Oracle 18c

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select * from V$VERSION;


BANNER
BANNER_FULL
BANNER_LEGACY
CON_ID


Oracle Database 18c Express Edition Release 18.0.0.0.0 - Production
Oracle Database 18c Express Edition Release 18.0.0.0.0 - Production
Version 18.4.0.0.0
Oracle Database 18c Express Edition Release 18.0.0.0.0 - Production
0


create table vals (id, val) as
    select trunc (rownum/3)+1, 'value'||rownum 
    from dual connect by level<=6
6 rows affected

select * from vals


ID
VAL


1
value1

1
value2

2
value3

2
value4

2
value5

3
value6


create or replace type listofvals is table of varchar2 (64)
with function mylistagg (vals listofvals, delimiter char := ',') return varchar2 is
    ret varchar2 (32767);
begin 
    for i in 1..vals.count loop 
        ret := ret||vals(i)||delimiter; end loop;
    return rtrim (ret, delimiter);
end; 
select id, mylistagg (cast (collect (val order by val) as listofvals), ' -> ') res
from vals
group by id


ID
RES


1
value1 -> value2

2
value3 -> value4 -> value5

3
value6