List all the titles created by user who was a subsriber when he created the list and have less than 50 movies in the list.
have less than 50 movies in the list refers to list_movie_number <50; was a subscriber refers to user_subscriber = 1
SELECT DISTINCT T2.list_title FROM lists_users AS T1 INNER JOIN lists AS T2 ON T1.list_id = T2.list_id WHERE T2.list_movie_number < 50 AND T1.user_subscriber = 1