SELECT *
FROM `posts` AS p
WHERE NOT EXISTS (SELECT * FROM `comments` AS c WHERE c.user_id=1 AND p.post_id=c.post_id)
AND p.post_id=c.post_id
SELECT c.id, c.text, u.id as user, p.id as post
FROM comments c
LEFT JOIN users u ON u.id=c.user_id
LEFT JOIN posts p ON p.id=c.post_id
WHERE u.id != 235