The package named STANDARD defines the PL/SQL atmosphere. The package specification globally declares the exceptions, types, and subprograms that are available automatically to every PL/SQL program. For illustration, the package STANDARD declares the built-in function below named as the ABS that returns the absolute value of its argument as:
FUNCTION ABS (n NUMBER) RETURN NUMBER;
The contents of the package STANDARD are directly noticeable to the applications. Therefore, you can call the ABS from stored subprograms, database triggers, 3GL applications, and different Oracle tools.
If you re-declare the ABS in a PL/SQL program, your local declaration overrides the global declaration. Though, you can still call the built-in function by using the dot notation, as shown below:
... STANDARD.ABS(x) ...
Most of the built-in functions are overloaded. For illustration, the package STANDARD contains the declarations a shown below:
FUNCTION TO_CHAR (right DATE) RETURN VARCHAR2;
FUNCTION TO_CHAR (left NUMBER) RETURN VARCHAR2;
FUNCTION TO_CHAR (left DATE, right VARCHAR2) RETURN VARCHAR2;
FUNCTION TO_CHAR (left NUMBER, right VARCHAR2) RETURN VARCHAR2;
The PL/SQL solves the call to the TO_CHAR by matching the number and datatypes of the formal and real parameters.