How many events has "Shanghai" hosted?
select count(*) from city as t1 join hosting_city as t2 on t1.city_id = t2.host_city where city = "Shanghai"
Give me the temperature of Shanghai in March.
select t2.mar from city as t1 join temperature as t2 on t1.city_id = t2.city_id where t1.city = "Shanghai"
How about the temperature in January?
select t2.jan from city as t1 join temperature as t2 on t1.city_id = t2.city_id where t1.city = "Shanghai"