正确连接多个多对多表-MySQL查询

问题描述 投票:2回答:2

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?

mysql sql
2个回答
2
投票

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)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;

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。


2
投票

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'
    )
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