从 SQL 查询中的计算列 select 创建新列

Question

我有一个特定日期从 2020 年到 2022 年的特定日期的数据 covid 死亡记录。我想使用国家/地区的分组依据计算每个国家 2 年的死亡率。

我尝试按原样进行查询，但查询不起作用，因为不允许将子查询用作表达式。我如何使这个查询工作？谢谢。

SELECT 
   location, 
   sum(total_deaths) as total_deaths, 
   sum(total_cases) as total_cases, 
   (select SUM(total_deaths) 
    FROM CovidDeaths$ 
    GROUP BY location
   )/
   (select sum(total_cases) 
    FROM CovidDeaths$ 
    GROUP BY location
   ) *100 as DeathPercentage 
FROM CovidDeaths$ 
WHERE NOT (total_deaths is null 
           OR total_cases is null 
           OR continent is null
          ) 
GROUP BY location
ORDER BY 1

*查询中的位置指的是国家

我尝试进行嵌套查询，但死亡百分比的值变为 0。

SELECT location, total_deaths1, total_cases1, total_deaths1/total_cases1*100 as Death_Percentage

FROM(

SELECT location, sum(total_deaths) as total_deaths1, sum(total_cases) as total_cases1

FROM CovidDeaths$ 

WHERE NOT (total_deaths is null OR total_cases is null OR continent is null) 

GROUP BY location

) as death

ORDER BY 1

Answer 1

你不需要在子查询中计算平均值，你可以在主查询中计算它

(SUM(total_deaths)/sum(total_cases))*100

更新

此计算列是您按位置分组的主查询的一部分：

SELECT 
location, 
sum(total_deaths) as total_deaths, 
sum(total_cases) as total_cases, 
(SUM(total_deaths)/sum(total_cases))*100
FROM CovidDeaths$ 
WHERE NOT (total_deaths is null 
       OR total_cases is null 
       OR continent is null
      ) 
GROUP BY location
ORDER BY 1

Answer 2

许多数据库在除法中使用整数时使用整数运算。由于 0 和 1 之间没有整数，这意味着在乘以 100.

之前，您的所有值都被四舍五入为 0

2 / 6 * 100 => 0 * 100 => 0

您可以简单地更改操作顺序

100 * 2 / 6 => 200 / 6 => 66

或者，将值隐式转换为 non-integer 类型

100.0 * 2 / 6 => 200.0 / 6 => 33.3333333

您还可以使用CAST()、CONVERT()或其他函数显式转换data-type（可能是DECIMAL(18,6)或FLOAT，取决于你的需要）.

具体语法取决于您使用的数据库
但是你还没有指定是哪个数据库

从 SQL 查询中的计算列 select 创建新列

To make a new column from calculated column select in SQL Query

sql

sql-server