hierarchical query help

Hi,

I am trying to write a query to reproduce "v$session.final_block_session" from the V$active_session_history. However, I am having a hard time getting the data return in the format I like.

Setup:

drop table pojen;

create table pojen(sample_id number, session_id number, blocking_session number,event varchar2(6));

insert into pojen values (1,101,109,'A');

insert into pojen values (1,102,109,'B');

insert into pojen values (1,103,109,'C');

insert into pojen values (1,109,108,'C');

insert into pojen values (1,108,107,'C');

-- insert into pojen values (1,110,111,'LOOP1');

-- insert into pojen values (1,111,110,'LOOP2');

insert into pojen values (1,121,129,'X');

insert into pojen values (1,131,null,null);