Oracle 11g Release 2

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create table temp_test (id varchar2(20), ord varchar2(20), test varchar2(20))




insert into temp_test (id,ord,test) values ('1','1','A')
1 rows affected

insert into temp_test (id,ord,test) values ('1','2','B')
1 rows affected

insert into temp_test (id,ord,test) values ('1','3','C')
1 rows affected

insert into temp_test (id,ord,test) values ('2','1','B');
1 rows affected

insert into temp_test (id,ord,test) values ('2','2','C');
1 rows affected

SELECT id,
       ord,
       LTRIM(sys_connect_by_path(test,'_'),'_') as test
FROM temp_test r2 START WITH
     ord = 1
CONNECT BY PRIOR id = id AND ord = PRIOR ord + 1;


ID
ORD
TEST


1
1
A

1
2
A_B

1
3
A_B_C

2
1
B

2
2
B_C