First not null (nnfirst)

[ 1 Description ] [ 2 How does it work in the search window? ] [ 3 How does it work in LINQ? ]

Description

Returns the first non-null value found in a specified column for each grouping occurrence. To determine the first value, Devo uses the eventdate time rather than the log time, that is, the date Devo receives it rather than the date it was generated.

If you want to include null values, you can use the First (first) operation.

How does it work in the search window?

Before being able to perform this operation, you have to group your data. Be aware that the columns used as arguments for the grouping operation will not be available to select as arguments for the aggregation operation.

After grouping the data, select Aggregation in the search window toolbar, then select the First not null operation. You need to specify one argument:

Argument	Data type

Argument	Data type
First of ^mandatory	Any

The data type of the aggregated values will be the same as the data type of the column used as an argument.

You cannot repeat the First not null operation to apply subsequent aggregations. Use the Minimum (min) operation instead to get the same result. Repeating the First not null operation returns an error because it requires access to the eventdate field. However, this field isn't available for subsequent aggregations.

Example

In the siem.logtrust.web.activity table, we want to get the first non-null IP address value in each 5-minute period. Before aggregating the data, the table must be grouped in 5-minute intervals. Then we will perform the aggregation using the First not null operation.

The arguments needed for the First not null operation are:

First of → clientIpAddress column

Click Aggregate function and you will see the following result:

How does it work in LINQ?

Group your data using the following structure:

group every server period by column1, column2...
every client period

Then, use select... as... to add the new column that will show the aggregated values. This is the syntax for the First not null operation:

nnfirst(column)

See Build a query using LINQ to learn more about grouping and aggregating your data using the LINQ language.

Example

You can copy the following LINQ syntax and try the example above on the siem.logtrust.web.activity table:

from siem.logtrust.web.activity
group every 5m
every 5m
  select nnfirst(city) as firstnotnull_cityfrom siem.logtrust.web.activity
group every 5m
every 5m
  select nnfirst(city) as firstnotnull_city