prev()

Returns the value of a specific column in a specified row. The specified row is at a specified offset from the current row in a serialized row set.

Syntax

prev(column, [ offset ], [ default_value ] )

Learn more about syntax conventions.

Parameters

Name	Type	Required	Description
column	string	✔️	The column from which to get the values.
offset	int		The offset to go back in rows. The default is 1.
default_value	scalar		The default value to be used when there are no previous rows from which to take the value. The default is null.

Examples

Filter data based on comparison between adjacent rows

The following query returns rows that show breaks longer than a quarter of a second between calls to sensor-9.

Run the query

TransformedSensorsData
| where SensorName == 'sensor-9'
| sort by Timestamp asc
| extend timeDiffInMilliseconds = datetime_diff('millisecond', Timestamp, prev(Timestamp, 1))
| where timeDiffInMilliseconds > 250

Output

Timestamp	SensorName	Value	PublisherId	MachineId	timeDiff
2022-04-13T00:58:53.048506Z	sensor-9	0.39217481975439894	fdbd39ab-82ac-4ca0-99ed-2f83daf3f9bb	M100	251
2022-04-13T01:07:09.63713Z	sensor-9	0.46645392778288297	e3ed081e-501b-4d59-8e60-8524633d9131	M100	313
2022-04-13T01:07:10.858267Z	sensor-9	0.693091598493419	278ca033-2b5e-4f2c-b493-00319b275aea	M100	254
2022-04-13T01:07:11.203834Z	sensor-9	0.52415808840249778	4ea27181-392d-4947-b811-ad5af02a54bb	M100	331
2022-04-13T01:07:14.431908Z	sensor-9	0.35430645405452	0af415c2-59dc-4a50-89c3-9a18ae5d621f	M100	268
...	...	...	...	...	...

Perform aggregation based on comparison between adjacent rows

The following query calculates the average time difference in milliseconds between calls to sensor-9.

Run the query

TransformedSensorsData
| where SensorName == 'sensor-9'
| sort by Timestamp asc
| extend timeDiffInMilliseconds = datetime_diff('millisecond', Timestamp, prev(Timestamp, 1))
| summarize avg(timeDiffInMilliseconds)

Output

avg_timeDiffInMilliseconds
30.726900061254298

Extend row with data from the previous row

In the following query, as part of the serialization done with the serialize operator, a new column previous_session_type is added with data from the previous row. Since there was no session prior to the first session, the column is empty in the first row.

Run the query

ConferenceSessions
| where conference == 'Build 2019'
| serialize previous_session_type = prev(session_type)
| project time_and_duration, session_title, session_type, previous_session_type

Output

time_and_duration	session_title	session_type	previous_session_type
Mon, May 6, 8:30-10:00 am	Vision Keynote - Satya Nadella	Keynote
Mon, May 6, 1:20-1:40 pm	Azure Data Explorer: Advanced Time Series analysis	Expo Session	Keynote
Mon, May 6, 2:00-3:00 pm	Azure's Data Platform - Powering Modern Applications and Cloud Scale Analytics at Petabyte Scale	Breakout	Expo Session
Mon, May 6, 4:00-4:20 pm	How BASF is using Azure Data Services	Expo Session	Breakout
Mon, May 6, 6:50 - 7:10 pm	Azure Data Explorer: Operationalize your ML models	Expo Session	Expo Session
...	...	...	...