select p_personid, p_firstname, p_lastname, m_messageid, COALESCE(m_ps_imagefile, m_content, '') AS content, m_creationdate
from person, message, knows
where
    p_personid = m_creatorid and
    m_creationdate < '2011-07-21T22:00:00' and
    k_person1id = 21990232556256 and
    k_person2id = p_personid
order by m_creationdate desc, m_messageid asc
limit 20