Help with ORDER BY using two colomns

[Richard]

Hello,
I've got a table which containes two date colomns.
The first one is called `date` and the second `update`
In the first one I put the ticket creation date, and on update I add or 
change the update value.
So the update colomn does not contain a value until the first update has 
been done.
I would like to order the tickets by their last update value. And if 
this value does not exist use the date value.

at the moment I use this :

ORDER BY `date` DESC"
and I would like to replace it by something like this :

ORDER (IF `update`!= '' BY UPDATE ELSE BY DATE)

I know this code is completly wrong, just to try and show you what I 
need ...

Here is an example of what I want to achieve

num	|	date	|	update
-------------------------------------------
1	|	1	|	
2	|	10	|	60
3	|	20	|	
4	|	30	|	
5	|	40	|	90
6	|	50	|	

The required result would be :

num	|	date	|	update
-------------------------------------------
5	|	40	|	90
2	|	10	|	60
6	|	50	|	
4	|	30	|	
3	|	20	|	
1	|	1	|	

Thanks in advance :)