Oracle 18c

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CREATE TABLE relationship ( customer_id, related_customer ) AS
--SELECT 1, 1 FROM DUAL UNION ALL
--SELECT 1, 2 FROM DUAL UNION ALL
--SELECT 1, 3 FROM DUAL UNION ALL
--SELECT 2, 1 FROM DUAL UNION ALL
--SELECT 2, 2 FROM DUAL UNION ALL
SELECT 2, 3 FROM DUAL UNION ALL
SELECT 3, 1 FROM DUAL UNION ALL
SELECT 3, 2 FROM DUAL UNION ALL
--SELECT 3, 3 FROM DUAL UNION ALL
--SELECT 11, 11 FROM DUAL UNION ALL
SELECT 11, 22 FROM DUAL-- UNION ALL
--SELECT 22, 11 FROM DUAL UNION ALL
--SELECT 22, 22 FROM DUAL;
4 rows affected

CREATE TABLE "CHECK" ( customer_id, check_flag ) AS
SELECT  1, 'y' FROM DUAL UNION ALL
SELECT  2, 'y' FROM DUAL UNION ALL
SELECT  3, 'n' FROM DUAL UNION ALL
SELECT 11, 'y' FROM DUAL UNION ALL
SELECT 22, 'y' FROM DUAL;
5 rows affected

SELECT customer_id,
       MIN(check_flag) AS check_flag
FROM   (
  SELECT CONNECT_BY_ROOT(c.customer_id) AS customer_id,
         c.check_flag AS check_flag
  FROM   "CHECK" c
         LEFT OUTER JOIN relationship r
         ON (r.customer_id = c.customer_id)
  WHERE  CONNECT_BY_ISLEAF = 1
  CONNECT BY NOCYCLE
         (  PRIOR r.related_customer = c.customer_id
         OR PRIOR c.customer_id = r.related_customer )
  AND    PRIOR c.check_flag = 'y'
)
GROUP BY
       customer_id
ORDER BY
       customer_id


CUSTOMER_ID
CHECK_FLAG


1
n

2
n

3
n

11
y

22
y