MySQL平均每1个月24小时

Question

So I have a table with 100's of thousands of rows like so:

+--+----------+-------------------+
|ID|metervalue  |datetime         |
|1   |10        |2016-02-23 01:00 |
|2   |20        |2016-02-23 02:00 |
|3   |5         |2016-02-23 03:00 |
|... |...       |...              |
|x   |x         |2016-02-24 01:00 |
|x   |x         |2016-02-24 02:00 |
|x   |x         |2016-02-24 03:00 |
|... |...       |...              |
|x   |x         |2016-03-01 01:00 |
|x   |x         |2016-03-01 02:00 |
|x   |x         |2016-03-01 03:00 |
+--+----------+-------------------+

So there is some meter reading taken every hour for many, many days.

Now I need to get data for a graph. Hourly graph data is easy because I can just query the data as it is, but for "monthly" data I need to query all the rows from a duration of a month so that every 24 hours is averaged into a single row.

So one row out of 30 (assuming month has 30 days) should contain 24 hours averaged into a 1 day.

I tried this solution, but it did not work as I would have wanted it to work: Average of data for every 5 minutes in the given times

I don't think the GROUP BY in that answer for example works because I don't want to average, say, every fifth hour of every day, but instead I want to average the 24 hours of a day to make up one day.

Answer 1

You still will GROUP BY but you do it by what the DATE() function returns which disregards the time part of the data, like so:

SELECT DATE(a.datetime), AVG(a.metervalue)
FROM MyInterestingTable a
GROUP BY DATE(a.datetime)