Invoices are a good example of the problem not having a simple solution. You want sequential numbering, so that Invoice 1001 clearly follows on immediately from Invoice 1000. If invoice 1000 can't be located by the issuer, it's missing for some reason. But the autonumber ability to increment each record by one and not more than one, and to ensure that invoice numbers don't duplicate, would be useful. There seems to be no data type that meets all the requirements together, and doesn't artificially inflate ID numbers when records are deleted (eg because of data entry error or pasting in a batch of incorrect entries) and replaced.
