The Azure Stream Analytics Lag function is reversed

Question

I am working with Stream analytics for a demo.

I have a physical device that is sendind coordinates as lat and long.
I want to add data about the speed of the device, so basicly take the coordidates difference (with ST_DISTANCE) and the time difference and divide them.

For this i used the query.

SELECT IoTHub.ConnectionDeviceId as deviceid,
EventEnqueuedUtcTime,
LAG(EventEnqueuedUtcTime, 1) OVER (PARTITION BY IoTHub.ConnectionDeviceId LIMIT DURATION(second, 29)) as previous
INTO timediffs
FROM mvpinput

coordinates are left out for now, i know how to add them.

In the editor i now see this as a result.

Where the first EventEnqueuedUtcTime in the list is the last data point send, But at the first item it can't determine the previous item. Every time has the next item.

So basicly it feels like the LAG function is reversed.

How would i fix this?

Answer 1

Hello @Oetze van den Broek ,
Thanks for the question and using MS Q&A platform.

As we understand the ask here is how to show the record as they arrived , please do let us know if its not accurate. I did some quick test against eventhub and I think the TIMSTAMP BY caluise should help .

SELECT EHInput.DeviceId as deviceid,
EventEnqueuedUtcTime,
LAG(EventEnqueuedUtcTime, 1) OVER (PARTITION BY EHInput.DeviceId LIMIT DURATION(second, 29)) as previous
into adlsgen2
FROM EHInput
TIMESTAMP BY EventEnqueuedUtcTime

Please do let me if you have any queries.
Thanks
Himanshu

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