Calculate average, variance and standard deviation of two numbers in two different rows/columns with sql / PHP on specific dates

Question

I have a Database with the following structure:

rowid       ID                  startTimestamp   endTimestamp   subject
1           00:50:c2:63:10:1a   ...1000          ...1090        entrance
2           00:50:c2:63:10:1a   ...1100          ...1270        entrance
3           00:50:c2:63:10:1a   ...1300          ...1310        door1
4           00:50:c2:63:10:1a   ...1370          ...1400        entrance
.
.
.

With this SQL-Query i can get the average differences between the endTime and the startTime between one row and the following row, sorted by subject and ID, with their min,max,variance and standard deviation:

SELECT ID,AVG(diff) AS average,
   AVG(diff*diff) - AVG(diff)*AVG(diff) AS variance,
   SQRT(AVG(diff*diff) - AVG(diff)*AVG(diff)) AS stdev,
   MIN(diff) AS minTime,
   MAX(diff) AS maxTime

FROM
(SELECT t1.id, t1.endTimestamp,
        min(t2.startTimeStamp) - t1.endTimestamp AS diff
FROM table1 t1
INNER JOIN table1 t2
ON t2.ID = t1.ID AND t2.subject = t1.subject
AND t2.startTimestamp > t1.startTimestamp  -- consider only later startTimestamps
WHERE t1.subject = 'entrance'
GROUP BY t1.id, t1.endTimestamp) AS diffs
GROUP BY ID

This works fine, if i have only a few rows on the same day with less time differences, you can see it in this sqlfiddle:

http://sqlfiddle.com/#!2/6de73/1

But when i have additional data on just a different day, i get bad values:

http://sqlfiddle.com/#!2/920b6/1

Therefore, I want to calculate the avg, min, max, variance, standard deviation for each single day.

I know that there are the DATE-Functions of MySQL but i couldn't get it done...can somebody help me? Or do i have to write a piece of PHP code that could handle this?

Answer 1

Is it as easy as adding the date into the group by. Here is syntax that should work in both MySQL and SQLite, basing the date on the end time and assuming the end time is stored as a datetime:

SELECT ID, thedate, AVG(diff) AS average,
   AVG(diff*diff) - AVG(diff)*AVG(diff) AS variance,
   SQRT(AVG(diff*diff) - AVG(diff)*AVG(diff)) AS stdev,
   MIN(diff) AS minTime,
   MAX(diff) AS maxTime
FROM (SELECT t1.id, t1.endTimestamp, DATE(endtimestamp) as thedate,
             min(t2.startTimeStamp) - t1.endTimestamp AS diff
      FROM table1 t1 INNER JOIN
           table1 t2
           ON t2.ID = t1.ID AND t2.subject = t1.subject AND
              t2.startTimestamp > t1.startTimestamp  -- consider only later startTimestamps
     WHERE t1.subject = 'entrance'
     GROUP BY t1.id, t1.endTimestamp
    ) AS diffs
GROUP BY ID, thedate

If stored as a time stamp, see Marty's comment.