PL/SQL User's Guide and Reference

Scope and Visibility

References to an identifier are resolved according to its scope and visibility. The scope of an identifier is that region of a program unit (block, subprogram, or package) from which you can reference the identifier. An identifier is visible only in the regions from which you can reference the identifier using an unqualified name. Figure 2 - 2 shows the scope and visibility of a variable named x, which is declared in an enclosing block, then redeclared in a sub-block.

Figure 2 - 2. Scope and Visibility

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 redeclared in a sub-block, both identifiers remain in scope. Within the sub-block, however, only the local identifier is visible because you must use a qualified name to reference the global identifier.

Although you cannot declare an identifier twice in the same block, you can declare the same identifier in two different blocks. The two objects represented by the identifier are distinct, and any change in one does not affect the other.

However, a block cannot reference identifiers declared in other blocks nested at the same level because those identifiers are neither local nor global to the block. The following example illustrates the scope rules:

DECLARE
   a CHAR;
   b REAL;
BEGIN
   -- identifiers available here: a (CHAR), b
   DECLARE
      a INTEGER;
      c REAL;
   BEGIN
      -- identifiers available here: a (INTEGER), b, c
   END;
   DECLARE
      d REAL;
   BEGIN
      -- identifiers available here: a (CHAR), b, d
   END;
   -- identifiers available here: a (CHAR), b
END;

Recall that global identifiers can be redeclared in a sub-block, in which 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 enclosing block, as the following example shows:

<<outer>>
DECLARE
   birthdate DATE;
BEGIN
   DECLARE
      birthdate DATE;
   BEGIN
      ...
      IF birthdate = outer.birthdate THEN ...

As the next example shows, the qualifier can also be the name of an enclosing subprogram:

PROCEDURE check_credit (...) IS
   rating NUMBER;
   FUNCTION valid (...) RETURN BOOLEAN IS
      rating NUMBER;
   BEGIN
      ...
      IF check_credit.rating < 3 THEN ...

However, within the same scope, a label and a subprogram cannot have the same name.