What countries are there that have roller coasters?
select t1.name from country as t1 join roller_coaster as t2 on t1.country_id = t2.country_id
Give me those that have more than one roller coaster.
select t1.name from country as t1 join roller_coaster as t2 on t1.country_id = t2.country_id group by t1.name having count(*) > 1