XXX表中最近一天和最近第三天的日期

SELECT MAX(today) f_day, min(today) l_day
  FROM (select today, row_number() over(order by today desc) rn
          from XXX
         where today between to_char(sysdate - 30, 'yyyy-mm-dd') and
               to_char(sysdate - 1, 'yyyy-mm-dd')
         group by today
        having(count(*)) > 5))
 where rn <= 3
 ORDER BY TODAY)

业务：找寻XXX表中最近一天和最近第三天的日期，当天记录数超过5行才算有效。
例如今天26号，周末没记录。
则结果为：
F_DAY                     L_DAY
------------------------- -------------------------
2013-02-21                2013-02-25