オンライン試験システムの試験用紙の生成と管理をサポートする MySQL テーブル構造をどのように設計するか?

CREATE TABLE `users` (
  `id` INT(10) NOT NULL AUTO_INCREMENT,
  `username` VARCHAR(50) NOT NULL,
  `password` VARCHAR(50) NOT NULL,
  `role` ENUM('admin', 'teacher', 'student') NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `username_UNIQUE` (`username`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE `papers` (
  `id` INT(10) NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(100) NOT NULL,
  `total_score` FLOAT NOT NULL,
  `creator_id` INT(10) NOT NULL,
  PRIMARY KEY (`id`),
  FOREIGN KEY (`creator_id`) REFERENCES `users` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE `questions` (
  `id` INT(10) NOT NULL AUTO_INCREMENT,
  `paper_id` INT(10) NOT NULL,
  `type` ENUM('single_choice', 'multiple_choice', 'true_false', 'short_answer') NOT NULL,
  `content` TEXT NOT NULL,
  `options` TEXT,
  `answer` TEXT NOT NULL,
  `score` FLOAT NOT NULL,
  PRIMARY KEY (`id`),
  FOREIGN KEY (`paper_id`) REFERENCES `papers` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE `exams` (
  `id` INT(10) NOT NULL AUTO_INCREMENT,
  `paper_id` INT(10) NOT NULL,
  `user_id` INT(10) NOT NULL,
  `start_time` DATETIME NOT NULL,
  `end_time` DATETIME,
  PRIMARY KEY (`id`),
  FOREIGN KEY (`paper_id`) REFERENCES `papers` (`id`),
  FOREIGN KEY (`user_id`) REFERENCES `users` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE `answers` (
  `id` INT(10) NOT NULL AUTO_INCREMENT,
  `exam_id` INT(10) NOT NULL,
  `question_id` INT(10) NOT NULL,
  `user_id` INT(10) NOT NULL,
  `answer` TEXT NOT NULL,
  PRIMARY KEY (`id`),
  FOREIGN KEY (`exam_id`) REFERENCES `exams` (`id`),
  FOREIGN KEY (`question_id`) REFERENCES `questions` (`id`),
  FOREIGN KEY (`user_id`) REFERENCES `users` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;