Ответ 1
SELECT comments.comment_id,
comments.content_id,
comments.user_id,
comments.`comment`,
comments.comment_time,
NULL
FROM (
SELECT content.content_id
FROM content
WHERE content.user_id = 1
LIMIT 10
) q
JOIN comments
ON comments.content_id = q.content_id
Вероятно, вы захотите добавить ORDER BY
во вложенный запрос.