第一种方法
SELECT f.film_id, f.title FROM film as f WHERE f.film_id not IN( select f1.film_id FROM film_category as fc, film as f1 where fc.film_id = f1.film_id )
第二种方法
SELECT f.film_id, f.title FROM film as f LEFT JOIN film_category as fc ON f.film_id=fc.film_id WHERE fc.category_id is NULL