Chapter 9. Guaranteeing Complex Relationships with Triggers

In this chapter, we’ll go over an advanced technique for mapping tricky real-world referential integrity constraints to the data layer. This technique uses database triggers, which allow you to run arbitrary code to check that new data is valid before inserting or updating rows. We’ll write a stored procedure that ensures an auditorium isn’t double-booked and create a trigger that runs our procedure whenever a movie showtime is added or changed. I’ll describe the basic structure of a function in Postgtres’s procedural language PL/pgSQL, so you can start building your own functions for a variety of uses, and cover the special properties of trigger functions.