MySQL 8.0

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create table movies (
  id int primary key,
  title varchar(100),
  genre varchar(50)
);

insert into movies
  select 1 as id, 'return' as title, 'Comedy' as genre
  union all
  select 2, 'Scooby', 'Documentary'
  union all
  select 3, 'Halo', 'Documentary';

Records: 3  Duplicates: 0  Warnings: 0


create table reviews (
  movie_id int,
  rating numeric(5,2)
);

insert into reviews
  select 1 as movide_id, 1 as rating
  union all
  select 1, 3
  union all
  select 2, 5
  union all
  select 2, 5
  union all
  select 3, 3;

Records: 5  Duplicates: 0  Warnings: 0


select
  genre,
  title,
  round(stars, 2) stars
from (
  select
    m.genre,
    m.title,
    avg(r.rating) stars,
    row_number() over(
      partition by m.genre
      order by avg(r.rating) desc
    ) rn
  from movies m
  join reviews r
  on r.movie_id = m.id
  group by
    m.genre,
    m.title
) s
where rn = 1
order by genre;


genre
title
stars


Comedy
return
2.00

Documentary
Scooby
5.00