How about this?

Code:
SELECT TO_CHAR( posting_date, 'YYYYMM' ) posting_date, 
           SUM( amount ) sum_amt
  FROM w_card_transaction
 WHERE posting_date > TO_DATE( '31-12-2003', 'DD-MM-YYYY' ) AND
       posting_date < TO_DATE( '01-01-2005', 'DD-MM-YYYY' )
 GROUP BY TO_CHAR( posting_date, 'YYYYMM' )
 ORDER BY TO_CHAR( posting_date, 'YYYYMM' );