SQL Functions

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.

Kline, Kevin and Daniel Kline. O'Reilly and Associates (2001). Articles>Information Design>Databases>SQL