text: SELECT AvgBefore1980-AvgAfter1980
  FROM 
  (
      SELECT avg(AvgsStars) AS AvgBefore1980
      FROM (
          SELECT avg(stars) AS AvgsStars
          FROM Rating, Movie
          WHERE Rating.mID = Movie.mID AND Movie.year < 1980
          GROUP BY Rating.mID
      )
  ),
  (
      SELECT avg(AvgsStars) AS AvgAfter1980
      FROM (
          SELECT avg(stars) AS AvgsStars
          FROM Rating, Movie
          WHERE Rating.mID = Movie.mID AND Movie.year > 1980
          GROUP BY Rating.mID
      )
  );