Show the name of all aircrafts.
select name from aircraft
Also show the number of people with a certificate on each one.
select t2.name, count(*) from certificate as t1 join aircraft as t2 on t2.aid = t1.aid group by t1.aid
Order the aircraft by these counts.
select t2.name from certificate as t1 join aircraft as t2 on t2.aid = t1.aid group by t1.aid order by count(*)
Which of them has the fewest?
select t2.name from certificate as t1 join aircraft as t2 on t2.aid = t1.aid group by t1.aid order by count(*) limit 1