SQL for selecting row with max timestamp

gistfile1.sql

SELECT n.node_id, n.lat, n.lng, n.timestamp
FROM nodes AS n
INNER JOIN (
  SELECT node_id, MAX(timestamp) AS timestamp
  FROM nodes
  GROUP BY node_id
) AS mn
ON n.node_id = mn.node_id
AND n.timestamp = mn.timestamp;

DELETE n.*
FROM nodes AS n
INNER JOIN (
  SELECT node_id, MAX(timestamp) AS timestamp
  FROM nodes
  GROUP BY node_id
) AS mn
ON n.node_id = mn.node_id
AND n.timestamp < (mn.timestamp - 30 seconds) -- Verify the subtration operation you need, I'm not sure what it is offhand