DATEPART you can determine the final week of the prior year and subtract the number of weeks to get the records needed.
You may need to adjust what day is the first day of the week for your count. See this post for more info on that.
DECLARE
#Year INT = 2022
, #WeekNum INT = 3;
SELECT
*
FROM
WeeklyResults
WHERE
Year = #Year
AND WeekNum <= #WeekNum
OR
(
#WeekNum < 6
AND Year = #Year - 1
AND WeekNum > DATEPART (WEEK, CONCAT ('12/31/', #Year - 1)) - #WeekNum
);