Ok, je suis venu avec la solution moi-même. J'ai utilisé une sous-requête dépendante pour résoudre.Voici ce que j'ai dans le résultat :
SELECT
item_discuss_threads.item_discuss_thread_id
, item_discuss_threads.item_discuss_thread_datetime
, item_discuss_threads.item_discuss_thread_title
, latest_posts.item_discuss_post_title
, latest_posts.item_discuss_post_datetime
, COUNT(item_discuss_posts.item_discuss_post_id) AS nb_posts
FROM
item_discuss_threads
INNER JOIN item_discuss_posts
ON item_discuss_threads.item_discuss_thread_id=item_discuss_posts.item_discuss_thread_id
INNER JOIN item_discuss_posts AS latest_posts
ON latest_posts.item_discuss_thread_id=item_discuss_threads.item_discuss_thread_id
WHERE
(
SELECT
item_discuss_post_id
FROM
item_discuss_posts AS p
WHERE
p.item_discuss_thread_id=item_discuss_posts.item_discuss_thread_id
ORDER BY
item_discuss_post_datetime DESC
LIMIT
1
)=latest_posts.item_discuss_post_id
GROUP BY
item_discuss_threads.item_discuss_thread_id
ORDER BY
latest_posts.item_discuss_post_datetime DESC
