Good day,

I am working in an online website for selling books.
There is a table where the sold books are stored, like this:

Purchases
======
id | int - autoincrement
Date
CustomerID (foreing key to table Customers)
BookID (foreign key to table Books)

How can I calculate the total quantity of sold books of each one?
For instance, having 'n' books, how many BookID = 1 to n have been sold (how many of each one)?

Thanks a lot!!!