Beräkna medelvärde, varians och standardavvikelse för två tal i två olika rader/kolumner med sql / PHP på specifika datum

Är det lika enkelt som att lägga till datumet i group by . Här är syntax som bör fungera i både MySQL och SQLite, baserat datumet på sluttiden och förutsatt att sluttiden lagras som en datumtid:

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

Om den lagras som en tidsstämpel, se Martys kommentar.