text: ALTER TABLE singer ADD PRIMARY KEY (singer_id);
  ALTER TABLE song ADD PRIMARY KEY (song_id);
  
  CREATE TABLE singer_to_song (
    singer_id INTEGER REFERENCES singer(singer_id),
    song_id INTEGER REFERENCES song(song_id)
  );
  
  INSERT INTO singer_to_song (singer_id, song_id)
  SELECT singer_id, song_id FROM song;
  
  ALTER TABLE song DROP COLUMN singer_id;
  