Context: CREATE TABLE roller_coaster (Speed INTEGER, Country_ID VARCHAR); CREATE TABLE country (Name VARCHAR, Country_ID VARCHAR)
Question: Show the names of countries and the average speed of roller coasters from each country.
Answer: 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