自然连接即可实现
select
        c.name,
        count(distinct a.film_id)
from
        film a,
        film_category b,
        category c,
        film_category d  --为了求出改种类数量，再加一个film_category ，进行group by 即可
where
        a.description like "%robot%"
        and a.film_id=b.film_id
        and b.category_id=c.category_id
        and c.category_id=d.category_id
group by c.name
having count(d.film_id)>=5;