SELECT
c.customer_id,
c.first_name,
c.last_name,
CEIL(AVG(TIMESTAMPDIFF(HOUR, r.rental_date, r.return_date))) AS average_rental_time
FROM
customer c
JOIN
rental r ON c.customer_id = r.customer_id
WHERE
r.return_date IS NOT NULL
GROUP BY
c.customer_id, c.first_name, c.last_name
ORDER BY
c.customer_id;