title: Show message box in PL/SQL procedure
body: <p>I have the following procedure, </p>
  
  <pre><code>CREATE OR REPLACE PROCEDURE Add_Shipment_Method(
  shipment_method_id_p IN NUMBER,
  shipment_description_p IN VARCHAR2)
  AS
  BEGIN
  INSERT INTO shipment_method 
      SELECT shipment_method_id_p, shipment_description_p
      FROM dual
      WHERE NOT EXISTS (SELECT * FROM SHIPMENT_METHOD WHERE SHIPMENT_METHOD_ID = shipment_method_id_p AND SHIPMENT_DESCRIPTION = shipment_description_p);
  COMMIT;
  EXCEPTION WHEN
  OTHERS THEN
  ROLLBACK;
  END;
  /
  </code></pre>
  
  <p>The procedure generally allows adding a new shipment method into the table shipment_method when the row doesn't exist. 
  I want to modify it and show a message box when the primary key 'shipment_method_id' does exist, and allows to write the row when it doesn't exist.
  Any suggestions?</p>