Show names for all aircrafts of which John Williams has certificates.
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"
names for all aircrafts = SELECT Aircraft.name; of which John Williams has certificates refers to WHERE Employee.name = 'John Williams';