I have this query

SELECT articles.*, 
       users.username AS `user` 
 FROM `articles` 
LEFT JOIN `users` ON articles.user_id = users.id 
 ORDER BY articles.timestamp 

Basically it returns the list of articles and the username that the article is associated to. Now if there is no entry in the users table for a particular user id, the users var is NULL. Is there anyway to make it that if its null it returns something like "User Not Found"? or would i have to do this using php?