CREATE TABLE students (
  id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
  username VARCHAR(64) NOT NULL,
  password VARCHAR(64),
  email VARCHAR(...)
  UNIQUE KEY (username)
);

CREATE TABLE courses (
  id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
  name VARCHAR(64) NOT NULL
);

CREATE TABLE enrollments (
  id INT NOT NULL PRIMARY KEY,
  student_id INT NOT NULL,
  course_id INT NOT NULL,
  UNIQUE KEY (student_id, course_id),
  FOREIGN KEY (student_id) REFERENCES students (id),
  FOREIGN KEY (course_id) REFERENCES courses (id)
);