Business logic. Sometimes it lives in the application, sometimes it lives in the database. I'll save the application/db argument for each individual case that I encounter. But if you do have significant business logic in your database, there will probably come a time when someone wants to incorporate some of that logic into a query they are writing.
I am often asked to improve the speed of queries that have User Defined Functions (UDF) in the where clause. Sometimes I am successful at tuning them, sometimes a major structural or logic change is required to get good performance.
