gistfile1.sql

UPDATE campaign,
  (SELECT
    c.id, IF(MAX(l.createTimestamp),
               MAX(l.createTimestamp),
               '0000-00-00') AS ts
    FROM
      campaign c
    LEFT JOIN
      lead l ON l.campaignID = c.id
    GROUP BY c.id) timestamps
SET
  campaign.lastActivity = timestamps.ts
WHERE
  campaign.id = timestamps.id;