How many cities are there
select count(*) from city
How many cities have hosted events?
select count(distinct city) from city as t1 join hosting_city as t2 on t1.city_id = t2.host_city
Wihch city hosted events in the most recent year?
select t1.city from city as t1 join hosting_city as t2 on t1.city_id = t2.host_city order by year desc limit 1
Show its id.
select host_city from hosting_city order by year desc limit 1