how many ships have more than one captain?
select count ( * ) from ( select * from ship as t1 join captain as t2 on t1.ship_id = t2.ship_id group by t2.ship_id having count ( * ) > 1 )
how many ships have a single captain?
select count ( * ) from ( select * from ship as t1 join captain as t2 on t1.ship_id = t2.ship_id group by t2.ship_id having count ( * ) = 1 )
what is the age of the oldest captain?
select age from captain order by age desc limit 1
what about the youngest?
select age from captain order by age limit 1