Prueba esta consulta
select * from
(select
@rn:=if(@prv=product_id, @rn+1, 1) as rId,
@prv:=product_id as product_id,
year,
month,
revenue
from tbl
join
(select @prv:=0, @rn:=0)tmp
order by
product_id, revenue desc) a
where rid<=5
SQL FIDDLE :
| RID | PRODUCT_ID | YEAR | MONTH | REVENUE |
---------------------------------------------
| 1 | 1 | 2013 | 1 | 100 |
| 2 | 1 | 2013 | 1 | 90 |
| 3 | 1 | 2013 | 1 | 70 |
| 4 | 1 | 2013 | 1 | 60 |
| 5 | 1 | 2013 | 1 | 50 |
| 1 | 2 | 2013 | 1 | 5550 |
| 2 | 2 | 2013 | 1 | 550 |
| 3 | 2 | 2013 | 1 | 520 |
| 4 | 2 | 2013 | 1 | 510 |
| 5 | 2 | 2013 | 1 | 150 |
