Google Sheet: Combining days into weeks in query - google-sheets

Question

I'm preparing a small data spreadsheet in Google Sheet, where by using pivot table with query I already managed to receive table as follows:
Day 1
Day 3
Day 8
Day 12
Article 1
Volume sold day 1
Volume sold day 3
Volume sold day 8
Volume sold day 12
Article 2
Volume sold day 1
Volume sold da3
Volume sold day 8
Volume sold day 12
Basically we're speaking about the table summing up sales with volumes sold each day (what's important, is that days are random).
I'm trying to sum up the volumes of each day to a whole calendar week so the outcome, I'm thinking of, is:
Week 1
Week 2
Article 1
Volume sold days 1 + 3
Volume sold days 8 + 12
Article 2
Volume sold days 1 + 3
Volume sold days 8 + 12
I'm filtering bigger table using query but this is not that important. To make it short I wrote following code:
=QUERY(<range>;
"select <article_column>,
sum(<volume_column>)
group by <article_column>
pivot <days_column>")
The question is: is there a way to combine week days from separate columns into a whole week and sum up the volumes sold within this particular week? Internet gives me a hint about using ARRAYFORMULA, but since my source database is kind of big, I'm trying to keep all the formulas as simple as it's possible.
Would you be able to recommend good resource to learn more about it? For last 2 hours I keep running in circles over the internet and I cannot find a good idea how to solve it.
Thanks in advance for your hints!

Answer 1

use:
=INDEX(QUERY({"Week "&WEEKNUM(A2:A100)\ B2:C100};
"select Col2,sum(Col3) where Col2 is not null group by Col2 pivot Col1"))