Unique Week (Monday - Sunday) Number Regardless Of What Year It Is - sql

Question

I would like to get the number of a week from a [Dates] column, starting with Monday as the first day of the week. I would like each week to be uniquely identified. Rather than rolling over at 52 to 1, to continue counting to week 54, 55, etc.
P.S. I need this for correct sort of [Week Date Range] column in Tableau.
Otherwise, Tableau sorts grouped data this way:
08/08/2022 - 08/14/2022
08/09/2021 - 08/15/2021
08/15/2022 - 08/21/2022
08/16/2021 - 08/22/2021
Code:
CREATE TABLE Dates
(Id INT, Dates DATE);
INSERT INTO Dates
VALUES
(1, '2022-12-19'),
(2, '2022-12-22'),
(3, '2022-12-25'),
(4, '2022-12-26'),
(5, '2022-12-29'),
(6, '2022-12-31'),
(7, '2023-01-01'),
(8, '2023-01-06'),
(9, '2023-01-07'),
(10, '2023-01-09')
SELECT *,
CONCAT(CONVERT(VARCHAR(10), DATEADD(DAY, DATEDIFF(DAY, '19000101', Dates) / 7 * 7, '19000101'), 101), ' - ', CONVERT(VARCHAR(10), DATEADD(DAY, (DATEDIFF(DAY, '19000101', Dates) / 7 + 1) * 7, '18991231'), 101)) AS [Week Date Range (Monday - Sunday)],
DATEPART(ISO_WEEK, Dates) AS [ISO Week Number]
FROM Dates
Id
Dates
Week Date Range (Monday - Sunday)
ISO Week Number
1
2022-12-19
12/19/2022 - 12/25/2022
51
2
2022-12-22
12/19/2022 - 12/25/2022
51
3
2022-12-25
12/19/2022 - 12/25/2022
51
4
2022-12-26
12/26/2022 - 01/01/2023
52
5
2022-12-29
12/26/2022 - 01/01/2023
52
6
2022-12-31
12/26/2022 - 01/01/2023
52
7
2023-01-01
12/26/2022 - 01/01/2023
52
8
2023-01-06
01/02/2023 - 01/08/2023
1
9
2023-01-07
01/02/2023 - 01/08/2023
1
10
2023-01-09
01/09/2023 - 01/15/2023
2

Answer 1

Your problem is not really that you are in need of a running week number. Your problem is that the "Week Date Range" that you have asked Tableau to sort is a string formatted in part as MM/DD/YYYY, which will never sort correctly across years.
The simplest solution is to provide either a true date value such as the calculated week start date or that same date as a string formatted as "yyyy-mm-dd" using CONVERT(VARCHAR(10), WeekStart, 120).
However, if you really want a unique week number, you can use the DATEDIFF(day, ...) / 7 function relative to a "reference Monday" to give you a continuous integer week number. (The "reference Monday" must precede all dates in the data to avoid negative diffs.)
The following includes both:
SET DATEFIRST 1
DECLARE #ReferenceMonday DATE = '1900-01-01' -- This just happens to be a Monday
SELECT
*,
DATENAME(weekday, DATES) AS Weekday,
CONCAT(CONVERT(VARCHAR(10), DATEADD(DAY, DATEDIFF(DAY, '19000101', Dates) / 7 * 7, '19000101'), 101), ' - ', CONVERT(VARCHAR(10), DATEADD(DAY, (DATEDIFF(DAY, '19000101', Dates) / 7 + 1) * 7, '18991231'), 101)) AS [Week Date Range (Monday - Sunday)],
DATEDIFF(day, #ReferenceMonday, Dates) / 7 AS SortableWeek,
CONVERT(VARCHAR(10), DATEADD(DAY, DATEDIFF(DAY, '19000101', Dates) / 7 * 7,'19000101'), 120) AS [SortableWeekStart]
FROM Dates
ORDER BY Id
(I have also added weekday name to the select list as a reference.)
The week start and finish date calculations can also be simplified by combining DATEPART(weekday,...) with DATEADD(day).
DATEADD(day, 1-DATEPART(weekday, D.Dates), D.Dates) AS WeekStart
DATEADD(day, 7-DATEPART(weekday, D.Dates), D.Dates) AS WeekFinish
A technique that may improve readability and reduce duplication of expressions is to use CROSS APPLY to separate calculations from the final select list.
Combining the above could result in a query rewritten as follows:
SET DATEFIRST 1
DECLARE #ReferenceMonday DATE = '1900-01-01' -- This just happens to be a Monday
SELECT
D.*,
DATENAME(weekday, DATES) AS Weekday,
DS.[Week Date Range (Monday - Sunday)],
WK.SortableWeek,
DS.SortableWeekStart
FROM Dates D
CROSS APPLY (
SELECT
DATEADD(day, 1-DATEPART(weekday, D.Dates), D.Dates) AS WeekStart,
DATEADD(day, 7-DATEPART(weekday, D.Dates), D.Dates) AS WeekFinish,
DATEDIFF(day, #ReferenceMonday, Dates) / 7 AS SortableWeek
) WK
CROSS APPLY (
SELECT
CONCAT(CONVERT(VARCHAR(10), WK.WeekStart, 101), ' - ', CONVERT(VARCHAR(10), WK.WeekFinish, 101)) AS [Week Date Range (Monday - Sunday)],
CONVERT(VARCHAR(10), WeekStart, 120) AS SortableWeekStart
) DS
ORDER BY Id
Both of the above queries yield the following results:
Id
Dates
Weekday
Week Date Range (Monday - Sunday)
SortableWeek
SortableWeekStart
1
2022-12-19
Monday
12/19/2022 - 12/25/2022
6416
2022-12-19
2
2022-12-22
Thursday
12/19/2022 - 12/25/2022
6416
2022-12-19
3
2022-12-25
Sunday
12/19/2022 - 12/25/2022
6416
2022-12-19
4
2022-12-26
Monday
12/26/2022 - 01/01/2023
6417
2022-12-26
5
2022-12-29
Thursday
12/26/2022 - 01/01/2023
6417
2022-12-26
6
2022-12-31
Saturday
12/26/2022 - 01/01/2023
6417
2022-12-26
7
2023-01-01
Sunday
12/26/2022 - 01/01/2023
6417
2022-12-26
8
2023-01-06
Friday
01/02/2023 - 01/08/2023
6418
2023-01-02
9
2023-01-07
Saturday
01/02/2023 - 01/08/2023
6418
2023-01-02
10
2023-01-09
Monday
01/09/2023 - 01/15/2023
6419
2023-01-09
See this db<>fiddle for a working example.

