如何设计一个可扩展的MySQL表结构来实现在线教育功能？

CREATE TABLE `user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `username` varchar(50) NOT NULL,
  `password` varchar(50) NOT NULL,
  `email` varchar(50) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `username` (`username`),
  UNIQUE KEY `email` (`email`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `course` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(100) NOT NULL,
  `description` text NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `video` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(100) NOT NULL,
  `url` varchar(255) NOT NULL,
  `course_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `course_id` (`course_id`),
  CONSTRAINT `fk_video_course` FOREIGN KEY (`course_id`) REFERENCES `course` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `comment` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `content` text NOT NULL,
  `create_time` datetime NOT NULL,
  `user_id` int(11) NOT NULL,
  `course_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `user_id` (`user_id`),
  KEY `course_id` (`course_id`),
  CONSTRAINT `fk_comment_user` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `fk_comment_course` FOREIGN KEY (`course_id`) REFERENCES `course` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `order` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `amount` decimal(10,2) NOT NULL,
  `create_time` datetime NOT NULL,
  `user_id` int(11) NOT NULL,
  `course_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `user_id` (`user_id`),
  KEY `course_id` (`course_id`),
  CONSTRAINT `fk_order_user` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `fk_order_course` FOREIGN KEY (`course_id`) REFERENCES `course` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;