is something like this possible? I get an error, but I thought it should have worked:

UPDATE EmpItems 
    LEFT JOIN HistItems ON HistItems.eiKey = EmpItems.eiKey
    WHERE HistItems.DelFlag = 0 
    SET EmpItems.itOnPosted = 1, EmpItems.itBalAmt = HistItems.itBalNew + HistItems.itPostCalc

Maybe I'm doing something wrong?