with x as
(select row_number() over(partition by customer,make order by offer desc) rn,
customer, make, zipcode, offer from tablename)
, y as (select customer, make, zipcode, offer from x where rn <=2)
, z as (select customer, make, zipcode,
case when rn = 1 then offer else 0 end as offer_1,
case when rn = 2 then offer else 0 end as offer_2
from y)
select customer, make, zipcode, offer_1, offer_2, offer_1+offer_2 total_offer
from z
Dette gør brug af rekursive cte'er til at udføre din opgave.