yes, >customers< is loaded before >sales< . i edited the question to make that more clear.
your proposal worked well, thank you.
the load of the >customers< list will be restricted by area, containing only users that match the area.
other users needed to be loaded later on, f.e. with outer join load.the
i only used where not exists before for incremental loads
where NOT exists (change_date)
WHERE EXISTS (subarea_code) worked fine.