ID   polID   expolid   userid

1    58912      -       MATT
2    12345     58912    SYSTEM
3    98765     12345    SYSTEM
.
.
10   XXX       YYYY    SYSTEM

SELECT CASE
          WHEN o.expolid IS NOT NULL
          THEN
             (SELECT userid
                FROM orders
               WHERE polid = o.expolid)
          ELSE
             o.userid
       END
          "UserID "
  FROM orders o

CREATE TABLE pol
  (
    id      NUMBER,
    polID   NUMBER,
    expolid NUMBER,
    userid  VARCHAR2(50)
  );
INSERT INTO pol VALUES
  (1,58912,NULL,'MATT'
  );
INSERT INTO pol VALUES
  (2,12345,58912 ,'SYSTEM'
  );
INSERT INTO pol VALUES
  (3,98765,12345 ,'SYSTEM2'
  );
INSERT INTO pol VALUES
  (4,123,NULL,'ALEX'
  );
INSERT INTO pol VALUES
  (5,1234,123 ,'SYSTEM3'
  );
INSERT INTO pol VALUES
  (6,98765,1234 ,'SYSTEM4'
  );
INSERT INTO pol VALUES
  (7,234,NULL,'CHRISTIAN'
  );
INSERT INTO pol VALUES
  (8,11111,234 ,'SYSTEM5'
  );
INSERT INTO pol VALUES
  (9,88888,234 ,'SYSTEM6'
  );
INSERT INTO pol VALUES
  (10,99999,88888 ,'SYSTEM7'
  );
INSERT INTO pol VALUES
  (11,99999,11111 ,'SYSTEM8'
  );
  
--Format
COLUMN root_userid format a12
COLUMN Path format a30
COLUMN PADDED_NAME format a20

-- Make a Tree
SELECT CONNECT_BY_ROOT(userid) AS root_userid,
  SYS_CONNECT_BY_PATH(userid, '/') "Path",
  lpad(' ', (level - 1) * 2) || userid as padded_name, 
  --level l1,
  --max(level) over(partition by id) max_l1,
  ID
FROM pol
  CONNECT BY PRIOR polid = expolid
  start with id in (select id from pol where expolid is null)
--ORDER BY id ASC
;