在 SQLite 中计算中位数

Question

我有下表：

我想在一个查询（SQLite）中计算每辆车的平均里程数。我该怎么做？

这是我最接近的：

SELECT Car, Mileage, ROW_NUMBER() OVER (PARTITION BY Car ORDER BY Mileage) RowNum FROM car_table ORDER BY Car, Mileage;

我的预期输出是：

汽车 |中位里程

Answer 1

您可以使用窗口函数 ROW_NUMBER()、COUNT() 和 AVG() 来完成：

SELECT DISTINCT Car,
       AVG(
         CASE counter % 2
           WHEN 0 THEN CASE WHEN rn IN (counter / 2, counter / 2 + 1) THEN mileage END 
           WHEN 1 THEN CASE WHEN rn = counter / 2 + 1 THEN mileage END
         END
       ) OVER (PARTITION BY car) median 
FROM (        
  SELECT *,
         ROW_NUMBER() OVER (PARTITION BY car ORDER BY mileage) rn,
         COUNT(*) OVER (PARTITION BY car) counter
  FROM car_table
)  

或者：

SELECT DISTINCT Car,
       AVG(CASE WHEN rn IN (counter / 2 + 1, counter / 2 + 1 - (counter % 2 = 0)) THEN mileage END) 
       OVER (PARTITION BY car) median 
FROM (        
  SELECT *,
         ROW_NUMBER() OVER (PARTITION BY car ORDER BY mileage) rn,
         COUNT(*) OVER (PARTITION BY car) counter
  FROM car_table
)

查看演示。
结果：

汽车	中位数
本田	11.85
丰田	9.8