Variable scope and Nested Blocks.

Variable scope and Nested Blocks:
Scope:

• The scope of an identifier is that region of a program unit (block, subprogram, or package) from which you can reference the identifier.
• Once the scope of the variable is lost it means that the life of the variable no more.
• All variables loose their scope as soon as the PL/SQL block ends or terminates.
• Oracle automatically release the space upon that variable once its scope is closed.

Concepts of scoping:

• With in the same scope, all identifiers must be unique.
• Even when the data types are differing, variables and parameters cannot share the same name.

Nested blocks:

• PL/SQL blocks can be nested wherever an executable statement is allowed.
• A nested block becomes a statement.
• An exception section can contain nested blocks.

Scope  and visibility diagram:
Ex:   
SQL>DECLARE
            x NUMBER;
         BEGIN
              DECLARE
                 y NUMBER;
              BEGIN
                 y:=x;
              END;
            …
         END;
SQL>DECLARE               
              m NUMBER:=250;
         BEGIN               
              DBMS_OUTPUT.PUT_LINE(‘The value of Outer m:’||m);
              DECLARE                   
                     m NUMBER:=550;
              BEGIN                  
                     DBMS_OUTPUT.PUT_LINE(‘The value of Outer m:’||m);
             END;               
         DBMS_OUTPUT.PUT_LINE(‘The value of Outer m:’||m);          
       END;

SQL>DECLARE               
               m NUMBER:=100;
         BEGIN
              m:=200;
              DBMS_OUTPUT.PUT_LINE(‘The value of Outer m:’||m);
              DECLARE
                   n NUMBER:=300;
                   v_tol NUMBER;
              BEGIN
                   m:=500;
                   n:=600;
                   v_tol:=m+n;
                  DBMS_OUTPUT.PUT_LINE(‘The sum of m,n is:’||v_tol);
             END;
           DBMS_OUTPUT.PUT_LINE(‘The value of Outer m:’||m) 
            END;