Thor:

There is a fine line between "business rules" (e.g. constraints) and "business logic" (e.g. if user enters something in this field, pop up this message)... It really needs to be determined on a case-by-case and application-by-application basis.  Some apps are, by nature, much more data-driven than others.  However, on the whole I've seen much less dependency on the database for constraints than I think there should be.  One of the primary points of the database is the enforce data integrity, yet most shops prefer to do that all in the application layer and use the database only as a data store.  That, IMO, is a mistake.

As for a way to pass an event from the db to an application, you can look into writing extended stored procedures.  Yet another part of the database that's under-used, IMO!