Ligne avec la dernière valeur par client et par mois

SELECT YEAR
      ,MONTH
      ,customer_id
      ,max(ACCOUNT_TYPE) keep(dense_rank FIRST ORDER BY date_changed DESC) LAST_ACC
 FROM (
  SELECT EXTRACT(YEAR FROM DATE_CHANGED) AS YEAR,
         EXTRACT(MONTH FROM DATE_CHANGED) AS MONTH,
         CUSTOMER_ID,
         date_changed,
         account_type
   FROM HISTORY
  )
GROUP BY YEAR, MONTH, customer_id
ORDER BY YEAR, MONTH, CUSTOMER_ID


| YEAR | MONTH | CUSTOMER_ID | LAST_ACC |
-----------------------------------------
| 2013 |     1 |         200 |  Premium |
| 2013 |     2 |         300 |     Free |
| 2013 |     3 |         100 |  Premium |
| 2013 |     3 |         200 | Standard |

http://sqlfiddle.com/#!4/e493a/15