Как да разделя интервала между две дати на детайли по месеци?

На Oracle 12c cross apply може да се използва клауза:

create table e_vents(
 name varchar2(10),
 startdate date,
 enddate date
);

insert all 
into e_vents values( 'A', date '2017-12-15', date '2018-01-17' )
into e_vents values( 'B', date '2017-12-15', date '2017-12-22' )
into e_vents values( 'C', date '2017-12-15', date '2018-05-22' )
select null from dual;

commit;

select e.name,
       case when e.startdate > x.s_date then e.startdate else x.s_date end as start_date,
       case when e.enddate < x.e_date then e.enddate else x.e_date end as end_date
from e_vents e
cross apply (
  select 
         trunc( e.startdate, 'mm') + (level-1) * interval '1' month as s_date,
         trunc( e.startdate + (level) * interval '1' month, 'mm') -1 as e_date 
  from dual
  connect by level <= months_between( trunc( e.enddate, 'mm'),trunc( e.startdate, 'mm')) + 1
) x

NAME       START_DATE END_DATE        
---------- ---------- ----------
A          2017-12-15 2017-12-31
A          2018-01-01 2018-01-17
B          2017-12-15 2017-12-22
C          2017-12-15 2017-12-31
C          2018-01-01 2018-01-31
C          2018-02-01 2018-02-28
C          2018-03-01 2018-03-31
C          2018-04-01 2018-04-30
C          2018-05-01 2018-05-22

9 rows selected.