Question

I’ve begun working on my first MySQL database, and I’ve run into a simple problem.

I have my movies categorized by genre in a ‘many-to-many’ relationship:

'movies' Table
+---------+-------------------+
|movie_id |movie_title        |
+---------+-------------------+
|1        |Indiana Jones      |
|2        |Shaun of the Dead  |
+---------+-------------------+

'genres' table
+---------+-----------+
|genre_id |genre_name |
+---------+-----------+
|1        |adventure  |
|2        |comedy     |
|3        |horror     |
+---------+-----------+

'movie_genres' table
+---------+---------+
|movie_id |genre_id |
+---------+---------+
|1        |1        |
|2        |2        |
|2        |3        |
+---------+---------+

What I am trying to do is search my movies table by genre, and display all the genres each movie is in, for instance:

Searching 'horror'

+------------------+---------------+
|movie_title       |genre_names    |
+------------------+---------------+
|Shaun of the Dead |comedy, horror |
+------------------+---------------+

Here is a query I would use to search for horror movies

    SELECT m.movie_title, GROUP_CONCAT(g.genre_name SEPARATOR ', ') as genre_names
    FROM movies m
    LEFT JOIN genre_movies gm ON gm.movie_id = m.movie_id
    LEFT JOIN genres g ON g.genre_id = gm.genre_id
    GROUP BY m.movie_id
    HAVING genre_names LIKE '%horror%'

The problem I have is that this query retrieves the entirety of all the tables before filtering it, which I gather is very inefficient. Is there a better way I could search by one genre while showing all associated genres?

Answer 1

Try this:

SELECT m.movie_title, A.genre_names
FROM movies m
INNER JOIN (SELECT gm.movie_id, GROUP_CONCAT(g.genre_name SEPARATOR ', ') AS genre_names
    FROM genre_movies gm INNER JOIN genres g ON g.genre_id = gm.genre_id
    GROUP BY gm.movie_id) AS A ON m.movie_id = A.movie_id 
WHERE genre_names LIKE '%horror%'