オンライン注文機能を実装するために保守可能な MySQL テーブル構造を設計するにはどうすればよいですか?

CREATE TABLE `User` (
  `user_id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  `username` VARCHAR(50) NOT NULL,
  `password` VARCHAR(50) NOT NULL,
  `phone` VARCHAR(20) NOT NULL,
  PRIMARY KEY (`user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `Merchant` (
  `merchant_id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  `merchant_name` VARCHAR(100) NOT NULL,
  `address` VARCHAR(200) NOT NULL,
  `phone` VARCHAR(20) NOT NULL,
  PRIMARY KEY (`merchant_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `Category` (
  `category_id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  `category_name` VARCHAR(50) NOT NULL,
  PRIMARY KEY (`category_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `Dish` (
  `dish_id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  `dish_name` VARCHAR(100) NOT NULL,
  `price` DECIMAL(10, 2) NOT NULL,
  `category_id` INT(10) UNSIGNED NULL,
  PRIMARY KEY (`dish_id`),
  CONSTRAINT `FK_Category` FOREIGN KEY (`category_id`) REFERENCES `Category` (`category_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `Order` (
  `order_id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  `user_id` INT(10) UNSIGNED NOT NULL,
  `merchant_id` INT(10) UNSIGNED NOT NULL,
  `order_time` DATETIME NOT NULL,
  `status` ENUM ('待付款', '已付款', '已取消') NOT NULL,
  PRIMARY KEY (`order_id`),
  CONSTRAINT `FK_User` FOREIGN KEY (`user_id`) REFERENCES `User` (`user_id`),
  CONSTRAINT `FK_Merchant` FOREIGN KEY (`merchant_id`) REFERENCES `Merchant` (`merchant_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;