SELECT country_code, size,
  CASE WHEN size > 50000000
            THEN 'large'
       WHEN size > 1000000
            THEN 'medium'
       ELSE 'small' END
       AS popsize_group
INTO pop_plus
FROM populations
WHERE year = 2015;

Select name, continent, geosize_group, popsize_group
-- 1. From countries_plus (alias as c)
From countries_plus as c
  -- 2. Join to  pop_plus (alias as p)
  inner Join pop_plus as p
    -- 3. Match on country code
    on c.code = p.country_code
-- 4. Order the table
order by geosize_group;