Question: Show the carriers of devices in stock at more than one shop.
Schema: create table stock (device_id varchar, PRIMARY KEY (device_id)); create table device (carrier varchar, device_id varchar, PRIMARY KEY (carrier))
SQL: select t2.carrier from stock as t1 join device as t2 on t1.device_id = t2.device_id group by t1.device_id having count(*) > 1