Trigger process.

Conceptually a database is a "server" and not a "client" so you cannot (AFAIK - and I'm always willing to be educated!) send an outbound message from the database to Appian. Assuming this is true, your next best option is to poll the database on a periodic basis. This would require a process model that would be triggered by a timer. Every time that timer triggered the process would see if there were any new items in the database (you'd need to provide a means by which to track which items were "new" and which had already been processed).

If there were new items, these would then be passed to a separate process model for processing. Once the items had been processed you would then update them in the database as having been processed so they would be excluded the next time the polling process model ran.

The key consideration here is how time-critical processing the new items is. This then determines how often your polling process model is triggered.