select mnu_code from lu_mnu_code where mnu_code = :mnu_code

DECLARE
   l_exist        CHAR (1);
   l_code_change   CHAR (1);
BEGIN
   SELECT CASE
             WHEN EXISTS (SELECT *
                            FROM lu_mnu_code
                           WHERE NVL (mnu_code, 'null') = :mnu_code
                         ) THEN 'Y'
             ELSE 'N'
          END
     INTO l_exist
     FROM DUAL;

   IF l_exist = 'Y' THEN
      RETURN FALSE;
   ELSE
      RETURN TRUE;
   END IF;
END;