Which cities have never hosted an event?
select city from city except select city from city as t1 join hosting_city as t2 on t1.city_id = t2.host_city
What is the average temperature in Dec of these cities?
select avg(dec) from city as t1 join temperature as t2 on t1.city_id = t2.city_id except select t3.city from city as t3 join hosting_city as t4 on t3.city_id = t4.host_city
How about the cities whose temperature in Mar is lower than that in Dec?
select t1.city from city as t1 join temperature as t2 on t1.city_id = t2.city_id where t2.mar < t2.dec except select t3.city from city as t3 join hosting_city as t4 on t3.city_id = t4.host_city