SQL> craete table test(id number(3), Name varchar2(30));
Table created.

SQL> insert into test values(1, 'a');
1 row created.
SQL> insert into test values(1, 'b');
1 row created.
SQL> insert into test values(1, 'b');
1 row created.
SQL> insert into test values(2, 'd');
1 row created.
SQL> insert into test values(2, 'e');
1 row created.
SQL> insert into test values(3, 'f');
1 row created.
SQL> insert into test values(4, 'g');
1 row created.

SQL> Select id, name from test;

Id    Name
1       a
1       b
1       c
2       d
2       e
3       f
4       g


I want output like this

Id    Name
1      a, b, c
2      d, e
3      f
4      g

select id,listagg(name,',') within group(order by id) test_str from test group by id;

 ID TEST_STR
--- ----------
  1 a,b,c
  2 d,e
  3 f
  4 g

with tab as (select 1 as id , 'a' as name from dual union 
select 1 as id , 'b' as name from dual union 
select 1 as id , 'c' as name from dual union 
select 2 as id , 'e' as name from dual union 
select 2 as id , 'e' as name from dual union 
select 3 as id , 'f' as name from dual union 
select 4 as id , 'g' as name from dual )
select ID, wm_concat(name) name from tab GROUP BY ID;

SQL*Plus: Release 11.1.0.6.0 - Production on Sat Sep 20 20:55:09 2014

Copyright (c) 1982, 2007, Oracle.  All rights reserved.


Connected to:
Oracle Database 11g Enterprise Edition Release 11.1.0.6.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> desc wm_concat
FUNCTION wm_concat RETURNS VARCHAR2
 Argument Name                  Type                    In/Out Default?
 ------------------------------ ----------------------- ------ --------
 P1                             VARCHAR2                IN

SQL>

with tab as (select 1 as id , 'a' as name from dual union 
select 1 as id , 'b' as name from dual union 
select 1 as id , 'c' as name from dual union 
select 2 as id , 'e' as name from dual union 
select 2 as id , 'e' as name from dual union 
select 3 as id , 'f' as name from dual union 
select 4 as id , 'g' as name from dual )
select  id,
        replace(ltrim(sys_connect_by_path(name,'|'),'|'),'|',', ') name
  from  (
         select  id,
                 name,
                 row_number() over(partition by id order by name) rn
           from  tab
        )
  where connect_by_isleaf = 1
  start with rn = 1
  connect by id = prior id
         and rn  = prior rn + 1
  order by id;

Id	Name
------------------------------------
1	a, b, c
2	e
3	f
4	g