So habe ich es nun gemacht
Code:
update DATEI_D set d_wert =COALESCE((                               
              SELECT SUM(CASE WHEN a.f1='J' then b.f2 else a.f2 end)
  FROM Datei_A a LEFT OUTER JOIN Datei_b b ON                       
               a.k1 = b.k1 and a.k2 = b.k2 and ...                  
                 LEFT OUTER JOIN Datei_c c ON                       
               a.k1 = c.k1 and ...                                  
 WHERE a.Fx = 1 and a.Fy  = 0    AND a.Fz = 0 and c.F4 = D.F4       
                and a.Fxx = 2019 AND d.Fx= 201901                   
 GROUP BY c.F4                     ), 0)                            
WHERE d.fx= 201901 AND d.fy =201901