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table account_stuff
account_id int,
...
host_key varchar(huge),
open_date date,
close_date date

- For each input file that's loaded       
  - Check it's list of primary keys against the account_stuff
...
    - If there's not a match, add a row.
  - Check the list of account_stuff 'source_file's host_keys against t
+he imported file.
    - If there are missing rows, update with a close_date