Count changes from specific value within time period - sql

Question

MS Access query question.
In its simplified version, I have a table containing a (support) ticketid column, status column and a timestamp column. The timestamp column indicates when the (new) status was set for the ticket.
My final goal is to count the number of tickets that changed status FROM status 18 within a certain time frame. This includes tickets that got status 18 within the time frame and 'lost' it again, but also tickets that already had status 18 at the start of the time frame.
My intermediate query below obviously is not correct, as it returns too many records. It does not just compare to the next record (in time), but any record.
SELECT *
FROM
(SELECT tkhtimestamp, tkhticketid, tkhstatusid FROM tickethistory WHERE tkhstatusid NOT IN (18) AND tkhtimestamp >= #3-jul-2014 9:00:00# AND tkhtimestamp <= #4-jul-2014 9:00:00#) AS new_status_records
INNER JOIN
(SELECT tkhtimestamp, tkhticketid, tkhstatusid FROM tickethistory WHERE tkhstatusid IN (18) AND tkhtimestamp <= #4-jul-2014 9:00:00#) AS old_status_records
ON old_status_records.tkhticketid = new_status_records.tkhticketid AND old_status_records.tkhtimestamp < new_status_records.tkhtimestamp
Can someone please advise on this, as I cannot seem to get this done. Other efforts using max/min and group by failed as well.
Thank you.

Answer 1

You had the right idea in using MAX/MIN and GROUP BY. For test data in [tickethistory]
tkhtimestamp tkhticketid tkhstatusid
------------------- ----------- -----------
2014-07-02 10:00:00 1 10
2014-07-02 12:00:00 2 18
2014-07-03 13:30:00 1 18
2014-07-03 13:35:00 1 99
2014-07-03 15:00:00 2 99
the following query will extract the current and previous timestamp for each new history record of each ticket:
SELECT
curr.tkhticketid,
curr.tkhtimestamp AS curr_timestamp,
MAX(prev.tkhtimestamp) AS prev_timestamp
FROM
tickethistory curr
INNER JOIN
tickethistory prev
ON curr.tkhticketid = prev.tkhticketid
AND curr.tkhtimestamp > prev.tkhtimestamp
GROUP BY
curr.tkhticketid,
curr.tkhtimestamp
It returns
tkhticketid curr_timestamp prev_timestamp
----------- ------------------- -------------------
1 2014-07-03 13:30:00 2014-07-02 10:00:00
1 2014-07-03 13:35:00 2014-07-03 13:30:00
2 2014-07-03 15:00:00 2014-07-02 12:00:00
We can then join that back to the [tickethistory] table (twice) to retrieve the current and previous status values
SELECT
tkh_curr.tkhticketid,
tkh_curr.tkhtimestamp AS curr_timestamp,
tkh_curr.tkhstatusid AS curr_status,
tkh_prev.tkhtimestamp AS prev_timestamp,
tkh_prev.tkhstatusid AS prev_status
FROM
(
(
SELECT
curr.tkhticketid,
curr.tkhtimestamp AS curr_timestamp,
MAX(prev.tkhtimestamp) AS prev_timestamp
FROM
tickethistory curr
INNER JOIN
tickethistory prev
ON curr.tkhticketid = prev.tkhticketid
AND curr.tkhtimestamp > prev.tkhtimestamp
GROUP BY
curr.tkhticketid,
curr.tkhtimestamp
) curr_prev
INNER JOIN
tickethistory tkh_curr
ON tkh_curr.tkhticketid = curr_prev.tkhticketid
AND tkh_curr.tkhtimestamp = curr_prev.curr_timestamp
)
INNER JOIN
tickethistory tkh_prev
ON tkh_prev.tkhticketid = curr_prev.tkhticketid
AND tkh_prev.tkhtimestamp = curr_prev.prev_timestamp
returning
tkhticketid curr_timestamp curr_status prev_timestamp prev_status
----------- ------------------- ----------- ------------------- -----------
1 2014-07-03 13:30:00 18 2014-07-02 10:00:00 10
1 2014-07-03 13:35:00 99 2014-07-03 13:30:00 18
2 2014-07-03 15:00:00 99 2014-07-02 12:00:00 18
If you save that query in Access with a name like [qryTicketHistoryChanges] then you should be able to run other queries against it to pull out the specific information you need.