text: -- How would Spotify pay it's artists?
  
  Select 
      sum(duration), 
      artist, 
      (select sum(duration) from tracks) (1.7%)
  from tracks
  group by artist;
  
  -- What is the problem with this?
  -- It's just about volume of listeners
  
  
  select 
      sum(select )
  
  -- musicbrainz 
  
  -- We were able to find the releases of bad by Michael Jackson (artis_credit='519')
  -- use just a select, then try a sub select, then try a join. 
  -- avoid using joins
  
  select (select sum(length) where artist_credit='519') mike/
  sum(length) 
  from tracks;
  
  
  