Question

我在MySQL 8.0中有这个丑陋的代码。

(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Adventure'
        group by genre_name, actor_id 
        ORDER BY count(actor_id) DESC
        limit 3)        
UNION 
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Music'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Fantasy'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Romance'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Family'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Comedy'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Short'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Animation'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Action'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Horror'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Sci-Fi'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Thriller'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Crime'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Drama'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Documentary'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Musical'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Mystery'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='War'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Western'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Adult'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3)
UNION
(SELECT genre_name, actor_id, count(actor_id)
       FROM movie_has_genre, role, movie, genre
        where role.movie_id=movie.movie_id
        and movie_has_genre.movie_id=movie.movie_id
        and genre.genre_id=movie_has_genre.genre_id
        and genre_name='Film-Noir'
        group by genre_name, actor_id
        ORDER BY count(actor_id) DESC
        limit 3);

我希望将其转换为更简洁。基本上，唯一改变的是genre_name条件。可以找到此条件的21个值：select genre_name from genre;如何遍历genre_name的21个值并将它们插入到select查询中，逐步构建UNION查询？感谢。

Answer 1

您可以使用ROW_NUMBER：

WITH cte AS (
  SELECT genre_name, actor_id, count(actor_id) AS cnt
    ,ROW_NUMBER() OVER(PARTITION BY genre_name ORDER BY count(actor_id) DESC) rn
  FROM movie
  JOIN movie_has_genre ON movie_has_genre.movie_id=movie.movie_id
  JOIN role            ON role.movie_id=movie.movie_id
  JOIN genre           ON genre.genre_id=movie_has_genre.genre_id  
  group by genre_name, actor_id 
)
SELECT *
FROM cte
WHERE rn <=3;

请不要使用过时的逗号连接语法。

工作原理：

计算每个genere_name和actor_id
每个genre_name获得3个最多值

Answer 2

在MySQL（8.0之前）中，您可以使用变量：

select genre_name, actor_id, cnt
from (select genre_name, actor_id, cnt,
             (@rn := if(@g = genre_name, @rn + 1,
                        if(@g := genre_name, 1, 1)
                       )
             ) as rn
      from (select g.genre_name, r.actor_id, count(*) as cnt
            from movie m join
                 role r
                 on r.movie_id = m.movie_id join
                 movie_has_genre mhg
                 on mhg.movie_id = m.movie_id join
                 genre g
                 on g.genre_id = mhg.genre_id
            group by g.genre_name, r.actor_id
            order by g.genre_name, cnt desc
           ) ga cross join
           (select @g := '', @rn := 0) params
      ) ga
where rn <= 3;

在8.0+中，使用row_number() ANSI标准方法来实现此功能。

注意：

从不在FROM子句中使用逗号。始终使用正确的JOIN语法。
使用表格别名表示您正在使用的列的缩写。
限定您正在使用的查询中的所有列名称，尤其是当FROM子句引用多个表时。

转换UNION选择在MySQL中使用循环单选

2 个答案: