What are the types of the ships that have both shiips with tonnage more than 6000 and those with tonnage less than 4000?
SELECT TYPE FROM ship WHERE Tonnage  >  6000 INTERSECT SELECT TYPE FROM ship WHERE Tonnage  <  4000
SELECT TYPE FROM ship WHERE Tonnage  >  6000 INTERSECT SELECT TYPE FROM ship WHERE Tonnage  <  4000
[{'Type': 'Battle ship'}]