Question

I have 3 tables. For the purposes of this example I will simplify it as much as I can.

First table contains movie ids and names, second table contains genre ids and genre names (action, drama, etc). 3rd table stores the genre id associated with each movie, since some movies have more than one. It has 2 columns, genre_id and movie_id. Fairy simple stuff. Im trying to output the list of movies, along with a list of genres associated with each movie.

    SELECT *         FROM movies         LEFT JOIN gen_pairs         ON movies.mov_id = gen_pairs.gen_movieid         LEFT JOIN categories         ON gen_pairs.gen_catid = categories.cat_id      GROUP BY mov_id  

This will obviously output a single genre of each film, even if it has multiple ones in the gen_pairs table. How would I get it to display a comma separated list of genres for each movie, without running a sub-query for each item?

Answer 1

Your select should build the Cartesian product, so you’ll get output like

MovieA   GenreA MovieA   GenreB MovieB   GenreA ... 

But it sounds like you want this instead:

MovieA   GenreA, GenreB MovieB   GenreA ... 

MySQL has a GROUP_CONCAT function to do what you want:

SELECT m.mov_id, GROUP_CONCAT(c.name)        FROM movies m   LEFT JOIN gen_pairs gp ON (m.mov_id = gp.gen_movieid)   LEFT JOIN categories c ON (gp.gen_catid = c.cat_id)   GROUP BY m.mov_id 

Note the GROUP_CONCAT and GROUP BY.