Problem

Given a table with the following schema:
CREATE TABLE customer_orders (
customer_id INTEGER,
product_id INTEGER,
quantity INTEGER
);
Write a SQL query that returns the total quantity of each product ordered by each customer.
Example input:
INSERT INTO customer_orders (customer_id, product_id, quantity)
VALUES (1, 1, 5),
(1, 2, 3),
(2, 1, 2),
(2, 2, 4);
Example output:
customer_id | product_id | quantity
1 | 1 | 5
1 | 2 | 3
2 | 1 | 2
2 | 2 | 4

Solution

SELECT customer_id, product_id, SUM (quantity) as Quantity
FROM customer_orders
GROUP BY customer_id, product_id
ORDER BY customer_id