access中的写法：
select * from
((spaceports as a left join flights as b on a.spaceport_id=b.departure_city)
left join passengers as c on b.flight_id = c.flight_id)
sqlserver中的写法：
select * from spaceports as a left join flights as b
on a.spaceport_id=b.departure_city
left join passengers as c on b.flight_id = c.flight_id
