What is the name of the client who has the largest quantity of rented material without returning it?
name refers to first_name, last_name; without returning a rented material refers to return_date is null
SELECT T.first_name FROM ( SELECT T2.first_name, COUNT(T1.rental_date) AS num FROM rental AS T1 INNER JOIN customer AS T2 ON T1.customer_id = T2.customer_id GROUP BY T2.first_name ) AS T ORDER BY T.num DESC LIMIT 1