Discriminator value sound in the music_data table.

The four unbounded sequence sublements are collectively stored as records in the midi table. Each midi record has a foreign key sound_id to the music_data table.

Column play.play_id contains the id attribute of the play element.

The subelements of the play element are stored in the single-inheritance table play_type whose discriminator values are ipa, mute, other play, and semi pitched.