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.

For users of other query languages

If you come from other query languages, this section explains how to adjust your existing queries to achieve the same results in APL.

Splunk SPL users

In Splunk SPL, you check for null values using isnull() function. APL’s isnull works the same way.

Splunk example

| where isnull(field)

ANSI SQL users

In ANSI SQL, you check for null values using IS NULL. APL’s isnull provides the same functionality with function syntax.

SQL example

SELECT * FROM logs WHERE field IS NULL;

Usage

Syntax

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 are not null.

Use case examples

Log analysis

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

Query

['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

Run in Playground

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.

List of related functions

• isnotnull: Returns true if a value is not 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.