SQL获得百分比

Question

我有以下代码来计算总人数和按PersonType分组。现在我想要一个列来计算每个死亡事故的百分比。这是为每个人的类型取值并除以总死亡人数。

Select distinct persontype as PerSsonType, sum(fatalities_in_crash) as  Total_Deaths

  from [CarCrash].[Source_Data_Staging] 
group by PersonType

Answer 1

您可以使用ANSI标准窗口函数（包括SQL Server在内的大多数数据库都支持）：

Select persontype as PerSsonType, sum(fatalities_in_crash) as Total_Deaths,
       sum(fatalities_in_crash) * 1.0 / sum(sum(fatalities_in_crash)) over () as ratio
from [CarCrash].[Source_Data_Staging] 
group by PersonType;

注意：您几乎不需要SELECT DISTINCT GROUP BY。 * 1.0是因为某些数据库使用整数除法 - 这会为所有行生成0。

Answer 2

您可以将此查询与选择死亡总和而不进行分组的查询交叉加入：

SELECT   persontype, 
         SUM(fatalities_in_crash) AS Total_Deaths,
         SUM(fatalities_in_crash) / all_deaths AS percentage
FROM     [CarCrash].[Source_Data_Staging] 
JOIN     (SELECT SUM(fatalities_in_crash) AS all_deaths
          FROM   [CarCrash].[Source_Data_Staging]) t
GROUP BY persontype