-- Database schema
| mission : Mission_ID [ INT ] primary_key , Ship_ID [ INT ] mission.Ship_ID = ship.Ship_ID , Code [ TEXT ] , Launched_Year [ INT ] , Location [ TEXT ] , Speed_knots [ INT ] , Fate [ TEXT ] | ship : Ship_ID [ INT ] primary_key , Name [ TEXT ] , Type [ TEXT ] , Nationality [ TEXT ] , Tonnage [ INT ] |
-- -- Show the types of ships that have both ships with tonnage larger than 6000 and ships with tonnage smaller than 4000.

SELECT TYPE FROM ship WHERE Tonnage  >  6000 INTERSECT SELECT TYPE FROM ship WHERE Tonnage  <  4000;