Copy One Field values into Another Field

static void CopyOneFieldToAnotherField(Args _args)

{
    TableA  t1;
    ;
    ttsbegin;
    while select forupdate t1 where t1.Num == t1.num
    {
         t1.SerialNumberFormat = t1.num;
         t1.Num = "";
         t1.update();
    }
    ttscommit;
}