Question

Problem:

I have a database of sensor readings with a timestamp for the time the sensor was read. Basically it looks like this:

Sensor | Timestamp | Value

Now I want to make a graph out of this data and I want to make several different graphs. Say I want one for the last day, one for the last week and one for the last month. The resolution of each graph will be different so for the day-graph the resolution would be 1 minute. For the week graph it would be one hour and for the month graph it would be one day, or quarter of a day.

So I would like an output that is the average of each resolution (eg. Day = Average over the minute, Week = Average over the hour and so on)

Ex:

Sensor | Start | End | Average

How do I do this easily and quickly in mySQL? I suspect it invoves creating a temporary table or sorts and joining the sensor data with that to get the average values of the sensor? But my knowledge of mySQL is limited at best.

Is there a really clever way to do this?

Answer 1

SELECT  DAY(Timestamp), HOUR(Timestamp), MINUTE(Timestamp), AVG(value)
FROM    mytable
GROUP BY
        DAY(Timestamp), HOUR(Timestamp), MINUTE(Timestamp) WITH ROLLUP

WITH ROLLUP clause here produces extra rows with averages for each HOUR and DAY, like this:

SELECT  DAY(ts), HOUR(ts), MINUTE(ts), COUNT(*)
FROM    (
        SELECT  CAST('2009-06-02 20:00:00' AS DATETIME) AS ts
        UNION ALL
        SELECT  CAST('2009-06-02 20:30:00' AS DATETIME) AS ts
        UNION ALL
        SELECT  CAST('2009-06-02 21:30:00' AS DATETIME) AS ts
        UNION ALL
        SELECT  CAST('2009-06-03 21:30:00' AS DATETIME) AS ts
        ) q
GROUP BY
        DAY(ts), HOUR(ts), MINUTE(ts) WITH ROLLUP

2, 20, 0, 1
2, 20, 30, 1
2, 20, NULL, 2
2, 21, 30, 1
2, 21, NULL, 1
2, NULL, NULL, 3
3, 21, 30, 1
3, 21, NULL, 1
3, NULL, NULL, 1
NULL, NULL, NULL, 4

2, 20, NULL, 2 here means that COUNT(*) is 2 for DAY = 2, HOUR = 20 and all minutes.