Refer to renamed columns in WHERE clause
You should use a View in your database

USE mydb;
CREATE VIEW name_capita AS
SELECT name, gdp/population as capita,continent FROM world;


Then, apply the WHERE clause to the View

USE mydb;
SELECT * FROM name_capita
WHERE continent = 'Europe' AND
capita &gt; (SELECT gdp/population FROM world WHERE name = 'United Kingdom');


or even re-using the view:

SELECT * FROM name_capita
WHERE continent = 'Europe' AND
capita &gt; (SELECT capita FROM name_capita WHERE name = 'United Kingdom');


GIVE IT A TRY !!!