List the shop names.
select shop_name from shop
Keep the names where there is a stock
select t1.shop_name from shop as t1 join stock as t2 on t1.shop_id = t2.shop_id
Keep the names of shops and the carriers of devices where there is a stock.
select t3.shop_name, t2.carrier from stock as t1 join device as t2 on t1.device_id = t2.device_id join shop as t3 on t1.shop_id = t3.shop_id