как выполнить группировку

select product_name,
      [b] (select t.country_name from country t where t.country_id = country_id) as country_name[/b]
from product
group by product_name, country_name

order by 2

select product_name,
         country_name
from(
select product_name,
       (select t.country_name from country t where t.country_id = country_id) as country_name
from product)
group by product_name, country_name

select product_name,
         country_name
from(
select product_name,
       (select t.country_name from country t where t.country_id = country_id) as country_name
from product)
group by product_name, country_name

select product_name,
       (select t.country_name from country t where t.country_id = p.country_id) as country_name
from product p
group by product_name, p.country_id

И, кстати, исходный запрос в принципе некорректен... В нем не соблюдены (или не учтены?) принципы "локальных и глобальных имен объектов"

select p.product_name,
         t.country_name 
from product p, country t 
where  t.country_id = t.country_id
group by product_name, country_name