Syntax:
CREATE [AGGREGATE] FUNCTION function_name RETURNS {STRING|INTEGER|REAL|DECIMAL}
    SONAME shared_library_name

A user-defined function (UDF) is a way to extend MySQL with a new
function that works like a native (built-in) MySQL function such as
ABS() or CONCAT().

function_name is the name that should be used in SQL statements to
invoke the function. The RETURNS clause indicates the type of the
function's return value. DECIMAL is a legal value after RETURNS, but
currently DECIMAL functions return string values and should be written
like STRING functions.

shared_library_name is the basename of the shared object file that
contains the code that implements the function. The file must be
located in the plugin directory. This directory is given by the value
of the plugin_dir system variable.

*Note*: This is a change in MySQL 5.1. For earlier versions of MySQL,
the shared object can be located in any directory that is searched by
your system's dynamic linker. For more information, see
http://dev.mysql.com/doc/refman/5.1/en/udf-compiling.html.

To create a function, you must have the INSERT privilege for the mysql
database. This is necessary because CREATE FUNCTION adds a row to the
mysql.func system table that records the function's name, type, and
shared library name. If you do not have this table, you should run the
mysql_upgrade command to create it. See
http://dev.mysql.com/doc/refman/5.1/en/mysql-upgrade.html.

URL: http://dev.mysql.com/doc/refman/5.1/en/create-function-udf.html

