SQL划分2列

Question

I need to calculate population density in SQL. I have a column with population and a column with area but I'm can't figure out how to execute it, I get a division by zero error.

Here is what I tried:

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

Answer 1

Why do you need group by ?
There is no need for grouping.
Even if you did need group by you couldn't select non aggregated columns like:
population, gdp, area .
Just check for null or 0 for the column 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

Use nullif() :

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

The group by doesn't seem necessary.

nullif() is a standard function that returns NULL when the two values passed in are equal. If your database does not support it, you can do the same thing with a case expression.

Answer 3

Just add in 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  

Or, You can handle your divisor area's value 0 via Case Statements...

  .......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;