You can JOIN the users
table to pull that info in.
-- [params]
-- topic_id :topic_id
SELECT
p.user_id,
u.username,
p.post_number,
p.raw,
p.created_at,
p.like_count
FROM posts p
JOIN users u ON u.id = p.user_id
WHERE p.topic_id = :topic_id
ORDER BY p.post_number