Oracle 11g Release 2

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with units(org_id, title, unit_valid_from, unit_valid_to, origin) as (
    select 101, 'T101A', date '2019-05-14', date '2019-05-17', 'different' from dual union all
    select 101, 'T101B', date '2019-05-15', date '2019-05-17', 'history'   from dual union all
    select 101, 'T101C', date '2019-05-16', date '2019-05-17', 'current'   from dual union all
    select 201, 'T201A', date '2019-05-14', date '2019-05-17', 'different' from dual union all
    select 201, 'T201B', date '2019-05-15', date '2019-05-17', 'history'   from dual union all
    select 301, 'T301A', date '2019-05-14', date '2019-05-17', 'different' from dual union all
    select 301, 'T301B', date '2019-05-15', date '2019-05-17', 'different' from dual )
select units.*, 
       case when 1 = 
                 row_number() over (
                     partition by org_id 
                     order by case origin when 'current' then 1 when 'history' then 2 else 3 end, 
                              unit_valid_from ) then 1 else 0 end as is_correct_version
  from units

ORG_ID	TITLE	UNIT_VALID_FROM	UNIT_VALID_TO	ORIGIN	IS_CORRECT_VERSION
101	T101C	16-MAY-19	17-MAY-19	current	1
101	T101B	15-MAY-19	17-MAY-19	history	0
101	T101A	14-MAY-19	17-MAY-19	different	0
201	T201B	15-MAY-19	17-MAY-19	history	1
201	T201A	14-MAY-19	17-MAY-19	different	0
301	T301A	14-MAY-19	17-MAY-19	different	1
301	T301B	15-MAY-19	17-MAY-19	different	0