Calculate dynamic price with many to many relationship using SQL - mysql

Question

I'm new to SQL and I have two tables with a many-to-many relationship. I know the math behind the calculation by am unable to transfer it into SQL language
Tables:
Table A like:
Item Name
Effective Date
Cancelled Date
Unit Price
Book
2010-01-02
2021-12-21
10
Book
2022-01-01
2028-01-01
15
Ice Cream
2018-01-01
2028-01-01
4
Table B like:
Item Name
Sales Date
Volume
Discounted Price
Book
2019-01-01
8
70
Book
2022-01-01
5
75
Ice Cream
2020-12-01
10
30
I want to calculate how much money each Item saved each month due to promotion, so my output should be:
Item Name
Month
MoneySaved
I came up with the following structure:
select a.price as price_at_that_time
from TableA join TableB on a.ItemName = b.ItemName
where SalesDate between EffectiveDate and CancelledDate
select to_char(SalesDate, 'YYYY-MM') as month,
a.ItemName,
sum(price_at_that_time * SalesVolume - discounted price) as MoneySaved
from TableA join TableB ...
group by to_char(SalesDate, 'YYYY-MM'),
a.fitm
However, I'm not able to combine these two steps into one because of the many to many relationship and the dynamic nature of the price. Items may have different unit prices in different months, and even within a month, the unit price can be different.
I tired code like:
select to_char(SalesDate, 'YYYY-MM') as month,
a.ItemName,
sum(a.UnitPrice * SalesVolume - DiscountedPrice) as MoneySaved
from TableA join TableB on a.ItemName = b.ItemName
where a.UnitPrice in
(select a.UnitPrice from TableA join TableB on a.ItemName = b.ItemName where SalesDate between EffectiveDate and CancelledDate)
group by to_char(SalesDate, 'YYYY-MM'),
a.fitm
I know the code is wrong but that's closest I can get.

Answer 1

Almost exactly as you described it:
With TableA as (
select *
from (values
('Book', '2010-01-02', '2021-12-21', 10)
,('Book', '2022-01-01', '2028-01-01', 15)
,('Ice Cream', '2018-01-01','2028-01-01', 4)
) T(ItemName,EffectiveDate,CancelledDate, UnitPrice)
)
, TableB as (
select *
from (values
('Book' ,'2019-01-01' ,8 ,70)
,('Book' ,'2022-01-01' ,5 ,75)
,('Ice Cream' ,'2020-12-01' ,10 ,30))
T(ItemName,SalesDate,Volume,DiscountedPrice)
)
select
Sales.ItemName
, sum((RegPRices.UnitPrice * Sales.Volume) -
Sales.DiscountedPrice) as SavedAmount
from
TableB as Sales
inner join
TableA RegPrices
on RegPrices.ItemName=Sales.ItemName
and Sales.SalesDate
between RegPrices.EffectiveDate and RegPrices.CancelledDate
group by Sales.ItemName
This returns:
ItemName
SavedAmount
Book
10
Ice Cream
10
.