1.首先筛选出查找描述信息中包含robot的电影--建立新film表
2.筛选出电影分类包含电影总数量>=5的电影分类--建立film_category表
3.使用内连接，连接三张表格+group by 统计出 电影分类和对应电影数量
学习到：having条件中的聚合函数，select 语句可以不包含
select category_id from film_category group by category_id
having count(film_id)>=5

select ct.name,count(film.film_id) from
(select * from film where description like '%robot%') film
inner join (select * from film_category where category_id in(
select category_id from film_category group by category_id
having count(film_id)>=5)) fc on film.film_id =fc.film_id
inner join category ct on fc.category_id=ct.category_id
group by ct.name