CREATE CURSOR cMovs (iid int autoinc, iAgentID int, yAmount y, dDate d)

RAND(-1)
FOR lnCounter=1 TO 100
  INSERT INTO cMovs (iAgentID, yAmount, dDate) ;
     VALUES ((RAND()*10)-1, RAND()*1000000,date(2005,01,01)+ (RAND()*45))
ENDFOR

CREATE CURSOR cAgent (iid int autoinc, yMonth y, nMonth int,  nYear int)
FOR lnCounter=1 TO 10
  INSERT INTO cAgent (yMonth, nMonth, nYear) ;
   VALUES (0,1,2005)
ENDFOR

* with inefficient solution
UPDATE A ;
  SET yMonth = cMovs.yAmount ;
  FROM cAgent A;
    JOIN (SELECT iAgentID , MONTH(dDate) nMonth , YEAR(dDate) nYear , SUM(yAmount) yAmount ;
                  FROM cMovs group by 1,2,3) cMovs;
         ON  cMovs.iAgentID = A.iID;
         AND cMovs.nMonth   = A.nMonth ;
         AND cMovs.nYear    = A.nYear;
  WHERE A.nMonth = 1 AND A.nYear=2005

BROWSE

* OR YOU OPTIMIZE IT WITH a literal correlation
UPDATE A ;
  SET yMonth = cMovs.yAmount ;
  FROM (SELECT iAgentID, SUM(yAmount) yAmount ;
                  FROM cMovs ;
                  WHERE MONTH(dDate)=1 AND YEAR(dDate)=2005;
                  group by 1) cMovs;
  		JOIN cAgent A;
        ON  cMovs.iAgentID = A.iID;
 		WHERE  A.nMonth = 1 AND A.nYear=2005

BROWSE 

* or use a subquery with a autojoin
* BUT THE cAgent definition is incorrect