Question

我有一张表评分：

part     rating     numReviews
----     ------     ----------
A1       100        5
A1       50         2
A2       90         4
A3       100        4

我需要找到每个部分的平均评分。在上面的例子中，A2是90，A3是100，因为它们是单行。容易。

但是，A1部分是多行，所以我需要为每行做评级* numReviews 然后除以总numReviews

这可能吗？这就是我正在尝试的（微软SQL）：

select part, 
cast((AVG((rating * numReviews) / SUM(numReviews))) as decimal(5,2)) as rating_average
from tableName group by part order by part

我收到以下错误：

Cannot perform an aggregate function on an expression containing an aggregate or a subquery.

Answer 1

如果评级列存储了各个评级的值（意味着您有4个评级值100，A3总计400，您想知道评级的平均值（计算为total per group / (rating value * rating count)）和输出应该是：

part AverageRating
---- ---------------------------------------
A1   85.71428571428571428571428571
A2   90.00000000000000000000000000
A3   100.00000000000000000000000000

然后这个查询就会这样做：

select 
    part, 
    sum(rating * numreviews)/sum(cast(numreviews as decimal(5,2))) as AverageRating
from tableName 
group by part

另一方面，如果评级栏存储评级的总和（意味着A3的值100应解释为4 * 25），并且您希望每组的平均评级具有此输出：

part rating_average
---- ---------------------------------------
A1   21.4285714285714285714
A2   22.5000000000000000000
A3   25.0000000000000000000

然后这个：

with ratings as (select 
    part, 
    sum(rating) r, 
    sum(numreviews) as c
from TableName
group by part
)
select part, avg(r/cast(c as decimal)) AS rating_average
from ratings 
group by part

我认为这是你要找的前者，而不是后者。

Answer 2

Select part, Sum(rating) as ratings,Sum(numReviews) as TotalReviews,
        (Sum(rating) /sum(numReview )) as rating_average
From tableName
Group By part

Answer 3

您只需使用SUM：

即可

SELECT part, 
    CAST(SUM(rating * numReviews) / SUM(numReviews) AS DECIMAL(5, 2)) as rating_average
FROM tableName
GROUP BY part
ORDER BY part

Answer 4

或者为了清楚起见，你可以一步一步地做到这一点

SELECT Level1.*,(Level1.ratings/Level1.TotalReviews) as rating_average
FROM
  (
     Select part, Sum(rating) as ratings,Sum(numReviews) as TotalReviews
     From tableName
      GroupBy part
   ) as Level1

找到多行字段的平均值

4 个答案: