多对多查询，MySQL

Question

I have 3 tables like this:

movies:

• movie_id (int)
• movie_title (varchar)
• movie_runtime (int)
• movie_releaseYear (int)
• ...

genres:

• genre_id (int)
• genre_name (varchar)

movieGenres:

• movieGenre_id (int)
• movieGenre_movieId (int)
• movieGenre_genreId (int)

A movie can include many genres.
I want to grab the movie title, runtime and release year together with its genres. How can I do that in a single query?

I tried to do it like this:

SELECT u.movie_title, u.movie_runtime, u.movie_releaseYear, a.genre_name 
FROM movieGenres ua 
LEFT JOIN movies u ON u.movie_id = ua.movieGenre_filmId 
LEFT JOIN genres a ON a.genre_id = ua.movieGenre_genreId
LIMIT 0,10

This return something like:

Robin Hood _ 101 _ 2001 _ Adventure
Robin Hood _ 101 _ 2001 _ Action
Robin Hood _ 101 _ 2001 _ Drama
Firewall _ 98 _ 2003 _ Action
Firewall _ 98 _ 2003 _ Drama

I don't want to repeat the information I already know. I want it to display like this:

Robin Hood _ 101 _ 2001 _ Adventure _ Action _ Drama
Firewall _ 98 _ 2003 _ Action _ Drama

Please help me figure this out:)

Answer 1

I had this problem a couple of weeks ago, and my professor gave me a pretty clever solution:

Use the Group_Concat function in a sub query to get the results as comma separated values, then use explode/split/whatever language equivalent to loop through the CSV results.

http://mahmudahsan.wordpress.com/2008/08/27/mysql-the-group_concat-function/

Hope this helps! This problem had me frustrated for awhile, but I think this is the simplest solution.

Answer 2

SELECT
    u.movie_title,
    u.movie_runtime,
    u.movie_releaseYear,
    GROUP_CONCAT(a.genre_name) genre
FROM
    movieGenres ua  
    INNER JOIN movies u ON u.movie_id = ua.movieGenre_filmId  
    INNER JOIN genres a ON a.genre_id = ua.movieGenre_genreId
GROUP BY
    u.movie_title,
    u.movie_runtime,
    u.movie_releaseYear
LIMIT 0,10 
;

This is essentially your query. I just added the GROUP BY clause and GROUP_CONCAT function. The default delimiter is a comma(,). If you want the list of genres space-separated, do this with the GROUP_CONCAT function:

SELECT
    u.movie_title,
    u.movie_runtime,
    u.movie_releaseYear,
    GROUP_CONCAT(a.genre_name SEPARATOR ' ') genre
FROM
    movieGenres ua  
    INNER JOIN movies u ON u.movie_id = ua.movieGenre_filmId  
    INNER JOIN genres a ON a.genre_id = ua.movieGenre_genreId
GROUP BY
    u.movie_title,
    u.movie_runtime,
    u.movie_releaseYear
LIMIT 0,10
;

Answer 3

This is the way databases work. There 'are' ways to do what you're wanting but honestly the database is usually not the right place to accomplish it. Is this purely just an ad-hoc query or are these results going to an application? The application would be the right place to format the results the way you want them if so.