Triggers — what are they and when should you use them?

A trigger runs a defined function in response to a table event, e.g. INSERT, UPDATE or DELETE. It can run for each row or once per statement, before or after the change.

Example: after a record is updated, I write the old and new values to an audit table. The audit write is part of the same transaction.

To be precise: a trigger can be useful when a rule must apply regardless of which application writes the data.

Pitfall: hidden logic complicates debugging and can increase write cost. For simple integrity I first consider CHECK, UNIQUE and FOREIGN KEY. I don't assume a trigger can safely make an external HTTP request.