Oracle 18c

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create table TEST_TABLE(id,pid,order_num) AS
SELECT 1, NULL, 1 FROM DUAL UNION ALL
SELECT 2, 1, 5 FROM DUAL UNION ALL
SELECT 3, NULL, 2 FROM DUAL UNION ALL
SELECT 4, 1, 4 FROM DUAL UNION ALL
SELECT 5, 3, 2 FROM DUAL UNION ALL
SELECT 1, 5, 5 FROM DUAL;
6 rows affected

select distinct * from test_table start with pid is null connect by prior id=pid order siblings by order_num;


ID
PID
ORDER_NUM


1
null
1

5
3
2

3
null
2

4
1
4

2
1
5

1
5
5


select * from test_table start with pid is null connect by prior id=pid order siblings by order_num;


ID
PID
ORDER_NUM


1
null
1

4
1
4

2
1
5

3
null
2

5
3
2

1
5
5

4
1
4

2
1
5


SELECT id, pid, order_num
FROM   (
  SELECT id, pid, order_num,
         ROW_NUMBER() OVER (PARTITION BY id, pid, order_num ORDER BY ROWNUM) AS rn
  FROM   (
    SELECT id, pid, order_num
    FROM   test_table
    START WITH pid IS NULL
    CONNECT BY PRIOR id = pid
    ORDER SIBLINGS BY order_num
  )
  ORDER BY ROWNUM
)
WHERE  rn = 1


ID
PID
ORDER_NUM


1
null
1

4
1
4

2
1
5

3
null
2

5
3
2

1
5
5

ID	PID	ORDER_NUM
1	null	1
5	3	2
3	null	2
4	1	4
2	1	5
1	5	5

ID	PID	ORDER_NUM
1	null	1
4	1	4
2	1	5
3	null	2
5	3	2
1	5	5
4	1	4
2	1	5

ID	PID	ORDER_NUM
1	null	1
4	1	4
2	1	5
3	null	2
5	3	2
1	5	5