Which carriers are active in Alberta?
SELECT T3.carrier FROM phone_market AS T1 JOIN market AS T2 ON T1.Market_ID   =   T2.Market_ID JOIN phone AS T3 ON T1.Phone_ID   =   T3.Phone_ID where T2.District  =  "Alberta"
carriers active refers to selecting carrier names; in Alberta refers to where District = 'Alberta'; joining tables to connect carriers to their market regions;