Databases :: SQL - Create A Variable That Holds The Value For Each Row Of The Timestamp?

I'm making a simple forum using MySQL and I use a repeater to display a list of all threads in it. I have two tables for posts, one for threads and one for replies, and I want to sort the threads by either when they were posted or when the last reply was posted in it. My SQL query is really long and hard to read so here's some psuedo code similar to it (easier to understand these column names, etc):

SELECT

threads.id,
threads.title,
threads.timestamp,
users.username,
(SELECT COUNT(*) FROM replies WHERE thread_id = threads.id) AS number_of_replies,
(SELECT COUNT(*) FROM views WHERE thread_id = threads.id) AS number_of_views,
(SELECT timestamp FROM replies WHERE thread_id = threads.id ORDER BY timestamp DESC LIMIT 1) AS last_reply_timestamp
FROM threads
LEFT JOIN users
ON threads.user_id = users.id
ORDER BY ************** DESC

What I do here is I fetch the values id, title, timestamp, the author's username, the number of replies and views for a specific thread, and I also get the timestamp for the last reply to this thread.

Then, what I want to do is sort the whole list so that the threads are displayed in an order matching when either it was posted or the last reply was posted in it (you know, like a normal forum) but I'm not sure how to accomplish this.

I tried this:

ORDER BY CASE WHEN last_reply_timestamp > threads.timestamp THEN last_reply_timestamp ELSE threads.timestamp END DESC

but that, I guess, maybe only looks at the first row and determines which one should be used so the order is not correct.

I'm not sure if it's possible to create a sort of variable that holds the value for each row of the timestamp that applies, if that's possible you could sort by that. For example:

(CASE WHEN last_reply_timestamp > threads.timestamp THEN last_reply_timestamp ELSE threads.timestamp END) AS last_activity_timestamp

...

ORDER BY last_activity_timestamp DESC

So each row has a field "last_activity_timestamp" containing either timestamp, whichever is the later than the other.

Can this be done somehow? How does everyone else solve this? It's so hard to search google because I just get lots of forums but no code.