Union and rownum

Hi,
I created a new table with two column: ID and description.

I want to feel the description with this query:

SELECT DISTINCT departure FROM tabTravels WHERE departure IS NOT NULL
UNION SELECT DISTINCT arrival FROM tabTravels WHERE arrival IS NOT NULL

How can I select a unique ROWNUM from this query (for the ID field)?

If someone is interested, I found the solution:

SELECT ROWUM id, departure FROM
(
SELECT departure FROM tabTravels WHERE departure IS NOT NULL
UNION SELECT arrival FROM tabTravels WHERE arrival IS NOT NULL
)

that will do the job, but some extra processing. Thanks for sharing.