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Scope and VisibilityThe References to an identifier are resolved according to its visibility and scope. The scope of an identifier is that area of a program unit (subprogram, block, or package) from which you can reference the identifier. An identifier is visible only in the areas from which you can reference the identifier using an unqualified name. The Figure shows the visibility and scope of a variable named x that is declared in an enclosing block, and then re-declared in a sub-block.The Identifiers declared in a PL/SQL block are considered local to that block and global to all its sub-blocks. If a global identifier is re-declared in a sub-block, both the identifiers remain in the scope. Within the sub-block, though, only the local identifier is visible as you must use a qualified name to reference the global identifier.Though you cannot declare an identifier twice in the similar block, you can declare the same identifier in two various blocks. The two items represented by the identifier are discrete, and any change in one does not affect the other. Though, a block cannot reference the identifiers declared in other blocks at the similar level as these identifiers are neither global nor local to the block.
Figure: Scope and Visibility
The example below describes the scope rules. Note that the identifiers declared in one sub-block cannot be referenced in another sub-block. That is because the block cannot reference the identifiers declared in another blocks nested at similar level.DECLAREa CHAR;b REAL;BEGIN-- identifiers available here: a (CHAR), bDECLAREa INTEGER;c REAL;BEGIN-- identifiers available here: a (INTEGER), b, cEND;DECLAREd REAL;BEGIN-- identifiers available here: a (CHAR), b, dEND;-- identifiers available here: a (CHAR), bEND;Remember that the global identifiers can be re-declared in a sub-block, in that case the local declaration prevails and the sub-block cannot reference the global identifier unless you use a qualified name. The qualifier can be the label of an enclose block, as the example below shows:<>DECLAREbirthdate DATE;BEGINDECLAREbirthdate DATE;BEGIN...IF birthdate = outer.birthdate THEN...As the next illustration shown below, the qualifier can also be the name of an enclosing subprogram:PROCEDURE check_credit (...) ISrating NUMBER;FUNCTION valid (...) RETURN BOOLEAN ISrating NUMBER;BEGIN...IF check_credit.rating < 3 THEN...Though, within the same scope, a label and a subprogram cannot have the similar name.
Package STANDARD The package named STANDARD defines the PL/SQL atmosphere. The package specification globally declares the exceptions, types, and subprograms that are available
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