SQL划分2列

Question

我需要在SQL中計算人口密度。 我有一列人口，一列面積，但我不知道如何執行它，我得到除以零誤差。

這是我嘗試過的：

select name, population, gdp, area, [population]/[area] as pop_density 
from country 
group by name 
order by gdp;

Answer 1

為什么需要group by ？
無需分組。
即使您確實需要group by ，也無法選擇非聚合列，例如：
population, gdp, area 。
只需檢查null或0的列area ：

select 
  name, population, gdp, area, 
  case coalesce(area, 0) 
    when 0 then null 
    else population / area 
  end as pop_density 
from country 
order by gdp

Answer 2

使用nullif() ：

select name, population, gdp, area,
       population / nullif(area, 0) as pop_density
from country
order by gdp;

group by似乎沒有必要。

nullif()是一個標准函數，當傳入的兩個值相等時會返回NULL 。 如果您的數據庫不支持它，則可以使用case表達式執行相同的操作。

Answer 3

只需在where clause as添加where clause as

       select name, population, gdp, area, 
      [population]/[area] as pop_density 
        from country where area
       NOT IN (NULL,0) group by name 
     order by gdp  

或者，您可以通過Case Statements處理除數區域的值0。

  .......Case when Area IN (Null,0)
         then 0
          else
           [population]/[area] 

Answer 4

select name, population, gdp, area, [population]/[area] as pop_density 
from country 
where area is not null and area is not 0
group by name 
order by gdp;