id : Auto-Increment
Product : String
Quantity : Number

id : Auto-Increment
Product : String
Quantity : Number

id | product | quantity
1 | A | 10
2 | B | 5
3 | A | 10
4 | C | 20
5 | C | 20
6 | A | 10
7 | C | 20
8 | B | 5
9 | B | 5

id | product | quantity
1 | A | 50
2 | B | 10
3 | C | 80

product | quantity_demanded | product_in_stock | rest
A | 30 | 50 | 20
B | 15 | 10 | -5
C | 60 | 80 | 20

SELECT s.product, sum(p.quantity) as quantity_demanded,
s.quantity as product_in_stock, s.quantity-sum(p.quantity) AS rest
from product as p inner join stock as s on p.product = s.product
group by s.product, s.quantity;