Oracle 18c

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CREATE TABLE T
    (clientid int, startdate date, enddate date)
;
    
    
insert into T 
  select 1, date'2017-10-10', date'2017-10-12' from dual union all
   select 1, date'2017-10-12', date'2017-10-13' from dual union all
   select 1, date'2017-10-13', date'2017-10-17' from dual union all  
   select 1, date'2017-11-10', date'2017-11-17' from dual union all  
   select 1, date'2017-11-17', date'2017-11-23' from dual union all  
   select 1, date'2017-12-12', date'2017-12-14' from dual union all
   select 2, date'2017-11-10', date'2017-11-15' from dual union all  
   select 2, date'2017-12-01', date'2017-12-02' from dual union all  
   select 2, date'2017-12-02', date'2017-12-05' from dual;
9 rows affected

 select clientid, 
        decode(nvl(lag(enddate) over 
                   (order by enddate),startdate),startdate,0,1) 
                   as grp, --> means prev. value equals or not 
        row_number() over (order by clientid, enddate) as rn,           
        startdate, enddate
    from t


CLIENTID
GRP
RN
STARTDATE
ENDDATE


1
0
1
10-OCT-17
12-OCT-17

1
0
2
12-OCT-17
13-OCT-17

1
0
3
13-OCT-17
17-OCT-17

1
1
4
10-NOV-17
17-NOV-17

1
0
5
17-NOV-17
23-NOV-17

1
1
6
12-DEC-17
14-DEC-17

2
1
7
10-NOV-17
15-NOV-17

2
1
8
01-DEC-17
02-DEC-17

2
0
9
02-DEC-17
05-DEC-17


select tt.*, sum(grp) over (order by clientid, startdate) sm 
  from
(
 select clientid, 
        decode(nvl(lag(enddate) over 
                   (order by enddate),startdate),startdate,0,1) 
                   as grp, --> means prev. value equals or not 
        row_number() over (order by clientid, enddate) as rn,           
        startdate, enddate
    from t
) tt
order by rn


CLIENTID
GRP
RN
STARTDATE
ENDDATE
SM


1
0
1
10-OCT-17
12-OCT-17
0

1
0
2
12-OCT-17
13-OCT-17
0

1
0
3
13-OCT-17
17-OCT-17
0

1
1
4
10-NOV-17
17-NOV-17
1

1
0
5
17-NOV-17
23-NOV-17
1

1
1
6
12-DEC-17
14-DEC-17
2

2
1
7
10-NOV-17
15-NOV-17
3

2
1
8
01-DEC-17
02-DEC-17
4

2
0
9
02-DEC-17
05-DEC-17
4


select clientid, min(startdate) as startdate, max(enddate) as enddate
  from
(
select tt.*, sum(grp) over (order by clientid, startdate) sm 
  from
(
 select clientid, 
        decode(nvl(lag(enddate) over 
                   (order by enddate),startdate),startdate,0,1) 
                   as grp, --> means prev. value equals or not 
        row_number() over (order by clientid, enddate) as rn,           
        startdate, enddate
    from t
) tt
order by rn
) 
group by clientid, sm 
order by clientid, enddate;


CLIENTID
STARTDATE
ENDDATE


1
10-OCT-17
17-OCT-17

1
10-NOV-17
23-NOV-17

1
12-DEC-17
14-DEC-17

2
10-NOV-17
15-NOV-17

2
01-DEC-17
05-DEC-17

CLIENTID	GRP	RN	STARTDATE	ENDDATE
1	0	1	10-OCT-17	12-OCT-17
1	0	2	12-OCT-17	13-OCT-17
1	0	3	13-OCT-17	17-OCT-17
1	1	4	10-NOV-17	17-NOV-17
1	0	5	17-NOV-17	23-NOV-17
1	1	6	12-DEC-17	14-DEC-17
2	1	7	10-NOV-17	15-NOV-17
2	1	8	01-DEC-17	02-DEC-17
2	0	9	02-DEC-17	05-DEC-17