Triggers

Triggers are procedural code that the database runs automatically when something changes. Insert a row, update a column, delete a record: if a trigger is attached to the table, it fires on its own, without the application asking it to. That sounds magical at first, and then uncomfortable once you realize what it means. A trigger is logic that runs without being called. The people writing the INSERT have no way to know it's there unless they go looking.

This chapter covers how triggers work, the patterns they're genuinely good at, and the traps that have made experienced engineers allergic to them. "Should I use a trigger for this?" is one of those questions where the right answer is almost always "explain the trade-offs," and that's exactly what interviewers are listening for. We'll build an audit logging trigger at the end and then talk honestly about what breaks at scale.