Question

I have a table with AIS marine data that gets updated very often.

What I need is the data from the last 30 minutes and from that result the newest rows and MMSI should be unique.

The query I have now:

select max(timestamp) as timestamp, mmsi, navstatus, rot, sog, lon, lat, cog,
thead, man, mtype from ais_cnb
where (timestamp > (now() - interval 30 minute))
group by mmsi
order by timestamp desc

It seems like all the data except the timestamp is old.

Answer 1

If you are wanting the latest row from the last 30 minutes for each unique “mmsi” that has one, then using a join to a subquery where you find the max timestamps first should work, like:

SELECT timestamp, a.mmsi, navstatus, rot, sog, lon, lat, cog, thead, man, mtype
FROM ais_cnb a INNER JOIN
(SELECT mmsi, MAX(timestamp) AS max_timestamp FROM ais_cnb
 WHERE timestamp > (now() - interval 30 minute)
 GROUP BY mmsi) t
ON ((timestamp = t.max_timestamp) AND (a.mmsi = t.mmsi))