Show the name for all aircraft.
select name from aircraft
For each, also show the names of employees with a certificate on it.
select t3.name, t1.name from employee as t1 join certificate as t2 on t1.eid = t2.eid join aircraft as t3 on t3.aid = t2.aid
What are the names of aircraft 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"