规范化的数据库-一对多-搜索所有联接的数据集 [英] Normalised database - ONE to MANY - Search through all joined data sets

问题描述

我正在尝试构造一个MySQL查询，该查询将允许我在数据库中同时具有Pop和Electronic类型的曲目中提取歌曲的歌曲标题.

I am trying to construct a MySQL query that will allow me to pull the song title of a track in the database that has a genre of both Pop and Electronic.

+----------------------------------------------------+
TABLE: SONG
+----------------------------------------------------+
song_id  | title       |  
1        | song name   | 


+----------------------------------------------------+
TABLE: GENRE
+----------------------------------------------------+
genre_id   | name       | 
1          | Pop        |    
2          | Electronic |   


+----------------------------------------------------+
TABLE: SONG_GENRE
+----------------------------------------------------+
genre_id  | song_id | 
1         | 1       | 
2         | 1       | 

此SQL无效，因为它显然永远不会返回genre_id 1和2，但这就是我遇到的问题.

This SQL doesn't work, as it is obviously never going to return both a genre_id of 1 and 2 but this is where I am stuck.

SELECT DISTINCT song.song_id, song.title
                    FROM song
                    LEFT JOIN song_genre ON song_genre.song_id = song.song_id
                    LEFT JOIN genre ON genre.genre_id = song_genre.genre_id
                    WHERE genre.genre_id ='1'
                    AND genre.genre_id='2'

如果有人能指出我正确的方向，我将非常感激！

If somebody could point me in the right direction I would be most greatful!

推荐答案

这里是一种方法:

    SELECT DISTINCT song.song_id, song.title
    FROM song
    INNER JOIN (SELECT songid FROM song_genre WHERE song_genre.genre_id ='1') genre1 
         ON genre1.song_id = song.song_id
    INNER JOIN (SELECT songid FROM song_genre WHERE song_genre.genre_id ='2') genre2 
         ON genre2.song_id = song.song_id

另一种可能更有效的方法.假设song_genre中没有dups. COUNT(*)= X，其中X等于列出的类型.

Another way that might be more efficient. This assumes there is no dups in song_genre. COUNT(*) = X where X equals the number of genres listed.

SELECT DISTINCT song.song_id, song.title
FROM song
INNER JOIN (SELECT songid, COUNT(*) FROM song_genre 
WHERE genre_id IN ('1','2') 
GROUP BY songid HAVING COUNT(*) = 2) genre1 ON genre1.song_id = song.song_id

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