CREATE TABLE ship (name VARCHAR2, CLASS VARCHAR2, ship_id VARCHAR2)
CREATE TABLE captain (name VARCHAR2, CLASS VARCHAR2, ship_id VARCHAR2)

what are the names and classes of the ships that do not have any captain yet?
SELECT name, CLASS FROM ship WHERE NOT ship_id IN (SELECT ship_id FROM captain)
SELECT "name", "CLASS" FROM "ship" WHERE NOT "ship_id" IN (SELECT "ship_id" FROM "captain")