How many countries have roller coaster?
SELECT count ( distinct T1.Name )  FROM country AS T1 JOIN roller_coaster AS T2 ON T1.Country_ID   =   T2.Country_ID
counting countries with roller coasters involves counting distinct country names (T1.Name) that are linked to roller coaster entries in the roller_coaster table through a JOIN operation on matching country IDs (T1.Country_ID = T2.Country_ID).