รถไฟเหาะมีกี่อัน?ไม่จำเป็นต้องใช้โต๊ะทั้งหมด
How many roller coasters are there?It is not neccessary to use all the tables.
 roller_coaster:  roller_coaster_id [ number]  name [ text]  park [ text]  country_id [ number]  length [ number]  height [ number]  speed [ text]  opened [ text]  status [ text] 
SELECT COUNT(*) FROM roller_coaster