Data Engineering

Sometimes called journal or audit tables. _A tables do the following magic trick: you can't screw up or delete your data in a way you can't recover.

In the most simple version possible you take your table foo, duplicate it's structure in a table named foo_A and add 2 columns: audit_dt and audit_user_id. Then you create triggers for update and deletes on the table foo to first write the old values as a new insert in the foo_A table.

Now even if you screw up your select and delete all of the contents of table foo. everything will still be in table foo_A. If you accidentally overwrite everything in foo with garbage data, the good data will still be in foo_A

The application nor any of the users need to know about the _A tables (unless you want to leverage stored procedures instead of triggers to create the _A table entries)