Q. https://solvesql.com/problems/sales-per-country/
SELECT customers.country,
(SUM(order_items.price * order_items.quantity)) - (SUM(CASE WHEN order_items.order_id LIKE 'C%' THEN order_items.price * order_items.quantity ELSE '0' END)) AS 'sales'
FROM customers LEFT JOIN orders ON customers.customer_id = orders.customer_id
LEFT JOIN order_items ON orders.order_id = order_items.order_id
WHERE order_date BETWEEN '2019-01-01' AND '2019-01-31'
GROUP BY customers.country
HAVING sales != '0'
ORDER BY sales DESC
