Calcule el promedio, la varianza y la desviación estándar de dos números en dos filas/columnas diferentes con sql/PHP en fechas específicas

¿Es tan fácil como agregar la fecha en el group by? . Esta es la sintaxis que debería funcionar tanto en MySQL como en SQLite, basando la fecha en la hora de finalización y asumiendo que la hora de finalización se almacena como una fecha y hora:

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

Si se almacena como una marca de tiempo, consulte el comentario de Marty.