SQL in a Nutshell, 2nd Edition [Electronic resources] نسخه متنی

Chapter 4. SQL Functions

A function
is a special type of command word in the SQL command set, and each
SQL dialect varies in its implementation of the command set. In
effect, functions are one-word commands that return single values.
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, such as the
function that returns the current system time,
CURRENT_TIME.

The ANSI standard supports a number of useful functions. This chapter
covers those functions, providing detailed descriptions and examples
for each platform. In addition, each database maintains a long list
of their own internal functions that are outside of the scope of the
standard SQL. This chapter provides parameters and descriptions for
each database implementation's internal functions.

In addition, most database platforms support the ability to create
user-defined functions (UDFs). For more information on UDFs, refer to
the CREATE/ALTER FUNCTION/PROCEDURE Statements.