Question

我需要编写一个SQL查询来返回所有电影的平均评分，而这些电影中的每一个都是“浪漫”，而不是“喜剧”。

下面是我的表格列表->

create table genres (genreid int,
             name text,
             primary key(genreid));

create table ratings(userid int,
           movieid int,
           rating numeric check(rating<=1 OR rating<=5),
           timestamp bigint,
            PRIMARY KEY (userid,movieid),
            FOREIGN KEY (userid) REFERENCES users (userid),
            FOREIGN KEY (movieid) REFERENCES movies (movieid));

create table hasagenre( movieid int,
               genreid int,
               FOREIGN KEY (movieid) REFERENCES movies (movieid),
               FOREIGN KEY (genreid) REFERENCES genres (genreid));

我尝试了以下代码，但得到的答案错误

  FROM ratings R,hasagenre H1,hasagenre H2,genres G1,genres G2
   WHERE R.movieid=H1.movieid AND H1.genreid=G1.genreid
   AND     R.movieid=H2.movieid AND H2.genreid=G2.genreid
   AND (G2.name <> 'Comedy' AND G1.name = 'Romance');

预期平均值：3.7429411764705882 实际结果：3.7458333333333333

如果我想获得所有电影的平均收视率，并且每部电影都是“浪漫”和“喜剧”，那么上面的代码可以很好地工作，方法是将<>'Comedy替换为='Comedy'

Answer 1

SELECT avg(R.rating)
FROM ratings R
WHERE movieid IN ( SELECT movieid
                   FROM hasagenre H  
                   JOIN genres G 
                     ON H.genreid = G.genreid
                   GROUP BY movieid
                   HAVING COUNT( CASE WHEN G.name = 'Comedy' THEN 1 END) = 1
                      AND COUNT( CASE WHEN G.name = 'Romance' THEN 1 END) = 0 )

我缺少什么加盟条件

1 个答案: