create table  test   (id int， name varchar(10) )
insert into test  values (1,'a')
insert into test  values (1,'b')
insert into test  values (1,'c')
insert into test  values (2,'a')
insert into test  values (2,'b')
insert into test  values (3,'a')
insert into test  values (3,'c')

select  id,sys_connect_by_path(name,',') from (
  select id,name, row_number() over(partition by id order by name)rn,
                     count(*) over(partition by id ) cnt from test
                 ) a  where level=cnt
              start with rn=1  connect by prior id=id and prior rn=rn-1