Subselect

SELECT ALL a.AKF_KTO_FLV_NR,

           a.AKF_KTO_JAHR,

           a.AKF_KTO_VKZF,

           sum (a.AKF_AKTUELLER_BETRAG) 

                 + (SELECT sum (b.AKF_AKTUELLER_BETRAG)

                      FROM AKFINANZ b

                     WHERE b.AKF_KTO_JAHR = 2007

                       AND (b.AKF_FINANZIERUNGSART = 29 

                               and b.AKF_FINANZIER = 2)

                       AND b.AKF_KTO_VKZF = a.AKF_KTO_VKZF

                    ) as Eigenleistung

  FROM AKFINANZ a

 WHERE a.AKF_KTO_JAHR = 2007

   AND (a.AKF_FINANZIERUNGSART = 29 AND a.AKF_FINANZIER = 1)

   AND a.AKF_KTO_VKZF = 586031

GROUP BY a.AKF_KTO_FLV_NR,

         a.AKF_KTO_JAHR,

         a.AKF_KTO_VKZF