By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
with a (unique_id, start_date, end_date, sequential_id) as (
select '001', date '2021-01-01', date '2021-01-15', null from dual union all
select '002', date '2021-02-01', date '2021-02-16', '001' from dual union all
select '003', date '2021-03-01', date '2021-03-17', '002' from dual union all
select '004', date '2021-03-10', date '2021-03-11', null from dual union all
select '005', date '2021-03-19', null, null from dual
)
, b as (
select
connect_by_root(unique_id) as unique_id
, connect_by_root(start_date) as start_date
, end_date
, connect_by_isleaf as l
from a
start with sequential_id is null
connect by prior unique_id = sequential_id
)
select
unique_id
, start_date
, end_date
from b
where l = 1
order by 1 asc
| UNIQUE_ID | START_DATE | END_DATE |
|---|---|---|
| 001 | 01-JAN-21 | 17-MAR-21 |
| 004 | 10-MAR-21 | 11-MAR-21 |
| 005 | 19-MAR-21 | null |