Question

I have a weather database which gets datasets about every 70 seconds (depends on when the weather-station delivers the data).
I want to graph it using Pchart but i have too many samples so the X axis is screwed up.
So i want the data for about every 5 minutes. (or every 30 minutes)
The query i currently have is this:

SELECT time, temp_out FROM wmr200 WHERE date(time) = curdate()

This gets the samples for the last 24 hours but there are too many.

Answer 1

The following will get you a sample consisting of any data with a timestamp at :00, :05, :10 …

SELECT time, temp_out FROM wmr200 WHERE date(time) = curdate()
    AND mod(minute(time),5) = 0

I’m using the modulo function to check if the minute part of the time is (0 or) divisible by 5.

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If you need only one result for each time segment, we’ve got to get quite a bit more complex.

SELECT concat(date(time),' ',hour(time),':',round(minute(time)/5,0)*5), 
       min(temp_out), avg(temp_out), max(temp_out) 
FROM wmr200 
GROUP BY date(time), hour(time), round(minute(time)/5,0)*5

I don’t have access to a MySQL instance to test it out right now, but here’s the idea. It groups by every five minutes, by grouping by date, hour, and round(minute(time)/5,0)*5 – which rounds minute to the nearest 5.

It selects the value of time that it should be grouped around, and the min, avg, and max temp_out, as I wasn’t sure which of these would best apply to your situation.

If, however, your DB doesn’t have data for a five minute stretch (or 30 minute stretch, if that’s what you’re using), it still may not have data for a sample point.