例如（示意图）：

CREATE TABLE users ( id PRIMARY KEY,
                     name,
                     email,
                     password,
                     role_id FOREIGN KEY roles(id),
                     school_id FOREIGN KEY schools(id),
                     type ENUM('Teacher', 'Student'),
                     teacher_id FOREIGN KEY users(id), -- NULL for teachers
                     CHECK (CASE WHEN type = 'Teacher'
                                 THEN teacher_id IS NULL
                                 ELSE teacher_id IS NOT NULL
                                 END)
                   );

不需要老师和学生的table。