For each phone, show its names and total number of stocks?
SELECT T2.Name ,  sum ( T1.Num_of_stock )  FROM phone_market AS T1 JOIN phone AS T2 ON T1.Phone_ID   =   T2.Phone_ID GROUP BY T2.Name
For each phone refers to GROUP BY Phone.Name; show its names refers to SELECT Phone.Name; total number of stocks refers to SUM(phone_market.Num_of_stock);