Subtraction Within Queries

Hi all,

I was wondering if somebody could help me with a query I'm struggling to write. I have written a query which shows customers who have booked capacity of my company's product. I have written another query which shows customers who have bought volumes of the product. I want to write a query which shows the remaining capacity which can be purchased by customers (e.g. capacity - purchased) but the query I have written for this excludes any customers who haven't purchased any product. Note I do not want to update the tables, I just want to take a view of the capacity position of our customers.

Example:

Booked Capacity
Customer A = 1000
Customer B = 4000
Customer C = 3000

Product Purchased
Customer B = 1000

At the moment my query gives following result:
Customer B = 3000

What I want it to do is:
Customer A = 1000
Customer B = 3000
Customer C = 3000

Any help would be much appreciated!!!!

Ste