A database function is a user-defined function. To create a database function, you define a database procedure with only one output parameter. You call a database function in the same way that you would call a predefined function.
SQL Reference Manual
Name of a Database Function
CREATE FUNCTION Statement (create_function_statement)
Database Function Call (dbfunction_call)
SQL Tutorial, Database Functions
Database Studio,
Creating Database Functions
Using Database Functions
Deleting Database Functions