ค้นหาตำแหน่งกัปตันที่มีกัปตันทั้งคลาส Cutter และ Armed schooner ไม่จำเป็นต้องใช้โต๊ะทั้งหมด
Find the captain rank that has some captains in both Cutter and Armed schooner classes.It is not neccessary to use all the tables.
 captain:  captain_id [ number]  name [ text]  ship_id [ number]  age [ text]  class [ text]  rank [ text] 
SELECT rank FROM captain WHERE class = 'Cutter' INTERSECT SELECT rank FROM captain WHERE class = 'Armed schooner'