text: DROP TABLE photos;
  
  CREATE TABLE photos (
    id SERIAL PRIMARY KEY,
    url VARCHAR(200),
    user_id INTEGER REFERENCES users(id) ON DELETE SET NULL
  );
  
  INSERT INTO photos (url, user_id)
  VALUES
    ('http:/one.jpg', 4),
    ('http:/754.jpg', 2),
    ('http:/35.jpg', 3),
    ('http:/256.jpg', 4);
  
  DELETE FROM users
  WHERE id = 4;
  
  SELECT * FROM photos;