Yes, how many different names.
select count ( distinct name ) from aircraft
Show me the name of the aircraft with the fewest people who have its certificate
select t2.name from certificate as t1 join aircraft as t2 on t2.aid = t1.aid group by t1.aid order by count ( * ) asc limit 1
yes
select count ( * ) from certificate as t1 join aircraft as t2 on t2.aid = t1.aid where t2.name = "Boeing 747-400"
Yes
select distance from aircraft where name = "Boeing 747-400"
What is the distance of the Boeing 737-800?
select distance from aircraft where name = "Boeing 737-800"