Agrupar registros hora por hora o día por día y llenar los espacios con cero o nulo

prueba:
primera consulta (por hora):

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');

segunda consulta (por día):

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');

tercera consulta (por mes):

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');