Oracle 18c

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alter session set nls_date_format = 'yyyy-mm-dd'
create table A(ID, ValidFrom, ValidTo) as (
    select 100, date '2009-01-01', date '2010-12-31' from dual union all
    select 100, date '2011-01-01', date '2012-03-31' from dual union all
    
    select 101, date '2013-08-01', date '2014-07-31' from dual union all
    select 101, date '2014-08-01', date '2014-08-31' from dual );
4 rows affected

create table B(ID, ValidFrom, ValidTo) as (
    select 100, date '2008-11-01', date '2011-12-31' from dual union all
    select 100, date '2012-02-01', date '2012-02-29' from dual union all
    
    select 101, date '2013-08-01', date '2014-06-30' from dual union all
    select 101, date '2014-07-01', date '2014-08-31' from dual );
4 rows affected

/*OUTPUT:
ID   ValidFrom   ValidTo
100  11/1/2008  12/31/2008
100  1/1/2009   12/31/2010
100  1/1/2011   12/31/2011
100  1/1/2012   1/31/2012
100  2/1/2012   2/29/2012
100  3/1/2012   3/31/2012
---------------------------
101  8/1/2013   6/30/2014
101  7/1/2014   7/31/2014
101  8/1/2014   8/31/2014 */
ORA-01742: comment not terminated properly

    select id, validfrom d1, 1 dir from a union all
    select id, validto,      2 dir from a union all
    select id, validfrom,    1 dir from b union all
    select id, validto,      2 dir from b
    order by 1, 2


ID
D1
DIR


100
2008-11-01
1

100
2009-01-01
1

100
2010-12-31
2

100
2011-01-01
1

100
2011-12-31
2

100
2012-02-01
1

100
2012-02-29
2

100
2012-03-31
2

101
2013-08-01
1

101
2013-08-01
1

101
2014-06-30
2

101
2014-07-01
1

101
2014-07-31
2

101
2014-08-01
1

101
2014-08-31
2

101
2014-08-31
2


select id, d1, d2, case dir when 3 then 'GAP' end note 
  from (
    select id, 
           case when dir = 2 
                 and lead(dir) over (partition by id order by dt) = 1
                 and lead(dt) over (partition by id order by dt) <> dt + 1
                then dt + 1 
                else dt
           end d1,
           case when dir = 2 
                 and lead(dir) over (partition by id order by dt) = 1
                 and lead(dt) over (partition by id order by dt) <> dt + 1
                then 3 
                else dir
           end dir,
           case when lead(dir) over (partition by id order by dt) = 1 
                then lead(dt)  over (partition by id order by dt) - 1
                else lead(dt)  over (partition by id order by dt) 
            end d2
      from (
        select * from a unpivot (dt for dir in (validfrom as 1, validto as 2)) union 
        select * from b unpivot (dt for dir in (validfrom as 1, validto as 2)) ) )
  where dir in (1, 3)


ID
D1
D2
NOTE


100
2008-11-01
2008-12-31
null

100
2009-01-01
2010-12-31
null

100
2011-01-01
2011-12-31
null

100
2012-01-01
2012-01-31
GAP

100
2012-02-01
2012-02-29
null

101
2013-08-01
2014-06-30
null

101
2014-07-01
2014-07-31
null

101
2014-08-01
2014-08-31
null

ID	D1	DIR
100	2008-11-01	1
100	2009-01-01	1
100	2010-12-31	2
100	2011-01-01	1
100	2011-12-31	2
100	2012-02-01	1
100	2012-02-29	2
100	2012-03-31	2
101	2013-08-01	1
101	2013-08-01	1
101	2014-06-30	2
101	2014-07-01	1
101	2014-07-31	2
101	2014-08-01	1
101	2014-08-31	2
101	2014-08-31	2

ID	D1	D2	NOTE
100	2008-11-01	2008-12-31	null
100	2009-01-01	2010-12-31	null
100	2011-01-01	2011-12-31	null
100	2012-01-01	2012-01-31	GAP
100	2012-02-01	2012-02-29	null
101	2013-08-01	2014-06-30	null
101	2014-07-01	2014-07-31	null
101	2014-08-01	2014-08-31	null