A UDR can either return values or not, as follows:
A user-defined function returns
one or more values and therefore can be used in SQL expressions.
Use the CREATE FUNCTION statement to
register the UDR in the system catalog
tables.
A user-defined procedure is
a routine that does not return any values. You cannot use a procedure
in SQL expressions because it does not return a value.
Use the CREATE PROCEDURE statement to
register the UDR in the system catalog
tables.