What is the  height of the highest roller coaster?
select max ( height ) from roller_coaster
Which park is this roller coaster located at?
select park from roller_coaster where height = ( select max ( height ) from roller_coaster )
What is the name of the country that has the highest roller coaster?
select t2.name from roller_coaster as t1 join country as t2 on t1.country_id = t2.country_id where t1.height = ( select max ( height ) from roller_coaster )
Hmm, I also want to know the population of this country.
select t2.name, t2.population from roller_coaster as t1 join country as t2 on t1.country_id = t2.country_id where t1.height = ( select max ( height ) from roller_coaster )