"unlike most database objects, when you invoke a user-defined function, you must always preface the name of the function with the"

Unlike most database objects, when you invoke a user-defined function, you must always preface the name of the function with the schema name.
A schema is a collection of database objects in SQL (collection of tables) associated with one particular database username (called the schema name).

Database uses the approach of programming while both the data and the processing instructions are stored in the database.

Answermc escher,;