Indicar os nomes dos países e a velocidade média das montanhas-russa de cada país.
CREATE TABLE roller_coaster (Speed INTEGER, Country_ID VARCHAR); CREATE TABLE country (Name VARCHAR, Country_ID VARCHAR)
SELECT T1.Name, AVG(T2.Speed) FROM country AS T1 JOIN roller_coaster AS T2 ON T1.Country_ID = T2.Country_ID GROUP BY T1.Name