CREATE OR REPLACE PACKAGE pkg_test
IS
v_a NUMBER :=  100 ;
PROCEDURE prc_test;
END;
/

CREATE OR REPLACE PACKAGE BODY pkg_test
IS

PROCEDURE prc_test
IS
v_b NUMBER;
BEGIN
v_b := v_a;
dbms_output.put_line(v_b);
END;
END;
/

begin
pkg_test.prc_test;
END;
/
 100 

CREATE OR REPLACE PACKAGE pkg_test
IS
v_a NUMBER :=  100 ;
PROCEDURE prc_test;
END;
/

CREATE OR REPLACE PACKAGE BODY pkg_test
IS
PROCEDURE prc_test
IS
v_a NUMBER :=  1000 ;
v_b NUMBER;
BEGIN
v_b := v_a;
dbms_output.put_line(v_b);
END;
END;
/
begin
pkg_test.prc_test;
END;
/ 
 1000 

create or replace procedure prc_emp
is
comm NUMBER :=  100000 ;
v_count NUMBER;
begin
SELECT count(*) INTO v_count
FROM emp WHERE sal > comm;
dbms_output.put_line(v_count);
END;
/
SELECT sal, comm from emp
/
SAL COMM 
 23   1  
 800   2  
 1600   2  
 1250   2  
 2975   2  
 1250   2  
 2850   2  
 2450   2  
 3000   2  
 5000   2  
 1500   2  
 1100   2  
 950   2  
 3000   2  
 1300   2  

begin
prc_emp;
end;
/
 15 

update emp
set sal =  900  where empno =  7369 
/
select sal from emp where empno =  7369 
/
 900 

create or replace trigger trg_test
before update on emp
for each row
BEGIN
:new.sal :=  1200 ;
end;

update emp
set sal =  800  where empno =  7369 
/
select sal from emp where empno =  7369 
/
 1200 

create or replace function fnc_emp
return number
is
v_sal number;
begin
select max(sal) into v_sal from emp;
return v_sal;
end;
/
create or replace view viw_test (empno, max_sal)
as select empno, fnc_emp from emp
/
select * from viw_test
/
EMPNO MAX_SAL 
 7369   5000  
 7499   5000  
 7521   5000  
 7566   5000  
 7654   5000  
 7698   5000  
 7782   5000  
 7788   5000  
 7839   5000  
 7844   5000  
 7876   5000  
 7900   5000  
 7902   5000  
 7934   5000  
 8888   5000  

create or replace function fnc_emp
return number
is
v_sal number;
begin
select min(sal) into v_sal from emp;
return v_sal;
end;
/

select * from viw_test
/
EMPNO MAX_SAL 
 7369   23  
 7499   23  
 7521   23  
 7566   23  
 7654   23  
 7698   23  
 7782   23  
 7788   23  
 7839   23  
 7844   23  
 7876   23  
 7900   23  
 7902   23  
 7934   23  
 8888   23  

CREATE TABLE employees2 AS SELECT last_name FROM employees;
<<main>>
DECLARE
   last_name VARCHAR2( 10 ) := 'King';
   v_last_name VARCHAR2( 10 ) := 'King';
BEGIN
-- deletes everyone, because both LAST_NAMEs refer to the column
   DELETE FROM employees2 WHERE last_name = last_name;
   DBMS_OUTPUT.PUT_LINE('Deleted ' || SQL%ROWCOUNT || ' rows.');
   ROLLBACK;
-- OK, column and variable have different names
   DELETE FROM employees2 WHERE last_name = v_last_name;
   DBMS_OUTPUT.PUT_LINE('Deleted ' || SQL%ROWCOUNT || ' rows.');
   ROLLBACK;
-- OK, block name specifies that 2nd last_name is a variable
   DELETE FROM employees2 WHERE last_name = main.last_name;
   DBMS_OUTPUT.PUT_LINE('Deleted ' || SQL%ROWCOUNT || ' rows.');
   ROLLBACK;
END;
/

DECLARE
   FUNCTION dept_name (department_id IN NUMBER)
      RETURN departments.department_name%TYPE
   IS
      department_name departments.department_name%TYPE;
   BEGIN
-- DEPT_NAME.department_name specifies the local variable
-- instead of the table column
      SELECT department_name INTO dept_name.department_name
         FROM departments
         WHERE department_id = dept_name.department_id;
      RETURN department_name;
   END;
BEGIN
   FOR item IN (SELECT department_id FROM departments)
   LOOP
      DBMS_OUTPUT.PUT_LINE('Department: ' || dept_name(item.department_id));
   END LOOP;
END;
/

SQL> set serveroutput on ;
SQL> 
SQL> declare
   2     a integer :=  1  ;
   3     b integer :=  2  ;
   4     c integer ;
   5   begin
   6     execute immediate 'select count(*) into :c from dual where :a = :a' into c using a, b ;
   7     dbms_output.put_line ( 'count = ' || c ) ;
   8   end ;
   9   /

count =  0 

PL/SQL procedure successfully completed

EXECUTE IMMEDIATE sql_stmt USING a, b, b, a;

sql_stmt := 'INSERT INTO payroll VALUES (:x, :x, :y, :x)';

you might code the corresponding USING clause as follows:

EXECUTE IMMEDIATE sql_stmt USING :х = a, :у = b;

DECLARE
   a NUMBER :=  4 ;
   b NUMBER :=  7 ;
BEGIN
   plsql_block := 'BEGIN calc_stats(:x, :x, :y, :x); END;'
   EXECUTE IMMEDIATE plsql_block USING a, b;
   ...
END;

declare
      a integer :=  1  ;
      b integer :=  2  ;
      c integer ;
    begin
      execute immediate '
           declare a integer := :a;
                   b integer := :a;
           begin 
           select count(*) into :c from dual where a = b;
           end; ' USING IN a, IN b, OUT c;
      dbms_output.put_line ( 'count = ' || c ) ;
    end ;
/
ORA- 01006 : bind variable does not exist 

declare
      a integer :=  1  ;
      b integer :=  2  ;
      c integer ;
    begin
      execute immediate '
           declare a integer := :a;
                   b integer := :b;
           begin 
           select count(*) into :c from dual where a = b;
           end; ' USING IN a, IN b, OUT c;
      dbms_output.put_line ( 'count = ' || c ) ;
    end ;
/
count =  0 

declare
      a integer :=  1  ;
      b integer :=  2  ;
      c integer ;
    begin
      execute immediate '
           declare a integer := :a;
                   b integer := :a;
           begin 
           select count(*) into :c from dual where a = b;
           end; ' USING IN a, OUT c;
      dbms_output.put_line ( 'count = ' || c ) ;
    end ;
/
count =  1 

declare
      a integer :=  1  ;
      b integer :=  2  ;
      c integer ;
    begin
      execute immediate '
           declare a integer := :a;
                   b integer := :b;
           begin 
           select count(*) into :c from dual where a > b;
           end; ' USING IN a, IN b, OUT c;
      dbms_output.put_line ( 'count = ' || c ) ;
    end ;

declare
      a integer :=  1  ;
      b integer :=  2  ;
      c integer ;
    begin
      execute immediate '
           declare a integer := :b;
                   b integer := :a;
           begin 
           select count(*) into :c from dual where a > b;
           end; ' USING IN a, IN b, OUT c;
      dbms_output.put_line ( 'count = ' || c ) ;
    end ;