左连接，取category_id为null的记录
使用join查询方式找出没有分类的电影id以及名称
http://www.nowcoder.com/questionTerminal/a158fa6e79274ac497832697b4b83658
select f1.film_id,f1.title
from film f1
left join(
select fc.film_id,fc.category_id
from film_category as fc
join category as c
on fc.category_id=c.category_id
) f2
on f1.film_id=f2.film_id
where category_id is null
