in which aircraft does John Williams have a certificate
SELECT T3.name FROM Employee AS T1 JOIN Certificate AS T2 ON T1.eid   =   T2.eid JOIN Aircraft AS T3 ON T3.aid   =   T2.aid WHERE T1.name   =   "John Williams"
John Williams refers to filtering by name in the Employee table; aircraft certificate refers to joining Employee with Certificate and then Aircraft to get the aircraft name;