SELECT     c.timestamp, c.wert, UNIX_TIMESTAMP(c.timestamp) AS unix_time, a.wert
  FROM (SELECT c.timestamp AS stamp1, max(a.Zeitstempel) AS stamp2
          FROM Counter.`99999910` as c
               LEFT JOIN alt.`statusanz` AS a ON c.timestamp >= a.Zeitstempel
      GROUP BY c.timestamp)
       LEFT JOIN c ON c.timestamp = stamp1
       LEFT JOIN a ON a.Zeitstempel = stamp2;