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(mysql.info)CREATE FUNCTION

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`CREATE FUNCTION/DROP FUNCTION' Syntax
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     CREATE [AGGREGATE] FUNCTION function_name RETURNS {STRING|REAL|INTEGER}
            SONAME shared_library_name
     
     DROP FUNCTION function_name

A user-definable function (UDF) is a way to extend MySQL with a new
function that works like native (built in) MySQL functions such as
`ABS()' and `CONCAT()'.

`AGGREGATE' is a new option for MySQL Version 3.23.  An `AGGREGATE'
function works exactly like a native MySQL `GROUP' function like `SUM'
or `COUNT()'.

`CREATE FUNCTION' saves the function's name, type, and shared library
name in the `mysql.func' system table.  You must have the *insert* and
*delete* privileges for the `mysql' database to create and drop
functions.

All active functions are reloaded each time the server starts, unless
you start `mysqld' with the `--skip-grant-tables' option.  In this
case, UDF initialization is skipped and UDFs are unavailable.  (An
active function is one that has been loaded with `CREATE FUNCTION' and
not removed with `DROP FUNCTION'.)

For instructions on writing user-definable functions, see Note: Adding
functions.  For the UDF mechanism to work, functions must be written
in C or C++, your operating system must support dynamic loading and you
must have compiled `mysqld' dynamically (not statically).