List all open PMs
sorted by recent activity
SELECT
t.id AS topic_id,
t.user_id
FROM
topics t
JOIN posts p ON t.id = p.topic_id
WHERE
t.archetype = 'private_message'
AND t.user_id > 0
AND t.reply_count > 1
AND NOT t.closed
GROUP BY
t.id
ORDER BY
t.updated_at DESC