按小时或一天一天分组记录小时，填补空白为零或为空(Grouping records hour by

我写了按小时计算的记录小时的查询：

select TO_CHAR(copied_timestamp, 'YYYY-MM-DD HH24'),count(*) from req group by
TO_CHAR(copied_timestamp, 'YYYY-MM-DD HH24');

其结果是：

2012-02-22 13    2280
2012-02-22 15    1250
2012-02-22 16    1245
2012-02-22 19    1258

但我需要一个像这样的结果：

2012-02-22 13    2280
2012-02-22 14    0
2012-02-22 15    1250
2012-02-22 16    1245
2012-02-22 17    0
2012-02-22 18    0
2012-02-22 19    1258

我也有这些疑问的日和月太那个组！

select TO_CHAR(copied_timestamp, 'YYYY-MM-DD'),count(*)  from req
group by TO_CHAR(copied_timestamp, 'YYYY-MM-DD');

select TO_CHAR(copied_timestamp, 'YYYY-MM'),count(*)  from req
group by TO_CHAR(copied_timestamp, 'YYYY-MM');

我需要他们的空白需要填补为零或为空过。任何帮助非常感谢。

尝试：
第一查询（由小时）：

with t as (
  select mnd + ((level-1)/24) ddd
  from
  (select trunc(min(copied_timestamp),'hh') mnd, trunc(max(copied_timestamp),'hh') mxd from req) v
  connect by mnd + ((level-1)/24) <= mxd
  )
select to_char(trunc(d1, 'hh'), 'yyyy-mm-dd hh24'), count(d2) from 
(select nvl(copied_timestamp, ddd) d1, copied_timestamp d2 from req right outer join (
  select ddd from t) ad on ddd = trunc(copied_timestamp, 'hh'))
group by trunc(d1, 'hh');

第二查询（按天）：

with t as (
      select mnd + level-1 ddd
      from
      (select trunc(min(copied_timestamp),'dd') mnd, trunc(max(copied_timestamp),'dd') mxd from req) v
      connect by mnd + level-1 <= mxd
      )
    select to_char(trunc(d1, 'dd'), 'yyyy-mm-dd'), count(d2) from 
    (select nvl(copied_timestamp, ddd) d1, copied_timestamp d2 from req right outer join (
      select ddd from t) ad on ddd = trunc(copied_timestamp, 'dd'))
    group by trunc(d1, 'dd');

第三个查询（每月）：

with t as (
      select add_months(mnd, level-1) ddd
      from
      (select trunc(min(copied_timestamp),'mm') mnd, trunc(max(copied_timestamp),'mm') mxd from req) v
      connect by add_months(mnd, level-1) <= mxd
      )
    select to_char(trunc(d1, 'mm'), 'yyyy-mm'), count(d2) from 
    (select nvl(copied_timestamp, ddd) d1, copied_timestamp d2 from req right outer join (
      select ddd from t) ad on ddd = trunc(copied_timestamp, 'mm'))
    group by trunc(d1, 'mm');