Из хелпа по SetSQLSelect:

If the new SELECT statement has a different table name in the FROM clause and the DataWindow object is updatable, then PowerBuilder must change the update information for the DataWindow object. PowerBuilder assumes the key columns are in the same positions as in the original definition. The following conditions will make the DataWindow not updatable:

· There is more than one table in the FROM clause.
· A DataWindow update column is a computed column in the SELECT statement.

If changing the SELECT statement makes the DataWindow object not updatable, the DataWindow control cannot execute an Update method call for the DataWindow object in the future.