Show me all the cities.
select * from city
Show me the official names of all the cities.
select official_name from city
Which of these cities have hosted competitions before?
select distinct official_name from city as t1 join farm_competition as t2 on t1.city_id = t2.host_city_id
Which of them have not?
select official_name from city where city_id not in (select host_city_id from farm_competition)