SELECT * FROM
                (SELECT discussions.created_at, discussions.id, discussions.title, discussions.stub,
                users.username AS created_by,
                COUNT(DISTINCT threads.id) OVER (partition BY discussions.created_at,
                                                  discussions.title,
                                                  users.username) AS replies,
                COUNT(DISTINCT discussion_views.discussion_id)
                    OVER (partition BY discussions.created_at,
                            discussions.title,
                            users.username) AS views,
                latest_poster.username AS latest_post_by,
                threads.updated_at AS latest_post_at,
                ROW_NUMBER() OVER (partition BY discussions.created_at,
                                    discussions.title,
                                    users.username
                       ORDER BY threads.id DESC) AS rn
            FROM discussions
            LEFT JOIN threads ON discussions.id=threads.discussion_id
            LEFT JOIN discussion_views ON discussions.id=discussion_views.discussion_id
            JOIN users ON users.id=discussions.user_id
            JOIN users latest_poster ON (latest_poster.id=threads.user_id)
            WHERE discussions.course_id={$course_id}
            ) sq
            WHERE rn=1
            ORDER BY created_at DESC"

SELECT
  *
FROM
(
  SELECT
    rownum AS row_id,
    <your other selects>
  FROM
    <your query>
  ORDER BY
    <whatever>
)
  DATA
WHERE
      DATA.row_id >= 123
  AND DATA.row_id <= 222