select t1
scan
    lnSum =  0 
    select t2
    CALCULATE SUM(t2.sumfield) FOR t2.id = t1.id TO lnSum
    select t1
    replace t1.sumfield with t1.sumfield + lnSum 
endscan

* Главная таблица    
CREATE CURSOR tabMain (tabMainID I, Summa N( 18 , 2 ))  
INDEX ON tabMainID TAG tabMainID  
SET ORDER TO  0   
INSERT INTO tabMain (tabMainID, Summa) VALUES ( 1 ,  0 )  
INSERT INTO tabMain (tabMainID, Summa) VALUES ( 2 ,  0 )  
INSERT INTO tabMain (tabMainID, Summa) VALUES ( 3 ,  0 )  
    
* Подчиненная таблица    
CREATE CURSOR tabChild (tabChildID I, tabMainID I, Qty I, Price N( 18 , 2 ))  
INDEX ON tabMainID TAG tabMainID  
SET ORDER TO  0   
INSERT INTO tabChild (tabChildID, tabMainID, Qty, Price) VALUES ( 1 ,  1 ,  10 ,  0 . 25 )  
INSERT INTO tabChild (tabChildID, tabMainID, Qty, Price) VALUES ( 2 ,  2 ,  15 ,  10 . 05 )  
INSERT INTO tabChild (tabChildID, tabMainID, Qty, Price) VALUES ( 3 ,  3 ,  30 ,  5 . 00 )
INSERT INTO tabChild (tabChildID, tabMainID, Qty, Price) VALUES ( 1 ,  1 ,  10 ,  0 . 25 )  
INSERT INTO tabChild (tabChildID, tabMainID, Qty, Price) VALUES ( 2 ,  2 ,  15 ,  10 . 05 )  
INSERT INTO tabChild (tabChildID, tabMainID, Qty, Price) VALUES ( 3 ,  3 ,  30 ,  5 . 00 )

* Собственно запрос

UPDATE tabMain ;  
SET Summa = Summa + tmp.sumPrice ;  
from (select ;
		tabMainID, ;
		Sum(tabChild.Qty*tabChild.Price) as sumPrice ;
	FROM tabChild ;  
	group by tabChild.tabMainID) as tmp ;
WHERE tabMain.tabMainID=tmp.tabMainID