Show the names of countries 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
What are the speed of all roller coasters?
select name, speed from roller_coaster
Give me a average of them in term of different countries.
select t1.name from country as t1 join roller_coaster as t2 on t1.country_id = t2.country_id