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you would not UPDATE amounts in a table to track this data, since that doesn't provide an accounting trail should a programming mistake, duplicate entry, or nafarious activity alter a value. these are derived amounts that you would calculate when needed. you would have an invoice_accounting (or similar named) table, that gets a row inserted for every amount that affects an invoice amount. you would query when needed (a UNION between the invoice(s) table and the invoice accounting table) to determine the current total +/-/0 for any invoice.