with tmp as
 (select date '2017-1-2' as begin_date, date '2017-1-8' as end_date
    from dual
  union all
  select date '2017-1-2' as begin_date, date '2017-1-4' as end_date
    from dual
  union all
  select date '2017-1-15' as begin_date, date '2017-1-20' as end_date
    from dual
  union all
  select date '2017-1-18' as begin_date, date '2017-1-28' as end_date
    from dual
  union all
  select date '2017-1-19' as begin_date, date '2017-1-20' as end_date
    from dual)
select sum (max(end_date) - min(begin_date)) as total_num
  from (select begin_date,
               end_date,
               max(rn) over(order by begin_date, end_date) as rn
          from (select begin_date,
                       end_date,
                       case
                         when begin_date <= max(end_date)
                          over(order by begin_date,
                                   end_date rows between unbounded
                                   preceding and 1 preceding) then
                          0
                         else
                          row_number() over(order by begin_date, end_date)
                       end as rn
                  from tmp))
 group by rn;