SELECT ?group ?last_date ?last_message
    (SELECT COUNT(?message) AS ?message_count WHERE { ?message example:group ?group })
WHERE {
    SELECT ?group ?last_date
        (SELECT ?message WHERE { ?message example:group ?group ; example:date ?date } ORDER BY DESC(?date) LIMIT 1) AS ?last_message
    WHERE {
        ?group example:lastDate ?last_date .
    } ORDER BY DESC(?last_date)
}