my question is more "theoretical" than practical - in other words, Im not really looking for a particular code for how to do something, but more like an advice about how to do it. Ive been thinking about it for some time but cannot come up with some feasible solution. So basically, I have a MySQL database that saves weather information from my weather station.
Column one contains date and time of measurement (Datetime format field), then there is a whole range of various columns like temp, humidity etc. The one I am interested in now is the one with the temperature. The data is sorted by date and time ascending, meaning the most recent value is always inserted to the end.
Now, what I want to do is using a PHP script, connect to the db and find temperature changes within a certain interval and then find the maximum. In other words, for example lets say I choose interval 3h. Then I would like to find the time, from all the values, where there was the most significant temperature change in those 3 h (or 5h, 1 day etc.).
The problem is that I dont really know how to do this. If I just get the values from the db, Im getting the values one by one, but I cant think of a way of getting a value that is lets say 3h from the current in the past. Then it would be easy, just subtracting them and get the date from the datetime field at that time, but how to get the values that are for example those 3 h apart (also, the problem is that it cannot just simply be a particular number of rows to the past as the intervals of data save are not regular and range between 5-10mins, so 3 h in the past could be various number of rows).
Any ideas how this could be done?
Thx alot
Not terribly hard actually. So I would assume it's a two column table with time and temp fields, where time is a DATETIME field
SELECT MAX(temp) FROM records
WHERE time >= "2013-10-14 12:00:00" and time <= "2013-10-14 15:00:00"
SELECT t1.*, ABS(t1.temperature - t2.temperature) as change
FROM tablename t1
JOIN tablename t2
ON t2.timecolumn <= (t1.timecolumn - INTERVAL 3 HOUR)
LEFT JOIN tablename t3
ON t3.timecolumn <= (t1.timecolumn - INTERVAL 3 HOUR)
AND t2.timecolumn > t3.timecolumn
WHERE
t3.some_non_nullable_column IS NULL
ORDER BY ABS(t1.temperature - t2.temperature) DESC
LIMIT 1;
1 table joined 2 times on itself, t2 is the quaranteed direct predecessor of t1 t2 is the closest record with offset 3h before or more. This could with the proper indexes, and a limited amount of data (where limited is in the eye of the beholder) be quite performant. However, if you need a lot of those queries in a big dataset, this is a prime candidate for denormalization, were you create a table which also stores the calculated offsets compared to the previous entry.