обыватель
тяжело было решить 28 задачку. но я справился. молодец. гордимся мной 10 минут
select avg(hd)
from product as a inner join pc on a.model=pc.model
where a.type='pc' and exists (select maker from product
where product.type='printer' and maker=a.maker)
select avg(hd)
from product as a inner join pc on a.model=pc.model
where a.type='pc' and exists (select maker from product
where product.type='printer' and maker=a.maker)
