Alternative query to this (avoid DISTINCT)
You could get that same result set with the following:

select country from countries where exists 
    (select countryid from users 
        where users.countryid = country.countryid)


OR

select country from countries where countryid in 
    (select countryid from users)


Neither of these use distinct but other than that I don't know how much "better" they are.