>>On many apps, the order number is not typically shown until the order is submitted (or data has been completed keyed) because it is not a valid order until then. Once that is done, then the order number is displayed.
>>
>If this satisfies the user requirements, then the problem is easy to solve. But if we do need to show this field in advance, then I don't see a simple solution.

If you have to be able to show the order number even before the order is submitted, then I would have a auxillary table that contains the order numbers. Basically, this is how it would work:

New order is requested:

1. Get next order number from orderlist table (ordered by ordernum)

FUNCTION GetNextOrderNumber
LOCAL lnNextOrderNum AS Integer
SELECT orderlist
LOCATE FOR DELETED()
IF FOUND()
  RECALL
  lnNextOrderNum = orderlist.ordernum
ELSE
  GOTO BOTTOM
  lnNextOrderNum = orderlist.ordernum + 1
  INSERT INTO orderlist (ordernum) VALUES lnNextOrderNum
ENDIF
RETURN lnNextOrderNum
ENDFUNC

This now gives you a order number that will be unique. If the user cancels the order request, then delete the record from the orderlist table:

FUNCTION DeleteOrderNumber
LPARAMETERS pnOrderNum
DELETE FROM orderlist WHERE ordernum = pnOrderNum
ENDFUNC

If the user commits the order, then the order number is what is used in your order header table and order items table as the key.