Extract duplicate rows and append them in single column

Question

hi,
my requirement is i have the data in a table like below
GIVEN DATA
req_id res_id
100 200
100 201
101 300
102 200
102 400
102 null
THE OUTPUT SHOULD BE
req_id res_id
100 200,201
101 300
102 200,400
can anybody please help me out in writing the query to get the desired output.
*****NOTE: REMEMBER THERE ARE 100'S OF RECORDS IN THE TABLE THE ABOVE IS ONLY SAMPLE DATA.
Thanks in advance
Best Regards,

Answer 1

SQL>  with t as
  2   (select 100 req_id,200 res_id from dual union all
  3    select 100,null from dual union all
  4    select 100,201 from dual union all
  5    select 100,null from dual union all
  6    select 101,null from dual union all
  7    select 101,null from dual)
  8   select req_id,
  9     ltrim(max(sys_connect_by_path(res_id,'-')),'-') res_id
10   from(
11      select req_id,res_id,
12        row_number() over(partition by req_id order by res_id nulls first) rn
13      from t)
14   start with rn = 1
15   connect by  rn = prior rn + 1
16   and  req_id = prior req_id
17   group by req_id;
    REQ_ID RES_ID
       100 200-201
       101