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5.4

returning rows from log file that are at least 'n' minutes apart

Asked by carled in MS SQL Server

Tags: sql

Hi all,

I have a log file that takes readings of events at periodic intervals which can be as little as every 10 seconds. Sometimes it can be as much as 10 minutes apart though, depending on what is going on...

For the sake of ease, consider the table as having the following structure:
eventID int
deviceID int
groupID int
eventDateTime datetime

Now, I have a small report I run on this file, that effectively does:
SELECT *
FROM EventFile
WHERE deviceID=x AND groupID=y
ORDER BY eventDateTime asc

The problem with this, is that as there could be a lot of events for a particular device & group, I only show the readout on my asp-driven report where the events are a minute apart. In the worst-case scenario, I could end up dropping 5 records out of every 6 I've read in and this strikes me as super-inefficient... why select rows I'm not going to use?

So... I need a way of doing the same select statement, but only returning rows where the eventdatetime is at least a minute apart from the previously-selected row. It's not possible to assume that the events matching the specified deviceID & groupID will be 10 seconds apart, so it can't be done by selecting every 6th row.

I tried doing this as follows:
SELECT A.eventID, A.eventdatetime
FROM eventFile A
INNER JOIN
eventFile B ON A.deviceID = b.deviceID AND A.groupID = B.groupID
WHERE
(DATEDIFF(n, B.eventdatetime, A.eventdatetime) >= 1) AND (A.deviceID = 18) AND (A.groupID = 3)

But it didn't work properly... the SQL statement seems to compare the "B.eventdatetime" against the first-read "A.eventDateTime" rather than the previously-read one so I get loads of duplicate records and it all goes a bit pear-shaped...

I tried adding all sorts of "distincts" and experimented with different join types, but I can't get the syntax right to achieve the effect I need...

Thanks for any help!

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20091118-EE-VQP-93

returning rows from log file that are at least 'n' minutes apart

Asked by carled in MS SQL Server

Tags: sql

About this solution