* 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)

* Для версии младше Visual FoxPro 9
SELECT tabMain
REPLACE FOR SEEK(tabMain.tabMainID,"tabChild","tabMainID") ;
    Summa WITH tabChild.Qty*tabChild.Price
* Или через команду UPDATE
UPDATE tabMain SET Summa = tabChild.Qty*tabChild.Price ;
    WHERE SEEK(tabMain.tabMainID,"tabChild","tabMainID") 
* Для Visual FoxPro 9
UPDATE tabMain ;
SET Summa =tabChild.Qty*tabChild.Price ;
FROM tabChild ;
WHERE tabMain.tabMainID=tabChild.tabMainID