List all the shop names.
select shop_name from shop
Which shop has the least quantity of devices, show the shop name.
select t2.shop_name from stock as t1 join shop as t2 on t1.shop_id = t2.shop_id group by t1.shop_id order by sum(t1.quantity) asc limit 1
What about the shop that has the most kind of devices in stock?
select t2.shop_name from stock as t1 join shop as t2 on t1.shop_id = t2.shop_id group by t1.shop_id order by count(*) desc limit 1