Tell me the phones in the city "Acua".
select t1.phone from address as t1 join city as t2 on t1.city_id = t2.city_id where t2.city = "Acua"
How about that of the the city "Bag"?
select t1.phone from address as t1 join city as t2 on t1.city_id = t2.city_id where t2.city = "Bag"
Which countries have at least 3 cities?
select t2.country from city as t1 join country as t2 on t1.country_id = t2.country_id group by t2.country_id having count(*) >= 3