SELECT
AVG(DATEDIFF(next_rental_date, return_date)) AS avg_days_between_rentals
FROM (
SELECT
inventory_id,
return_date,
LEAD(rental_date) OVER (PARTITION BY inventory_id ORDER BY rental_date) AS next_rental_date
FROM rental
WHERE return_date IS NOT NULL
) AS sub
WHERE next_rental_date IS NOT NULL;