DECLARE
   v_pub_date_cnt   NUMBER;
   v_pub_date_min   erp_booking_hist.publish_date%TYPE;
BEGIN
   BEGIN
      SELECT   COUNT (DISTINCT publish_date), MIN (publish_date)
        INTO   v_pub_date_cnt, v_pub_date_min
        FROM   erp_booking_hist;
   EXCEPTION
      WHEN OTHERS
      THEN
         v_pub_date_cnt := 0;
   END;

   IF v_pub_date_cnt > 5
   THEN
      DELETE FROM   erp_booking_hist
            WHERE   publish_date = v_pub_date_min;

      COMMIT;
   END IF;
END;

DELETE FROM erp_booking_hist
WHERE  publish_date = (SELECT min_publish_date
                       FROM   (SELECT MIN(publish_date) min_publish_date,
                                      COUNT(publish_date)
                               FROM   erp_booking_hist
                               HAVING COUNT(publish_date) > 5))

/  

SELECT min_publish_date
FROM   (SELECT MIN(publish_date) min_publish_date,
               COUNT(publish_date)
        FROM   erp_booking_hist
        HAVING COUNT(publish_date) > 5)
/