Consider the following database table design: CREATE TABLE `notifications` ( `id` INT NOT NULL AUTO_INCREMENT, `type` INT(8) NOT NULL, `notifiable_id` INT unsigned NOT NULL, `notifiable_type` VARCHAR(10) NOT NULL, `relation_id_1` INT unsigned, `relation_type_1` VARCHAR(10), `relation_id_2` INT unsigned, `relation_type_2` VARCHAR(10), `updated_at` TIMESTAMP NOT NULL, `created_at` TIMESTAMP NOT NULL, PRIMARY KEY (`id`) ); What is wrong with the above and how could it be improved?