-- Modified so the inventory_id is nullable.
CREATE TABLE `rental` (
  `rental_id` int(11) NOT NULL auto_increment,
  `rental_date` datetime NOT NULL,
  `inventory_id` mediumint(8) unsigned default NULL,
  `customer_id` smallint(5) unsigned NOT NULL,
  `return_date` datetime default NULL,
  `staff_id` tinyint(3) unsigned NOT NULL,
  `last_update` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,
  PRIMARY KEY  (`rental_id`),
  UNIQUE KEY `rental_date` (`rental_date`,`inventory_id`,`customer_id`),
  KEY `idx_fk_inventory_id` (`inventory_id`),
  KEY `idx_fk_customer_id` (`customer_id`),
  KEY `idx_fk_staff_id` (`staff_id`),
  CONSTRAINT `fk_rental_customer` FOREIGN KEY (`customer_id`) REFERENCES `customer` (`customer_id`) ON UPDATE CASCADE,
  CONSTRAINT `fk_rental_inventory` FOREIGN KEY (`inventory_id`) REFERENCES `inventory` (`inventory_id`) ON UPDATE CASCADE,
  CONSTRAINT `fk_rental_staff` FOREIGN KEY (`staff_id`) REFERENCES `staff` (`staff_id`) ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8