Find the entire tree from the root giving any node

Question

How do I find the entire tree given a node of a tree?

Example of tree:

       100
  101        102
1010 1011   1020  1021


select level, employee_id         


        

Answer 1

Why not just:

select level, employee_id, last_name, manager_id ,
connect_by_root manager_id as root_id
from employees
connect by prior employee_id = manager_id
start with manager_id = 100

Here is a fiddle

EDIT
Here is another try (After understanding the full problem):

with t as (
select case when mgr.employee_id is null then
1 else 0 end is_root, emp.employee_id employee, emp.manager_id manager, emp.last_name last_name

from employees mgr right outer join employees emp
on mgr.employee_id = emp.manager_id
),
tmp as (

select level, employee, last_name, manager ,
connect_by_root manager as root_id,
manager||sys_connect_by_path(employee,
',') cbp

from t
connect by prior employee = manager
start with t.is_root =
1 )
select * from tmp
where tmp.root_id in (select root_id from tmp where employee= 101 or manager = 101)

I checked it with 100, 101 and 1010 and it worked well
Here is a fiddle

Answer 2

select 
     level, 
     employee_id, 
     last_name, manager_id ,
connect_by_root employee_id as root_id
from employees
connect by prior employee_id = manager_id
start with employee_id in  ( 
  select employee_id from employees 
  where manager_id is null )

Answer 3

You need to first traverse up the tree to get all managers then traverse down to fetch all employees:

select level, employee_id, last_name, manager_id ,
       connect_by_root employee_id as root_id
   from employees
connect by prior employee_id = manager_id -- down the tree
start with manager_id in ( -- list up the tree
     select manager_id 
       from employees
     connect by employee_id = prior manager_id -- up the tree
     start with employee_id = 101
     )
;

See http://www.sqlfiddle.com/#!4/d15e7/18

Edit:

If the given node might also be the root node, extend the query to include the given node in the list of parent nodes:

Example for non-root node:

select distinct employee_id, last_name, manager_id 
   from employees
connect by prior employee_id = manager_id -- down the tree
start with manager_id in ( -- list up the tree
     select manager_id 
       from employees
     connect by employee_id = prior manager_id -- up the tree
     start with employee_id = 101
     union 
     select manager_id -- in case we are the root node
       from employees
     where manager_id = 101
     )
;

Example for root node:

select distinct employee_id, last_name, manager_id 
   from employees
connect by prior employee_id = manager_id -- down the tree
start with manager_id in ( -- list up the tree
     select manager_id 
       from employees
     connect by employee_id = prior manager_id -- up the tree
     start with employee_id = 100
     union 
     select manager_id -- in case we are the root node
       from employees
     where manager_id = 100
     )
;

Fiddle at http://www.sqlfiddle.com/#!4/d15e7/32