How to limit the max counts - mysql

Question

I want to find for each genre of movie, find the N actors who have played in most movies of the 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)
With this query I can find the actors who played on the most movies of the same genre.
select t1.genre_name, t1.actor_id, t1.max_value
from
(
select g.genre_name, a.actor_id, count(*) as max_value
from genre g
inner join movie_has_genre mhg on mhg.genre_id = g.genre_id
inner join movie m on mhg.movie_id = m.movie_id
inner join role r on m.movie_id = r.movie_id
inner join actor a on a.actor_id = r.actor_id
group by g.genre_name, a.actor_id
) t1
inner join
(
select genre_name, MAX(max_value) AS max_value
from
(
select g.genre_name, a.actor_id, count(*) as max_value
from genre g
inner join movie_has_genre mhg on mhg.genre_id = g.genre_id
inner join movie m on mhg.movie_id = m.movie_id
inner join role r on m.movie_id = r.movie_id
inner join actor a on a.actor_id = r.actor_id
group by g.genre_name, a.actor_id
) t
GROUP BY genre_name
) t2
ON t1.genre_name = t2.genre_name and t1.max_value = t2.max_value
ORDER BY
t1.max_value desc;
But I want to limit the number of the actors to 1.So how can I do that?
Example:
Results I get:
genre_name | actor_id | max_value
==================================
Thriller | 22591 | 7
Drama | 22591 | 6
Crime | 65536 | 3
Horror | 22591 | 3
Action | 292028 | 3
Action | 378578 | 3
Action | 388698 | 3
Results I want:
genre_name | actor_id | max_value
==================================
Thriller | 22591 | 7
Drama | 22591 | 6
Crime | 65536 | 3
Horror | 22591 | 3
Action | 292028 | 3

Answer 1

If you want just one actor selected randomly just add the following line to your code:
select genre_name, actor_id, max_value
from
(
select g.genre_name, a.actor_id, count(*) as max_value
from genre g
inner join movie_has_genre mhg on mhg.genre_id = g.genre_id
inner join movie m on mhg.movie_id = m.movie_id
inner join role r on m.movie_id = r.movie_id
inner join actor a on a.actor_id = r.actor_id
group by g.genre_name, a.actor_id
) t1
inner join
(
select genre_name, MAX(max_value) AS max_value
from
(
select g.genre_name, a.actor_id, count(*) as max_value
from genre g
inner join movie_has_genre mhg on mhg.genre_id = g.genre_id
inner join movie m on mhg.movie_id = m.movie_id
inner join role r on m.movie_id = r.movie_id
inner join actor a on a.actor_id = r.actor_id
group by g.genre_name, a.actor_id
) t
GROUP BY genre_name
) t2
USING(genre_name,max_value)
GROUP BY genre_name, max_value
ORDER BY max_value desc;

Answer 2

Some of the joins you used are redundant.
SELECT
U.genre_name, U.actor_id, U.actor_genre_count
FROM
(SELECT
A.genre_id, A.genre_name, C.actor_id, count(*) actor_genre_count
FROM genre A
JOIN movie_has_genre B
ON A.genre_id=B.genre_id
JOIN role C
ON C.movie_id=B.movie_id
GROUP BY A.genre_id, A.genre_name, C.actor_id) U
JOIN
(SELECT
S.genre_id, S.genre_name, MAX(S.actor_genre_count) max_actor_genre
FROM
(SELECT
A.genre_id, A.genre_name, C.actor_id, count(*) actor_genre_count
FROM genre A
JOIN movie_has_genre B
ON A.genre_id=B.genre_id
JOIN role C
ON C.movie_id=B.movie_id
GROUP BY A.genre_id, A.genre_name, C.actor_id) S
GROUP BY S.genre_id, S.genre_name) V
ON U.genre_name=V.genre_name AND U.actor_genre_count=V.max_actor_genre;

Answer 3

This solution is adapted from this Stack Overflow answer about limiting results by name. I attempted to do a similar query that should choose the first actor_id and only return it.
SELECT id, CategoryName, image, date_listed, item_id
SELECT t1.genre_name, t1.actor_id, t1.actor_movie_count
FROM
(
SELECT g.genre_name, r.actor_id, COUNT(*) as actor_movie_count
FROM genre g
INNER JOIN movie_has_genre mhg ON mhg.genre_id = g.genre_id
INNER JOIN role r ON m.movie_id = r.movie_id
GROUP BY g.genre_name, r.actor_id
) t1
LEFT JOIN
(
SELECT genre_name, actor_id, MAX(actor_movie_count) AS max_actor_movie_count
FROM
(
SELECT g.genre_name, r.actor_id, COUNT(*) AS actor_movie_count
FROM genre g
INNER JOIN movie_has_genre mhg ON mhg.genre_id = g.genre_id
INNER JOIN role r ON m.movie_id = r.movie_id
GROUP BY g.genre_name, r.actor_id
)
GROUP BY genre_name
) t2
ON t1.genre_name = t2.genre_name AND t1.actor_movie_count = t2.max_actor_movie_count AND (t1.actor_id > t2.actor_id)
WHERE t2.genre_id IS NULL
ORDER BY t1.actor_movie_count DESC
If this still doesn't solve your problem, other similar questions with explanations are described below:
SO answer about returning 1 row per group
SO question about limiting query answer to N results per group
SO question about selecting N items per category
External Article: Finding the max/first of a particular group in SQL

Answer 4

You can use a correlated LIMIT 1 subquery to get the id of the actor who played that genre most.
select g.genre_name, (
select r.actor_id
from movie_has_genre mg
join role r on r.movie_id = mg.movie_id
where mg.genre_id = g.genre_id
group by r.actor_id
order by count(*) desc,
r.actor_id asc -- on tie least actor_id wins
) as actor_id
from genre g
The result would be like:
genre_name | actor_id
======================
Thriller | 22591
Drama | 22591
Crime | 65536
Horror | 22591
Action | 292028
As you see, the count is not included. If you need the count, the simple way would be to return it in the same string column with actor_id
Change the SELECT clause in the subquery to
select concat(r.actor_id, ':', count(*)) as actor_id_count
This wil return the actor_id and the count in a single string column like
genre_name | actor_id_count
===========================
Thriller | 22591:7
You can then parse it (with split, explode or what ever) in your application code.
A solution with CTE (Common Table Expression) and ROW_NUMBER() (window functions) (supported by MySQL 8 and MariaDB 10.2) could be:
with cte as (
select g.genre_name, r.actor_id, count(*) as max_value,
row_number() over (partition by g.genre_name order by count(*) desc, r.actor_id) as rn
from genre g
inner join movie_has_genre mhg on mhg.genre_id = g.genre_id
inner join role r on mhg.movie_id = r.movie_id
group by g.genre_name, r.actor_id
)
select genre_name, actor_id, max_value from cte where rn = 1