- Track is dependent on Album. Can't exist without it (in this data model).
- Adopting album PK as part of Track PK reflects this.
- Could have invented track id as column, made it PK, but still need the album_id foreign key.
- Album is dependent on Artist, so it should be possible to get
rid of album_id. I.e., instead of identifying Album by album_id use (artist_id, title).
(Assuming artists don't ever reuse album titles.)