SQL Functions
http://www.oreilly.com/catalog/sqlnut/chapter/ch04.html

Kline, Kevin and Daniel Kline
O'Reilly and Associates
2001

Abstract:

A function is a special type of command word in the SQL99 command set. In effect, functions are one-word commands that return a single value. The value of a function can be determined by input parameters, as with a function that averages a list of database values. But many functions do not use any type of input parameter.