Oracle 11g Release 2

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create table T_CUSTOMER_RELATIONSHIP (CUST_ID VARCHAR2(10), OTHER_CUST_ID VARCHAR2(10))
BEGIN
insert into T_CUSTOMER_RELATIONSHIP values ('CUST1', 'CUST1');
insert into T_CUSTOMER_RELATIONSHIP values ('CUST1', 'CUST2');
insert into T_CUSTOMER_RELATIONSHIP values ('CUST1', 'CUST3');
insert into T_CUSTOMER_RELATIONSHIP values ('CUST1', 'CUST4');
insert into T_CUSTOMER_RELATIONSHIP values ('CUST5', 'CUST5');
insert into T_CUSTOMER_RELATIONSHIP values ('CUST5', 'CUST4');
insert into T_CUSTOMER_RELATIONSHIP values ('CUST5', 'CUST6');
insert into T_CUSTOMER_RELATIONSHIP values ('CUST7', 'CUST7');
insert into T_CUSTOMER_RELATIONSHIP values ('CUST7', 'CUST6');
insert into T_CUSTOMER_RELATIONSHIP values ('CUST8', 'CUST8');
insert into T_CUSTOMER_RELATIONSHIP values ('CUST8', 'CUST9');
insert into T_CUSTOMER_RELATIONSHIP values ('CUST8', 'CUST10');
END;
/
1 rows affected

SELECT DISTINCT
       CONNECT_BY_ROOT( cust_id ) AS cust_id,
       other_cust_id
FROM   T_CUSTOMER_RELATIONSHIP
START WITH cust_id = 'CUST1'
CONNECT BY NOCYCLE
       (  PRIOR other_cust_id IN ( cust_id, other_cust_id )
       OR PRIOR cust_id       IN ( cust_id, other_cust_id ) )
ORDER BY cust_id, other_cust_id;


CUST_ID
OTHER_CUST_ID


CUST1
CUST1

CUST1
CUST2

CUST1
CUST3

CUST1
CUST4

CUST1
CUST5

CUST1
CUST6

CUST1
CUST7

CUST_ID	OTHER_CUST_ID
CUST1	CUST1
CUST1	CUST2
CUST1	CUST3
CUST1	CUST4
CUST1	CUST5
CUST1	CUST6
CUST1	CUST7