28.4.1 Features of the User-Defined Function Interface
The MySQL interface for user-defined functions provides the following features and capabilities:
Functions can return string, integer, or real values and can accept arguments of those same types.
You can define simple functions that operate on a single row at a time, or aggregate functions that operate on groups of rows.
Information is provided to functions that enables them to check the number, types, and names of the arguments passed to them.
You can tell MySQL to coerce arguments to a given type before passing them to a function.
You can indicate that a function returns
NULLor that an error occurred.