Find the addresses of the shops where they have happy hour.
select t1.address from shop as t1 join happy_hour as t2 on t1.shop_id = t2.shop_id
also return the staff numbers.
select t1.address, t1.num_of_staff from shop as t1 join happy_hour as t2 on t1.shop_id = t2.shop_id
how about those where there is no happy hour?
select address, num_of_staff from shop where shop_id not in (select shop_id from happy_hour)