即使在 where 函数中实现了也无法过滤 null

Question

三张表：-Rating (rID, mID, stars, ratingDate) Movie (mID, title, year, director)Reviewer (rID, name)

找出 1980 年之前发行的电影的平均评分与 1980 年之后发行的电影的平均评分之间的差异。

（确保计算每部电影的平均评分，然后计算 1980 年之前和之后的电影的平均值。不要只计算 1980 年之前和之后的总体平均评分。）

但总是有null，即使我在where函数中过滤了null

''''
select r.mID,sum(stars),
count(case when year>1980 and stars is not null then stars end )as "numaf",
count(case when year<1980 and stars is not null then stars end )as "numbf",
sum(case when year>1980 and stars is not null then stars end )as "sumaf",
sum(case when year<1980 and stars is not null then stars end )as "sumbf",
(("sumaf"/"numaf")-("sumbf"/"numbf")) as "difof"
from Rating r left join movie m on r.mID = m.mID 
where stars is not null
group by r.mID
''''

and this is output, always has "NULL":
101 9   0   3   <NULL>  9   <NULL>
103 5   0   2   <NULL>  5   <NULL>
104 5   2   0   5   <NULL>  <NULL>
106 9   0   2   <NULL>  9   <NULL>
107 8   2   0   8   <NULL>  <NULL>
108 10  3   0   10  <NULL>  <NULL>

Answer 1

电影的平均评分是：

select r.mID, avg(stars) as avg_stars
from Rating r 
group by r.mID;

然后，要回答这个问题，您可以将其加入到电影中并使用条件聚合：

select avg(case when year < 1980 then avg_stars) as avg_stars_pre1980,
       avg(case when year > 1980 then avg_stars) as avg_stars_post1980,
       (avg(case when year < 1980 then avg_stars) -
        avg(case when year > 1980 then avg_stars) a
       ) as diff       
from (select r.mID, avg(stars) as avg_stars
      from Rating r 
      group by r.mID
     ) r join
     movie m
     on r.mID = m.mID 
group by r.mID;

您获得NULL 的原因是因为您认为您正在划分SELECT 中定义的列别名。但是，您实际上是在划分字符串。所以：

(("sumaf"/"numaf")-("sumbf"/"numbf")) as "difof"

只是：

(('sumaf'/'numaf')-('sumbf'/'numbf')) as difof

字符串被解释为数字，在数字常量中，前导数字转换为数字。这些没有前导数字，因此这些值都是0 - 导致被零除。 MySQL 返回NULL 除以零而不是产生错误。

Answer 2

要获得 1980 年之前和之后的电影的平均评分，您可以执行以下操作：

SELECT SUM(avgRating)/Count(mID), yearSection
FROM (
    SELECT 
       r.mid,
       round(sum(stars),2)/count(r.mID) as avgRating,
       case when year > 1980 then 1 else 0 end as yearSection
from Rating r left join movie m on r.mID = m.mID 
where stars is not null
group by r.mID
     )AS foo
GROUP BY yearSection

这不会产生任何空值。此外，您应该需要将总和四舍五入为小数，否则您的平均值将不正确。