Хоба, ловите всратый sql запрос. Ваши ответы срите в комменты
SELECT u.name AS "покупатель", SUM(p.price) AS "итоговая сумма" FROM orders AS o
INNER JOIN users as u ON o.user_id = u.id
INNER JOIN products as p ON o.product_id = p.id
GROUP BY u.name
HAVING COUNT(o.quantity) > 5;