SQL Server 2012:複数の基準に基づいて上位 n を選択

row_number() でこれを行うことができます :

select region, month, MemberId, sales
from (select region, month, MemberId, sum(sales) as sales
             row_number() over (partition by region, month order by sum(sales) desc) as seqnum
      from table t
      group by region, month, MemberId
     ) t
where seqnum <= 2;