Group by start and end of the week date - sql

Question

I have the below promotion dates:
Group: Ambient
Start Date: 03/11/2020 End Date: 09/11/2020
Start Date: 10/11/2020 End Date: 16/11/2020
Group: Chilled
Start Date: 04/11/2020 End Date: 10/11/2020
Start Date: 11/11/2020 End Date: 17/11/2020
etc
My data consists of the below columns:
Dates (Daily), Group (Ambient/Chilled), Net Sales, Volume Sold
I want to group the data like the below:
Weekly Date, Group, Net Sales, Volume Sold
The Weekly date I want the week to start from Tuesday and ending on Monday based on the type of group it is. If the product is chilled I want the week to start from Wednesday and end on Tuesday. I want to do this and include a historical view to compare previous weeks to promotional weeks Example
Week Date Group Net Sales Volume
Row 1 - 27/10/2020 Ambient £900 30
Row 2 - 03/11/2020 Ambient £1000 50
Row 3 - 04/11/2020 Chilled £2000 40
Row 4 - 11/11/2020 Chilled £1000 30
What is the SQL query for changing the week dates?

Answer 1

There might be other ways to do this, but I would it first creating some functions that return me the DOW (Day Of Week) as I wanted (for Tuesday 0, ... Monday 6) and then another function FDOW (First Day Of Week) that use DOW. ie:
-- Tuesday = 0...Monday = 6
CREATE FUNCTION [dbo].[DOW]
(
#date DATETIME
)
RETURNS INT
AS
BEGIN
-- Add the T-SQL statements to compute the return value here
RETURN (##DATEFIRST + DATEPART(dw, #date) - 1 - 2) % 7;
END;
CREATE FUNCTION [dbo].[FDOW]
(
#date DATETIME
)
RETURNS DATETIME
AS
BEGIN
RETURN DATEADD(d, -dbo.DOW(#date), CAST(#date AS DATE));
END;
Having such functions it is easy to group on "WeekStart" which is our FDOW function:
Select dbo.FDOW(Dates) as WeekOf, [Group],
sum([Net Sales]) [Net Sales],
sum([Volume Sold]) [Volume]
from myTable
group by dbo.FDOW(Dates) as WeekOf, [Group];