Oracle 18c

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CREATE TABLE t
    (cusId1 int, cusId2 int, flg int)
;

INSERT ALL 
    INTO t (cusId1, cusId2, flg) VALUES (1, 2, 1)
    INTO t (cusId1, cusId2, flg) VALUES (2, 3, 1)
    INTO t (cusId1, cusId2, flg) VALUES (3, 4, 2)
    INTO t (cusId1, cusId2, flg) VALUES (5, 1, 2)
    INTO t (cusId1, cusId2, flg) VALUES (6, 7, 1)
    INTO t (cusId1, cusId2, flg) VALUES (7, 8, 1)
    INTO t (cusId1, cusId2, flg) VALUES (8, 6, 1)
SELECT * FROM dual
;
7 rows affected

select * from t


CUSID1
CUSID2
FLG


1
2
1

2
3
1

3
4
2

5
1
2

6
7
1

7
8
1

8
6
1


with alledges as (
      select cusid1, cusid2
      from t
      where flg = 1
      union all
      select cusid2, cusid1
      from t
      where flg = 1
      union all
      (select cusid1, cusid1
       from t
       where not exists (select 1 from t t2 where t2.flg = 1 and t.cusid1 in (t2.cusid1, t2.cusid2))
       union
       select cusid2, cusid2
       from t
       where not exists (select 1 from t t2 where t2.flg = 1 and t.cusid2 in (t2.cusid1, t2.cusid2))
      )
     ),
     cte (parentid, childid, visited) as (
      select distinct cusid1, cusid1, ',' || cusid1 || ','
      from alledges
      union all
      select cte.parentid, ae.cusid2, cte.visited || ae.cusid2 || ','
      from cte join
           alledges ae
           on cte.childid = ae.cusid1
      where cte.visited not like '%,' || ae.cusid2 || ',%'
     )
select childid, min(parentid) 
from cte
group by childid
order by childid


CHILDID
MIN(PARENTID)


1
1

2
1

3
1

4
4

5
5

6
6

7
6

8
6