Show average values for large amount of data - mysql

Question

I have a MySQL database named WindData which looks like this:
timestamp
temperature
windspeed
winddirection
I add a new row every 2-3 minutes so over a year time there will be lots of rows.
Now I want to present the data as a chart within a certain timeframe, (4 days ago, last month, last 6 month, 2011-2012...). Say that I want to display how the temperature has varied for the past year, using Google Charts to display this. Then Google chart has a maximum limit of the amount of datapoints that you may use.
I would then like a SQL query where I specify the timerange (2012-01-01 -- 2013-10-10) that gives me
A fixed number of rows (for example 200)
Every row contains the average and max value over that interval.
An ascii art example:
...............1..............2...............+..............199..............200
Where . is one row in my table, and the numbers represent average and maxvalue of the previous dots.
Some psudocode that might show what I am trying to accomplish is:
SELECT AVG(temperature)
FROM WindData
WHERE timestamp > 2012-01-01 AND timestamp < 2013-10-10
This would just give me one result where I get the average value of the whole timerange.
So maybe there is a way to create one more SQL statement which runs the above sql statement 200 times with different time-range.
SELECT AVG(temperature)
FROM WindData
WHERE timestamp > 2012-01-01 AND timestamp < 2013-02-1
SELECT AVG(temperature)
FROM WindData
WHERE timestamp > 2012-02-01 AND timestamp < 2013-03-1
SELECT AVG(temperature)
FROM WindData
WHERE timestamp > 2012-03-01 AND timestamp < 2013-04-1
SELECT AVG(temperature)
FROM WindData
WHERE timestamp > 2012-04-01 AND timestamp < 2013-05-1
...and so on.
If anyone is interested, I will use the help here to present better diagrams on www.surfvind.se, which displays weather data from a homebuilt weather station.

Answer 1

You can get a fixed number of rows using something like this:-
SELECT units.i + tens.i * 10 + hundreds.i AS aNumber
FROM (SELECT 0 AS i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) units
CROSS JOIN (SELECT 0 AS i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) tens
CROSS JOIN (SELECT 0 AS i UNION SELECT 1 ) hundreds
You can use something like this to get the different ranges, and join it against you data to get the number of values within each range.
EDIT - To go with the details you have added:-
SELECT Sub1.aDate, AVG(temperature)
FROM
(
SELECT DATE_ADD('2012-01-01', INTERVAL units.i + tens.i * 10 + hundreds.i * 100 DAY) AS aDate
FROM (SELECT 0 AS i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) units
CROSS JOIN (SELECT 0 AS i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) tens
CROSS JOIN (SELECT 0 AS i UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) hundreds
) Sub1
LEFT OUTER JOIN
WindData
ON Sub1.aDate = DATE(WindData.`timestamp`)
GROUP BY Sub1.aDate
This is getting a range of 1000 days starting from 2012-01-01 (you can easily limit that range in the subselect if you want), and matching that against the temp values for a day and getting the average group by date.