Someone

SELECT 
	t.id,
	t.headline,
	t.posted_at,
	r.id
	r.posted_at,
        CASE
          WHEN r.posted_at > t.posted_at THEN r.posted_at
          ELSE t.posted_at
        END AS LastPostedAt
FROM threads AS t
JOIN replies AS r ON r.thread_id = t.id
ORDER BY
	CASE
		WHEN r.posted_at > t.posted_at THEN r.posted_at
		ELSE t.posted_at
	END DESC
LIMIT 100