Find the cities which were once a host city after 2009?
select t1.city from city as t1 join hosting_city as t2 on t1.city_id = t2.host_city where t2.year > 2009
How many cities in this table?
select count ( distinct city ) from city
Find average GDP of this table?
select avg ( gdp ) from city
Yes.
select max ( gdp ) from city