A package named STANDARD defines the PL/SQL environment. The package specification declares public types, variables, exceptions, subprograms, which are available automatically to PL/SQL programs. For example, package STANDARD declares function ABS, which returns the absolute value of its argument, as follows:

FUNCTION ABS (n NUMBER) RETURN NUMBER;

The contents of package STANDARD are directly visible to applications. You need not qualify references to its contents by prefixing the package name. For example, you might invoke ABS from a database trigger, stored subprogram, Oracle tool, or 3GL application, as follows:

abs_diff := ABS(x - y);

If you declare your own version of ABS, your local declaration overrides the public declaration. You can still invoke the SQL function by specifying its full name:

abs_diff := STANDARD.ABS(x - y);

Most SQL functions are overloaded. For example, package STANDARD contains these declarations:

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;

PL/SQL resolves an invocation of TO_CHAR by matching the number and data types of the formal and actual parameters.