if you try to reference a variable before you declare it because PL/SQL requires an identifier be declared before we use it in our code

  
SQL>
SQL>
SQL> declare
  2    myNumber number := another_number;
  3    another_number number := 10;
  4  begin
  5    null;
  6  end;
  7  /
  myNumber number := another_number;
                     *
ERROR at line 2:
ORA-06550: line 2, column 22:
PLS-00320: the declaration of the type of this expression is incomplete or malformed
ORA-06550: line 2, column 12:
PL/SQL: Item ignored


SQL>
SQL>
SQL> declare
  2    another_number number := 10;
  3    myNumber number := another_number;
  4  begin
  5    null;
  6  end;
  7  /

PL/SQL procedure successfully completed.

SQL>
SQL>

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5.	This type is local to this block
6.	Reference Type in another block as well
7.	variable scope with nested block
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if you try to reference a variable before you declare it because PL/SQL requires an identifier be declared before we use it in our code : Variable Scope « PL SQL « Oracle PL / SQL

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