How to run a query in PL/SQL with Serializable transaction isolation I want to run this SQL in Oracle 11g:

declare a number;
  begin 
  select count(*) into a  from item w where w.Item_num='MOH601' ; 
  dbms_output.put_line(a);
  end ; 


How to run this query with isolation Serializable? As I have learnt before, Oracle will lock the rows with w.Item_num='MOH601' when using Serializable.
Alter the session:

declare a number;
  begin 
    EXECUTE IMMEDIATE 'ALTER SESSION SET ISOLATION_LEVEL SERIALIZABLE';
    select count(*) into a  from item w where w.Item_num='MOH601' ; 
    dbms_output.put_line(a);
  end ;