CREATE OR REPLACE FUNCTION public.inventory_held_by_customer(p_inventory_id integer)
 RETURNS integer
 LANGUAGE plpgsql
AS $function$
DECLARE
    v_customer_id INTEGER;
BEGIN

  SELECT customer_id INTO v_customer_id
  FROM rental
  WHERE return_date IS NULL
  AND inventory_id = p_inventory_id;

  RETURN v_customer_id;
END $function$