Question

I have a table like this:

Index , PersonID  , ItemCount         , UnixTimeStamp
1     , 1         , 1             , 1296000000
2     , 1         , 2             , 1296000100
3     , 2         , 4             , 1296003230
4     , 2         , 6             , 1296093949
5     , 1         , 0             , 1296093295

Time and index always go up. Its basically a logging table to log the itemcount each time it changes. I get the most recent ItemCount for each Person like this:

SELECT *
FROM table a
INNER JOIN
(
 SELECT MAX(index) as i
 FROM table
 GROUP BY PersonID) b
ON a.index = b.i;

What I want to do is get get the most recent record for each PersonID that is at least 24 hours older than the most recent record for each Person ID. Then I want to take the difference in ItemCount between these two to get a change in itemcount for each person over the last 24 hours:

personID ChangeInItemCountOverAtLeast24Hours

    1        3
    2        -11
    3        6

Im sort of stuck with what to do next. How can I join another itemcount based on latest adjusted timestamp of individual rows?

Answer 1

You might want to add some composite indexes to help the query, depending (among others) on the the read:write ratio.

SELECT lsG.PersonID,
  lsG.CrItemCount,
  lsI.ItemCount,
  IF(lsI.ItemCount IS NULL, 0, lsG.CrItemCount  - lsI.ItemCount) Change
FROM (
  SELECT cr.PersonID, MAX(ls.index) MaxLsIndex, cr.ItemCount CrItemCount
  FROM (
    SELECT crI.Index, crI.PersonID, crI.ItemCount, crI.UnixTimeStamp
    FROM table crI JOIN (
      SELECT PersonID, MAX(index) MaxIndex
      FROM table
      GROUP BY PersonID
    ) crG ON crI.Index = crG.MaxIndex
  ) cr LEFT JOIN table ls ON cr.PersonID = ls.PersonID
        AND ls.UnixTimeStamp < (cr.UnixTimeStamp - 86400 /*24 hours*/)
  GROUP BY cr.PersonID
) lsG LEFT JOIN table lsI ON lsG.MaxLsIndex IS NOT NULL
        AND lsG.MaxLsIndex = lsI.index

I named the table aliases:

• cr = Current
• ls = Last (most recent record before most recent – 24 hours)
• Suffix G = Groupped
• Suffix I = Item (the record itself found by the groups MaxID)