Calculating time difference between activity timestamps in a query

Question

I'm reasonably new to Access and having trouble solving what should be (I hope) a simple problem - think I may be looking at it through Excel goggles.

I have a table named importedData into which I (not so surprisingly) import a log file each day. This log file is from a simple data-logging application on some mining equipment, and essentially it saves a timestamp and status for the point at which the current activity changes to a new activity.

A sample of the data looks like this:

This information is then filtered using a query to define the range I want to see information for, say from 29/11/2013 06:00:00 AM until 29/11/2013 06:00:00 PM

Now the object of this is to take a status entry's timestamp and get the time difference between it and the record on the subsequent row of the query results. As the equipment works for a 12hr shift, I should then be able to build a picture of how much time the equipment spent doing each activity during that shift.

In the above example, the equipment was in status "START_SHIFT" for 00:01:00, in status "DELAY_WAIT_PIT" for 06:08:26 and so-on. I would then build a unique list of the status entries for the period selected, and sum the total time for each status to get my shift summary.

Answer 1

You can use a correlated subquery to fetch the next timestamp for each row.

SELECT
    i.status,
    i.timestamp,
    (
        SELECT Min([timestamp])
        FROM importedData
        WHERE [timestamp] > i.timestamp
    ) AS next_timestamp
FROM importedData AS i
WHERE i.timestamp BETWEEN #2013-11-29 06:00:00#
    AND #2013-11-29 18:00:00#;

Then you can use that query as a subquery in another query where you compute the duration between timestamp and next_timestamp. And then use that entire new query as a subquery in a third where you GROUP BY status and compute the total duration for each status.

Here's my version which I tested in Access 2007 ...

SELECT
    sub2.status,
    Format(Sum(Nz(sub2.duration,0)), 'hh:nn:ss') AS SumOfduration
FROM
    (
        SELECT
            sub1.status,
            (sub1.next_timestamp - sub1.timestamp) AS duration
        FROM
            (
                SELECT
                    i.status,
                    i.timestamp,
                    (
                        SELECT Min([timestamp])
                        FROM importedData
                        WHERE [timestamp] > i.timestamp
                    ) AS next_timestamp
                FROM importedData AS i
                WHERE i.timestamp BETWEEN #2013-11-29 06:00:00#
                    AND #2013-11-29 18:00:00#
            ) AS sub1
    )  AS sub2
GROUP BY sub2.status;

If you run into trouble or need to modify it, break out the innermost subquery, sub1, and test that by itself. Then do the same for sub2. I suspect you will want to change the WHERE clause to use parameters instead of hard-coded times.

Note the query Format expression would not be appropriate if your durations exceed 24 hours. Here is an Immediate window session which illustrates the problem ...

' duration greater than one day:
? #2013-11-30 02:00# - #2013-11-29 01:00#
 1.04166666667152 
' this Format() makes the 25 hr. duration appear as 1 hr.:
? Format(#2013-11-30 02:00# - #2013-11-29 01:00#, "hh:nn:ss")
01:00:00

However, if you're dealing exclusively with data from 12 hr. shifts, this should not be a problem. Keep it in mind in case you ever need to analyze data which spans more than 24 hrs.

If subqueries are unfamiliar, see Allen Browne's page: Subquery basics. He discusses correlated subqueries in the section titled Get the value in another record.