- 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:
- Part of the PK of track.
- FK to album.
- This implies that track represents a weak
entity: A track cannot exist independently of
an album.