一个看似通用的SQL查询确实让我毫无头绪。就是这种情况。我有3个通用表(此处为简化表):
Movie
id | title
-----------------------
1 | Evil Dead
-----------------------
2 | Bohemian Rhapsody
....
Genre
id | title
-----------------------
1 | Horror
-----------------------
2 | Comedy
....
Rating
id | title
-----------------------
1 | PG-13
-----------------------
2 | R
....
和2个多对多表将它们连接:
Movie_Genre
movie_id | genre_id
Movie_Rating
movie_id | rating_id
[最初的挑战是编写一个查询,使我能够提取属于多种流派的电影(例如恐怖喜剧或科幻动作)。
非常感谢,我能够在这里找到此解决方案MySQL: Select records where joined table matches ALL values
但是,获取属于多个表的记录的正确选择是什么?例如。评分为R的恐怖喜剧。没有子查询(或仅一个子查询),有没有办法做到这一点?
一种方法使用相关子查询:
select m.*
from movies m
where (select count(*)
from movie_genre mg
where mg.movie_id = m.id
) > 1 and
(select count(*)
from movie_rating mr
where mr.movie_id = m.id
) > 1 ;
[在movie_genre(movie_id)
和movie_rating(movie_id)
上具有索引,这可能具有相当合理的性能。
以上可能是最有效的方法。但是,如果要避免子查询,一种方法是:
select mg.movie_id
from movie_genres mg join
movie_ratings mr
on mg.movie_id = mr.movie_id
group by mg.movie_id
having count(distinct mg.genre_id) > 0 and
count(distinct mr.genre_id) > 0;
比join
之前的聚合效率更高:
select mg.movie_id
from (select movie_id
from mg_genres
group by movie_id
having count(*) >= 2
) mg join
(select movie_id
from mg_ratings
group by movie_id
having count(*) >= 2
) mr
on mg.movie_id = mr.movie_id;
尽管您声明要避免子查询,但具有讽刺意味的是,没有子查询的版本可能在这三个选项中性能最差。
例如评分最高的R恐怖喜剧
您可以将join
所有表放在一起,按电影汇总并使用HAVING
子句进行过滤:
select m.id, m.title
from movies m
inner join movie_genre mg on mg.movid_id = m.id
inner join genre g on g.id = mg.genre_id
inner join movie_rating mr on mr.movie_id = m.id
inner join rating r on r.id = mr.rating_id
group by m.id, m.title
having
max(r.title = 'R') = 1
and max(g.title = 'Horror') = 1
and max(g.title = 'Comedy') = 1
您还可以使用几个exists
条件以及相关的子查询:
select m.*
from movie m
where
exists (
select 1
from movie_genre mg
inner join genre g on g.id = mg.genre_id
where mg.movie_id = m.id and g.title = 'R')
and exists (
select 1
from movie_rating mr
inner join rating r on r.id = mr.rating_id
where mr.movie_id = m.id and r.title = 'Horror'
)
and exists (
select 1
from movie_rating mr
inner join rating r on r.id = mr.rating_id
where mr.movie_id = m.id and r.title = 'Comedy'
)