--1 sposob
select ProductName, UnitPrice, (select avg(UnitPrice) from Products ) as Average,
       unitprice - (select avg(UnitPrice) from Products ) as Difference
from Products
    --2 sposob
select ProductName, UnitPrice, Average, Difference
from (select ProductName, UnitPrice,(select avg(UnitPrice) from Products ) as Average,
     unitprice - (select avg(UnitPrice) from Products ) as Difference
     from Products) as p