How to separate the maximum count for each genre of something - mysql

Question

I want to find for each
genre of movie, find the N actors who have played in most movies
of the genre
I have done this:
select genre.genre_name,actor.actor_id,count(genre.genre_name) from genre
inner join movie_has_genre on movie_has_genre.genre_id=genre.genre_id
inner join movie on movie_has_genre.movie_id=movie.movie_id
inner join role on movie.movie_id=role.movie_id
inner join actor on actor.actor_id=role.actor_id
group by genre.genre_name,actor.actor_id;
which gives as a result for each genre how many movies of that genre every actor has played and now i want to find for each genre the actor that has played the most moviesof that genre.
Tables and their columns:
actor(actor_id,name)
role(actor_id,movie_id)
movie(movie_id,title)
movie_has_genre(movie_id,genre_id)
genre(genre_id,genre_name)
Also the result should be something like this:
Action 22591 7
Horror 25863 3
Horror 24867 3
Comedy 23476 2
Drama 14536 1
Drama 19634 1
Drama 17563 1

Answer 1

Man, what I'd do is the next (supposing your code is working well):
-- Notice this is your code with some aliases, nothing else.
-- Just for making mi job easier.
create view frequency as
select genre.genre_name as genre_ name,
actor.actor_id as actor_id,
count(genre.genre_name) as freq
from genre
inner join movie_has_genre on movie_has_genre.genre_id=genre.genre_id
inner join movie on movie_has_genre.movie_id=movie.movie_id
inner join role on movie.movie_id=role.movie_id
inner join actor on actor.actor_id=role.actor_id
group by genre.genre_name,actor.actor_id;
-- And this is my proposal
-- Take the max frequency per each category
-- and find the guy who possesses it (maybe 2 or more...)
select genre.genre_name,actor.actor_id
from frequency as tbl1 inner join
(
-- The max frequency in a genre.
select f.genre_name,
max(f.freq) as max_freq
from frequency f
group by(genre_name)
) as tbl2 on (tbl1.genre_name = tbl2.genre_name)
where tbl1.freq = tbl2.max_freq;
And well, there's one problem: It may return more than one actor per category, if there's a tie. But how can I know who is the winner? I let it for you. Maybe it's wrong, I don't think so, but we're both learning! Hope I'd help you.

Answer 2

You need to use the MAX() function. Some SQL implementations (such as Oracle) allow you to do this: SELECT MAX(COUNT(whatever)) but MySQL isn't one of them.
One way to do what you want is this:
select genre_name, actor_id, max(genrecount)
from (
select genre.genre_name, actor.actor_id, count(genre.genre_name) as genrecount
from genre
inner join movie_has_genre on movie_has_genre.genre_id=genre.genre_id
inner join movie on movie_has_genre.movie_id=movie.movie_id
inner join role on movie.movie_id=role.movie_id
group by genre.genre_name,actor.actor_id
) as topactor
This does the outer SELECT on the table derived from the inner SELECT.