NOT IN vs IN Do Not Return Complimentary Results

Question

Hi I am working through example #7 from the sql zoo tutorial: SELECT within SELECT. In the following question

\"Find each country that belongs to a continent where all p

Answer 1

why use a sub query?

try using:

SELECT name, continent, population FROM world 
WHERE population > 25000000

and/or

SELECT name, continent, population FROM world 
WHERE population <= 25000000

the column of your condition: "population" is in the FROM table: "world". There is no need to use a sub query of the same table "world" again, just use the "population" column directly in the WHERE

or are you trying to do this:

SELECT name, continent, population FROM world 
WHERE continent NOT IN (
    SELECT continent FROM world
    GROUP BY continent 
    HAVING SUM(population) > 25000000)

notice the: SUM(), GROUP BY, and HAVING