Question: Among all movies in the list, calculate the percentage of movies that were never been rated?
SQL: 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