Answer 2

I think you're mostly on the right path. Logic as follows
Find the start of the week
With that start of the week, calculate the relevant week number
Convert that week number into a 'grouping' value that is sorted
You've done the first two steps above; to get the third, I get the 'grouping value' for Tableau to be an int value calculated as 100 * YEAR(date) + (week number) - meaning that in your data above, the grouping value will range from 202251 to 202302.
Note that this needs to be done on the 'WeekStart' field, not 'Dates', because 20230101 will return a YEAR of 2023 rather than 2022.
-- Make Monday the first day of week
SET DATEFIRST 1;
-- Calculate 'ISOwkRef' as sort variable
WITH Dates_with_weekstart AS
(SELECT ID,
Dates,
DATEADD(day, -1 * (DATEPART(weekday, dates)-1), Dates) AS WeekStart
FROM dates)
SELECT *,
100*YEAR(Weekstart) + DATEPART(iso_week, WeekStart) AS ISOwkRef
FROM Dates_with_weekstart
ORDER BY ISOwkRef;
db<>fiddle with answer above here.
Results
ID Dates WeekStart ISOwkRef
1 2022-12-19 2022-12-19 202251
2 2022-12-22 2022-12-19 202251
3 2022-12-25 2022-12-19 202251
4 2022-12-26 2022-12-26 202252
5 2022-12-29 2022-12-26 202252
6 2022-12-31 2022-12-26 202252
7 2023-01-01 2022-12-26 202252
8 2023-01-06 2023-01-02 202301
9 2023-01-07 2023-01-02 202301
10 2023-01-09 2023-01-09 202302
Edit: Alternatively, if you prefer to have the weeks numbered from the first week and then increasingly, you can do it similar to above, but
First calculate the week number for the earliest week
Calculate the difference in weeks from that earliest week, and add it to the number
I think it's a bit more convoluted, but here is an approach to do it:
WITH Dates_with_weekstart AS
(SELECT ID,
Dates,
DATEADD(day, -1 * (DATEPART(weekday, dates)-1), Dates) AS WeekStart
FROM dates)
SELECT Dates_with_weekstart.ID,
Dates_with_weekstart.Dates,
Dates_with_weekstart.Weekstart,
DATEPART(ISO_week, First_Weekstart) + DATEDIFF(week, First_Weekstart, Dates_with_weekstart.Weekstart) AS ISOWkNum_rolling
FROM Dates_with_weekstart
CROSS JOIN
(SELECT MIN(WeekStart) AS First_Weekstart
FROM Dates_with_weekstart
) AS FirstWkStart
ORDER BY ISOWkNum_rolling;
Results:
ID Dates Weekstart ISOWkNum_rolling
1 2022-12-19 2022-12-19 51
2 2022-12-22 2022-12-19 51
3 2022-12-25 2022-12-19 51
4 2022-12-26 2022-12-26 52
5 2022-12-29 2022-12-26 52
6 2022-12-31 2022-12-26 52
7 2023-01-01 2022-12-26 52
8 2023-01-06 2023-01-02 53
9 2023-01-07 2023-01-02 53
10 2023-01-09 2023-01-09 54
Fiddle above has been adjusted with this second approach.
Feel free to add the varchar 'Date range' field you have; this answer is just working with the key data.