bq042.sql

SELECT
  age.country_name,
  SUM(age.population) AS under_25,
  pop.midyear_population AS total,
  ROUND((SUM(age.population) / pop.midyear_population) * 100,2) AS pct_under_25
FROM (
  SELECT country_name, population, country_code
  FROM `bigquery-public-data.census_bureau_international.midyear_population_agespecific`
  WHERE year =2017 AND age < 25) age
INNER JOIN (
  SELECT midyear_population, country_code
  FROM `bigquery-public-data.census_bureau_international.midyear_population`
  WHERE year = 2017) pop
ON age.country_code = pop.country_code
GROUP BY 1,3
ORDER BY 4 DESC