query

I have two tables.
orders table and ship table
For orders table,
ordernum
------------
1
2
3
4

For ship table,
ordernum        tracking_num
------------
1                           t1    
3                           t3


I want to have the Result looks like
----------------
ordernum      tracking_num
-----------      ----------------
1                         t1
2
3                         t3
4

if i do like,

select orders(*), ship(*)
from orders, ship
where orders.ordernum = ship.ordernum

it only displays two results.

but i want to display all of ordernum with tracking_num(it's ok even though it is null)

how can i do that??