Oracle 11g Release 2

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CREATE TABLE TBL_IORELATE (ID, MainID, RelatedID) AS
SELECT 1, 'I-225', 'I-20' FROM DUAL UNION ALL
SELECT 2, 'I-225', 'I-35' FROM DUAL UNION ALL
SELECT 3, 'I-225', 'I-300' FROM DUAL UNION ALL
SELECT 4, 'I-410', 'I-20' FROM DUAL UNION ALL
SELECT 5, 'I-410', 'I-50' FROM DUAL UNION ALL
SELECT 6, 'I-300', 'I-95' FROM DUAL UNION ALL
SELECT 7, 'I-455', 'I-300' FROM DUAL UNION ALL
SELECT 8, 'I-20', 'I-390' FROM DUAL;
8 rows affected

SELECT id,
       LISTAGG(value, ',') WITHIN GROUP (ORDER BY value) AS io_related
FROM   (
  SELECT DISTINCT id, value
  FROM   (
    SELECT CONNECT_BY_ROOT MainID AS id,
           MainID,
           RelatedID
    FROM   TBL_IORELATE
    START WITH MainID  IN ('I-225')
    CONNECT BY NOCYCLE
       PRIOR MainID IN (MainID)
    OR PRIOR RelatedID IN (RelatedID)
  )
  UNPIVOT(value FOR key IN (MainID, RelatedID))
  WHERE id <> value
)
GROUP BY id


ID
IO_RELATED


I-225
I-20,I-300,I-35,I-410,I-455,I-50


SELECT id,
       LISTAGG(value, ',') WITHIN GROUP (ORDER BY value) AS io_related
FROM   (
  SELECT DISTINCT id, value
  FROM   (
    SELECT CONNECT_BY_ROOT MainID AS id,
           MainID,
           RelatedID
    FROM   TBL_IORELATE
    START WITH MainID  IN ('I-225')
    CONNECT BY NOCYCLE
       PRIOR MainID IN (RelatedID, MainID)
    OR PRIOR RelatedID IN (RelatedID, MainID)
  )
  UNPIVOT(value FOR key IN (MainID, RelatedID))
  WHERE id <> value
)
GROUP BY id


ID
IO_RELATED


I-225
I-20,I-300,I-35,I-390,I-410,I-455,I-50,I-95