MySQL Query for book cost of a class

Question

I just need a single MySQL line query for the following.

Lets say there are 2 simple tables: Class and Books<

Answer 1

This should do the trick:

SELECT SUM(cost) AS cost FROM Books WHERE ClassId=? GROUP BY ClassId

Where the question makr is either the ID of the class or part of a prepared statement where you feed it the class ID.

You can retrieve the sum with column name "cost".