Normalised database - ONE to MANY - Search through all joined data sets

问题

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       |

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'

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

回答1:

Here's one way to do it:

    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

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

回答2:

Assuming the data is normalized and character as in your sample:

SELECT
   song.song_id, 
   song.title
FROM 
   song
      INNER JOIN song_genre ON song_genre.song_id = song.song_id
      INNER JOIN genre ON genre.genre_id= song_genre.genre_id
WHERE 
   genre.genre_id in ('1', '2')

Modified according to your comment:

SELECT
   song.song_id, 
   song.title
FROM 
   song
      INNER JOIN song_genre ON song_genre.song_id = song.song_id
      INNER JOIN genre g1 ON g1.genre_id = song_genre.genre_id
      INNER JOIN genre g2 ON g2.genre_id = song_genre.genre_id
WHERE 
   g1.genre_id = '1' and
   g2.genre_id = '2'

回答3:

                WHERE genre.genre_id ='1'
                AND genre.genre_id='2'

is your query right?

if, then this means it have to be true in both cases but you have per row only one genre_id! perhaps chain this with 'OR' or... genre.genre_id IN (1,2)

来源：https://stackoverflow.com/questions/9592038/normalised-database-one-to-many-search-through-all-joined-data-sets

标签

mysql

sql

relational-database

normalization