• In album: artist_id ... references artist is a foreign key.

  • Every album.artist_id value must match artist_id in some row of artist. (artist_id is the PK of artist.)

  • Similarly, track.album_id is a foreign key to album.

  • track.album_id plays two roles:
    1. Part of the PK of track.
    2. FK to album.

  • This implies that track represents a weak entity: A track cannot exist independently of an album.