Hibernate中的数据库更新

Question

 SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
             session =sessionFactory.openSession();                 
             org.hibernate.Transaction tx = session.beginTransaction();
             
             String newName="",oldName="";
             newName=sname;
             oldName=ds_name;
             String hqlUpdate = "update Graduate  set DS_ID = (select DS_ID from Teacher where DS_NAME=:newName )where SNAME = :oldName";
             
             System.out.println(hqlUpdate);
             int updatedEntities = session.createQuery( hqlUpdate )
                         .setString( "newName", newName )
                         .setString( "oldName", oldName ).executeUpdate();             System.out.println("UserDemo object persisted to the database........");
             tx.commit();
             session.flush();
             session.close(); }catch(Exception e){
e.getStackTrace();
System.out.println("Error " + e.getMessage()); }为什么没有执行更新
报一个异常