% diff of 2 columns in SQL - sql

Question

I have 2 tables - countries (id, name, continent) and population_years (id, population, year, country_id). The data goes from 2000-2010 and I'm trying to calculate the percentage diff in average population for each continent across this time period. I'm trying to do it by creating a temporary table which produces an output of:
But when I try to calculate the % diff (as you can see from my code below), I don't know how to reference the 'avg pop 2000' and 'avg pop 2010' columns in the code as they haven't been assigned a variable that I can reference. In the code, I've used avg_pop_2010 and avg_pop_2000 to reference these columns - obviously this doesn't actually work.
WITH avg_pop AS( SELECT countries.continent,
ROUND(AVG(CASE WHEN population_years.year = 2000 THEN population_years.population END), 2) as 'avg pop 2000',
ROUND(AVG(CASE WHEN population_years.year = 2010 THEN population_years.population END), 2) as 'avg pop 2010'
FROM countries
JOIN population_years
WHERE population_years.country_id = countries.id
GROUP BY 1)
SELECT countries.continent, ROUND(((avg_pop_2010 - avg_pop_2000)/avg_pop_2000)*100.0, 2) AS '%diff'
FROM avg_pop;

Answer 1

Another option is to repeat the expressions - with a little optimization:
select
c.continent,
round(avg(case when py.year = 2000 then py.population end), 2) avg_pop_2000,
round(avg(case when py.year = 2010 then py.population end), 2) avg_pop_2010,
round(
100.* avg(case when py.year = 2010 then py.population else - py.population end)
/ avg(case when py.year = 2000 then py.population end),
2
) percent_diff
from countries c
inner join population_years py on py.country_id = c.id
where py.year in (2010, 2020)
group by c.continent
Side notes:
don't use single quotes for identifiers! They stand for literal strings in standard SQL; in general, you should prefer identifiers that do not need to be quoted. If quoting is needed, use standard double quotes ("), which SQLite recognizes
prefiltering the relevant years with a where clause makes the query more efficient
use standard join syntax; the join condition goes to the on clause of the join, not to the where clause
rounding, then computing the percent difference is not accurate; compute first, then round
table aliases make the query easier to write and read

Answer 2

WITH avg_pop AS( SELECT countries.continent,
ROUND(AVG(CASE WHEN population_years.year = 2000 THEN population_years.population END), 2) as avg_pop_2000,
ROUND(AVG(CASE WHEN population_years.year = 2010 THEN population_years.population END), 2) as avg_pop_2010
FROM countries
JOIN population_years
WHERE population_years.country_id = countries.id
GROUP BY 1)
SELECT continent, ROUND((( avg_pop_2000 - avg_pop_2010)/avg_pop_2010)*100, 2) AS '%diff'
FROM avg_pop;
Here is a small demo:
DEMO
When checking on other comments and answer I realized you need to give different aliases without '' and all will work...I have updated my answer and demo.