Da MySQL keine Fensterfunktionen unterstützt, gibt es meines Erachtens keine Möglichkeit, dies ohne eine Unterabfrage zu tun:
SELECT f.id AS forum_id,
f.name AS forum_name,
t.id AS thread_id,
t.topic AS thread_topic,
t.ts AS thread_timestamp,
p.id AS post_id,
p.content AS post_content,
p.ts AS post_timestamp
FROM forums f
JOIN (SELECT t2.forum_id, max(p2.ts) as ts
FROM posts p2
JOIN threads t2 ON p2.thread_id = t2.id
GROUP BY t2.forum_id) max_p ON f.id = max_p.forum_id
JOIN posts p ON max_p.ts = p.ts
JOIN threads t ON f.id = t.forum_id AND p.thread_id = t.id
ORDER BY p.ts
Wenn Sie die neuesten Ergebnisse zwischenspeichern, können Sie dies natürlich ohne die Leistungseinbußen beim Aufrufen von MAX() tun, aber mit den richtigen Indizes sollte dies kein großes Problem darstellen ...
AKTUALISIEREN
Die prägnanteste Art, Threads ohne Beiträge und Foren ohne Threads einzubinden, wäre die Verwendung von LEFT JOINs anstelle von INNER JOINs:
SELECT f.id AS forum_id,
f.name AS forum_name,
t.id AS thread_id,
t.topic AS thread_topic,
t.ts AS thread_timestamp,
p.id AS post_id,
p.content AS post_content,
p.ts AS post_timestamp
FROM forums f
LEFT JOIN (SELECT t2.forum_id, max(COALESCE(p2.ts, t2.ts)) as ts, COUNT(p2.ts) as post_count
FROM threads t2
LEFT JOIN posts p2 ON p2.thread_id = t2.id
GROUP BY t2.forum_id) max_p ON f.id = max_p.forum_id
LEFT JOIN posts p ON max_p.ts = p.ts
LEFT JOIN threads t ON f.id = t.forum_id AND (max_p.post_count = 0 OR p.thread_id = t.id)
ORDER BY p.ts
