Oracle 11g Release 2

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ALTER SESSION SET NLS_DATE_FORMAT = 'dd-MM-yy';
create table mytable (
  rn int,
  old_date_row date
);
insert into mytable
select 1 as rn,   '01-JUN-2018' as old_date_row from dual union all
select 2,   null from dual union all
select 3,   null from dual union all
select 4,   null from dual union all
select 5,   '22-JUN-2018' from dual union all
select 6,   null from dual union all
select 7,   null from dual union all
select 8,   null from dual union all
select 9,   '25-JUN-2019' from dual union all
select 10,  null from dual union all
select 11,  null from dual union all
select 12,  '25-JUN-2020' from dual ;
12 rows affected

select old_date_row, rid, round(rid/2)
from (
  select old_date_row, row_number() over (order by rn) rid
  from mytable t
  where old_date_row is not null
)


OLD_DATE_ROW
RID
ROUND(RID/2)


01-06-18
1
1

22-06-18
2
1

25-06-19
3
2

25-06-20
4
2


select min(old_date_row) as start_date, max(old_date_row) as end_date
from (
  select old_date_row, row_number() over (order by rn) rid
  from mytable t
  where old_date_row is not null
)
group by round(rid/2)


START_DATE
END_DATE


01-06-18
22-06-18

25-06-19
25-06-20