SQL加权平均值

Question

有一张如下表格。

make   | model | engine | cars_checked | avg_mileage
---------------------------------------|--------
suzuki | sx4   | petrol | 11           | 12
suzuki | sx4   | diesel | 150          | 16
suzuki | swift | petrol | 140          | 15
suzuki | swift | diesel | 18           | 19
toyota | prius | petrol | 16           | 17
toyota | prius | hybrid | 250          | 24

所需的输出是

1. 发动机的平均里程（汽油，柴油）
2. 乘以平均里程
3. 按型号计算的平均里程数

无法做一个简单的group by，因为需要考虑每个记录（cars_checked）的样本数量的权重年龄，以避免平均值问题的平均值。

实现它的正确方法是什么？有没有办法考虑在group by中进行加权平均的样本数？

更新 - 为上面的＃1添加的输出格式为例

engine   | mileage_by_engine
--------------------------
petrol   | xx.z
diesel   | yy.z

Answer 1

SELECT engine, SUM(cars_checked * avg_mileage) / SUM(cars_checked) AS avgMilageByEngine
FROM [YOUR_TABLE]
GROUP BY engine

SELECT make, SUM(cars_checked * avg_mileage) / SUM(cars_checked) AS avgMilageByMake
FROM [YOUR_TABLE]
GROUP BY make

SELECT model, SUM(cars_checked * avg_mileage) / SUM(cars_checked) AS avgMilageByModel
FROM [YOUR_TABLE]
GROUP BY model

Answer 2

简化查询的一种方法是使用grouping sets：

select engine, make, model,
       sum(cars_check * avg_mileage) / sum(cars_checked) as avgMilage
from t
group by grouping sets ((engine), (make), (model));

输出格式仅在聚合 列中具有非NULL值。