Oracle 11g Release 2

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

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

with cte(id,ord,test,concat_val) 
  as (select id,ord,test,test as concat_val
       from temp_test
       where ord=1
      union all
      select a.id,a.ord,a.test,b.concat_val||'_'||a.test
        from temp_test a
        join cte b
          on a.id=b.id
         and a.ord=b.ord+1 
      )
select * from cte order by id,ord


ID
ORD
TEST
CONCAT_VAL


1
1
A
A

1
2
B
A_B

1
3
C
A_B_C

2
1
B
B

2
2
C
B_C