Untitled

SELECT t.name,
       SUM(i.quantity),
       pt.playlist_id,
       pl.name AS playlist-- укажите необходимое поле
FROM track AS t
INNER JOIN invoice_line AS i ON t.track_id=i.track_id
INNER JOIN playlist_track AS pt ON t.track_id=pt.track_id
INNER JOIN playlist AS pl ON pt.playlist_id = pl.playlist_id
GROUP BY t.name, pt.playlist_id, pl.playlist_id 
LIMIT 20-- добавьте новое поле для группировки
-- добавьте ещё одно присоединение