Useful SQL queries

Find unique Senses that had been uploading data in the last 2 hours, since 2014-11-01

SELECT sq.device_id, sq.id, sq.diff, sq.max_ts FROM  
(SELECT M.device_id AS device_id, 
  MAX(M.id) AS id, MAX(D.ts) AS max_ts, 
  now() - MAX(D.ts) AS diff 
  FROM account_device_map M 
  LEFT OUTER JOIN device_sensors_master D ON M.id = D.device_id 
  WHERE D.ts > '2014-11-01 00:00:00' 
  GROUP BY M.device_id) AS sq 
WHERE sq.diff < '2 hours';

note: both tables need to be in the same database