SUM a grouped field

I need to SUM the contents of a column which is already worked out using GROUP BYs.. How exactly would you go about that?

The group should be based on the user name, not the entire contents of the result set. I believe this essentially a group by on that username field, but that i believe would break how the query currently works..

Example below:

   SELECT A1.USERNAME, DATE_FORMAT(FROM_UNIXTIME(A1.TIME_STAMP),'%Y-%m-%d') AS DTTM, A1.ACCTSESSIONID,
          MAX(IFNULL(A1.ACCTINPUTGW,0)  * POW(2,32) + IFNULL(A1.ACC开发者_运维技巧TINPUTOCT, 0)) - MAX(IFNULL(A2.ACCTINPUTGW,0)  * POW(2,32) + IFNULL(A2.ACCTINPUTOCT, 0)) as TOTAL_UPLOAD,
          MAX(IFNULL(A1.ACCTOUTPUTGW,0) * POW(2,32) + IFNULL(A1.ACCTOUTPUTOCT, 0)) - MAX(IFNULL(A2.ACCTOUTPUTGW,0) * POW(2,32) + IFNULL(A2.ACCTOUTPUTOCT, 0)) as TOTAL_DOWNLOAD

     FROM ACCOUNTING A1   

LEFT JOIN ACCOUNTING A2
       ON A1.ACCTSESSIONID = A2.ACCTSESSIONID
      AND DATE_FORMAT(FROM_UNIXTIME(A2.TIME_STAMP), '%Y-%m-%d') = '2011-07-04'   

    WHERE DATE_FORMAT(FROM_UNIXTIME(A1.TIME_STAMP), '%Y-%m-%d') = '2011-07-05'   
 GROUP BY A1.ACCTSESSIONID,A2.ACCTSESSIONID
 ORDER BY A1.USERNAME

Edit:

The columns would be: TOTAL_DOWNLOAD and TOTAL_UPLOAD

Thanks @ ypercube, worked a treat

SELECT A3.USERNAME
     , SUM(A3.TOTAL_UPLOAD) AS FINAL_UPLOAD
     , SUM(A3.TOTAL_DOWNLOAD) AS FINAL_DOWNLOAD
FROM 
  ( SELECT
        A1.USERNAME
      , DATE_FORMAT(FROM_UNIXTIME(A1.TIME_STAMP),'%Y-%m-%d') AS DTTM
      , A1.ACCTSESSIONID
      , MAX(IFNULL(A1.ACCTINPUTGW,0) * POW(2,32) + IFNULL(A1.ACCTINPUTOCT, 0))
        - MAX(IFNULL(A2.ACCTINPUTGW,0) * POW(2,32) + IFNULL(A2.ACCTINPUTOCT, 0))
        AS TOTAL_UPLOAD
      , MAX(IFNULL(A1.ACCTOUTPUTGW,0) * POW(2,32) + IFNULL(A1.ACCTOUTPUTOCT, 0))
        - MAX(IFNULL(A2.ACCTOUTPUTGW,0) * POW(2,32) + IFNULL(A2.ACCTOUTPUTOCT, 0))
        AS TOTAL_DOWNLOAD             
    FROM ACCOUNTING A1   
      LEFT JOIN ACCOUNTING A2
        ON A1.ACCTSESSIONID = A2.ACCTSESSIONID
        AND DATE_FORMAT(FROM_UNIXTIME(A2.TIME_STAMP), '%Y-%m-%d') = '2011-07-04'   
    WHERE DATE_FORMAT(FROM_UNIXTIME(A1.TIME_STAMP), '%Y-%m-%d') = '2011-07-05'   
    GROUP BY A1.ACCTSESSIONID,A2.ACCTSESSIONID
    ORDER BY A1.USERNAME
  ) AS A3    
GROUP BY A3.USERNAME