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