[RESOLVED] Help with sort on left join

I have this sql statement:

SELECT *
FROM `posts`
LEFT JOIN comments ON comments.post_id = posts.id
ORDER BY comments.`timestamp`,posts.`timestamp` DESC

this gives me first the latest posts with comments and then all the posts with no comments

this is not what I need. I need that if the comment timestamp is null than it will be sorted by the posts timestamp

is that possible in one query?
thanks!

order by coalesce(comments.`timestamp`,posts.`timestamp`) desc

edit: lars beat me to it and showed me something new as well.

Like this?

ORDER BY CASE WHEN comments.timestamp IS NOT NULL THEN comments.timestamp END ASC, posts.timestamp DESC

that's so cool thanks!
Now I need to learn what coalesce do...