isnull

This page explains how to use the isnull function in APL.

The isnull function evaluates its argument and returns true if the argument is null. Use this function to identify missing data, filter out incomplete records, or validate that optional fields are absent.

Usage

Syntax

APL

isnull(value)

Parameters

Name	Type	Required	Description
value	scalar	Yes	The value to check for null.

Returns

Returns true if the value is null, otherwise returns false. Note that empty strings return false because they're not null.

Use case examples

Identify HTTP requests with missing duration information to assess data quality and completeness.

Query

APLRun in Playground

['sample-http-logs']
| extend missing_duration = isnull(req_duration_ms)
| summarize total_requests = count(),
            missing_duration_count = countif(missing_duration),
            missing_percentage = round(100.0 * countif(missing_duration) / count(), 2) by status
| sort by missing_duration_count desc
| limit 10

Output

status	total_requests	missing_duration_count	missing_percentage
500	1234	123	9.97
200	8765	87	0.99
404	2341	23	0.98

This query identifies the percentage of requests missing duration data by status code, helping assess logging infrastructure reliability and identify potential issues.

Find traces with missing duration information to identify instrumentation problems.

Query

APLRun in Playground

['otel-demo-traces']
| extend null_duration = isnull(duration)
| where null_duration
| summarize incomplete_spans = count() by ['service.name'], kind
| sort by incomplete_spans desc
| limit 10

Output

service.name	kind	incomplete_spans
product-catalog	server	234
cart	internal	123
checkout	client	89

This query identifies services with incomplete trace data, helping pinpoint instrumentation issues where duration information is not being captured properly.

Identify anonymous access attempts by finding requests without user identification.

Query

APLRun in Playground

['sample-http-logs']
| extend anonymous = isnull(id)
| summarize total_failures = count(),
            anonymous_failures = countif(anonymous) by status, ['geo.country']
| extend anonymous_rate = round(100.0 * anonymous_failures / total_failures, 2)
| where anonymous_failures > 10
| sort by anonymous_failures desc
| limit 10

Output

status	geo.country	total_failures	anonymous_failures	anonymous_rate
401	Unknown	567	345	60.85
403	Russia	234	189	80.77
401	China	198	156	78.79

This query identifies patterns of anonymous failed access attempts by country, helping security teams detect automated attacks or scanning activity.

isnotnull: Returns true if a value isn't null. Use this for the inverse check of isnull.
isempty: Checks if a value is empty or null. Use this when you need to check for both null and empty strings.
coalesce: Returns the first non-null value from a list. Use this to provide default values for null fields.
gettype: Returns the type of a value. Use this to distinguish between null and other types.

isnull

Usage

Syntax

Parameters

Returns

Use case examples

List of related functions

Other query languages