code_1	code_2	year_month	value
A	10001	201511	100
A	10002	201512	200
B	20001	201509	300
B	20002	201510	400

SELECT
  t2.code_1
  , t2.code_2
  , t2.yaer_month
  , t2.value
FROM
  target_table t2
  INNER JOIN (
              SELECT
                code_1
                , MAX(year_month) AS year_month
              FROM
                target_table
              GROUP BY
                code_1
             ) t1
          ON t1.code_1 = t2.code_1
         AND t1.year_month = t2.year_month
ORDER BY
  t2.code_1
  , t2.code_2
  , t2.yaer_month

以下のようにcode_1、yaer_monthのみでは、重複レコードを排除できない状態でcode_2が最大のレコードを取得する。

code_1	code_2	year_month	value
A	10001	201511	100
A	10002	201512	200
A	10003	201512	300

SELECT
  t4.code_1
  , t4.code_2
  , t4.yaer_month
  , t4.value
FROM
  target_table t4
  INNER JOIN (
              SELECT
                t2.code_1
                , t2.yaer_month
                , MAX(t2.code_2) AS code_2
              FROM
                target_table t2
                INNER JOIN (
                            SELECT
                              code_1
                              , MAX(year_month) AS year_month
                            FROM
                              target_table
                            GROUP BY
                              code_1
                           ) t1
                        ON t1.code_1 = t2.code_1
                       AND t1.year_month = t2.year_month
              GROUP BY
                t2.code_1
                , t2.yaer_month
             ) t3
          ON t3.code_1 = t4.code_1
         AND t3.yaer_month = t4.yaer_month
         AND t3.code_2 = t4. code_2
ORDER BY
  t4.code_1
  , t4.code_2
  , t4.yaer_month