Among all movies in the list, calculate the percentage of movies that were never been rated?
percentage of movies that were never been rated refers to DIVIDE(COUNT(main_movies.movie_id ! = main_ratings.movie_id),COUNT(movie_id))
SELECT CAST(SUM(CASE WHEN T2.movie_id IS NULL THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(T2.movie_id) FROM ratings AS T1 INNER JOIN movies AS T2 ON T1.movie_id = T2.movie_id