Опиши проектирование БД для системы расписания (сущности: уроки, записи, связи, обработка замен занятий)

CREATE TABLE lessons (
  id SERIAL PRIMARY KEY,
  subject_id INT REFERENCES subjects(id),
  teacher_id INT REFERENCES teachers(id),
  room_id INT REFERENCES rooms(id),
  start_time TIMESTAMP,
  duration INTERVAL
);

CREATE TABLE bookings (
  id SERIAL PRIMARY KEY,
  lesson_id INT REFERENCES lessons(id),
  student_id INT REFERENCES students(id),
  status VARCHAR(20) -- booked/cancelled
);

CREATE TABLE lesson_changes (
  id SERIAL PRIMARY KEY,
  original_lesson_id INT REFERENCES lessons(id),
  new_teacher_id INT NULL,
  new_room_id INT NULL,
  change_date DATE
);