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```json
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"path": "/apl/scalar-functions/conversion-functions/ensure-field",
"feedback": "Description of the issue"
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```
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# ensure_field
> This page explains how to use the ensure_field function in APL.
Use the `ensure_field` function to safely access a field that may or may not exist in your data. The function returns the field’s value if it exists, or a typed nil if it doesn’t. This helps you write queries that work even when fields are missing, making your queries more robust and future-proof.
You typically use `ensure_field` when working with schemaless or evolving data where fields might be absent, or when you want to write queries that handle missing fields gracefully without errors.
## 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.
In Splunk, you can use `if` expressions with `isnull` or check field existence with `isnull(field)`. In APL, `ensure_field` provides a more type-safe way to handle missing fields by returning a typed nil that matches the expected field type.
```sql Splunk example theme={null}
... | eval status = if(isnull(status_code), null(), status_code)
```
```kusto APL equivalent theme={null}
... | extend status = ensure_field('status_code', typeof(string))
```
In standard SQL, you use `COALESCE` or `ISNULL` functions to handle missing values, but these don't check for field existence. In APL, `ensure_field` checks if a field exists and returns a typed nil if it doesn't, allowing you to write queries that work with optional fields.
```sql SQL example theme={null}
SELECT COALESCE(optional_field, NULL) AS field_value FROM logs;
```
```kusto APL equivalent theme={null}
['sample-http-logs']
| extend field_value = ensure_field('optional_field', typeof(string))
```
## Usage
### Syntax
```kusto theme={null}
ensure_field(field_name, field_type)
```
### Parameters
| Name | Type | Description |
| ----------- | ------ | ------------------------------------------------------------------------------------------------------ |
| field\_name | string | The name of the field to ensure exists. |
| field\_type | type | The type of the field. See [scalar data types](/apl/data-types/scalar-data-types) for supported types. |
### Returns
This function returns the value of the specified field if it exists, otherwise it returns a typed nil that matches the specified type.
## Use case examples
Handle missing fields gracefully when analyzing HTTP logs where some fields might not be present in all records.
**Query**
```kusto theme={null}
['sample-http-logs']
| extend user_agent = ensure_field('user_agent', typeof(string))
| extend referer = ensure_field('referer', typeof(string))
| where isnotnull(user_agent) or isnotnull(referer)
| project _time, ['uri'], user_agent, referer
```
[Run in Playground](https://play.axiom.co/axiom-play-qf1k/query?initForm=%7B%22apl%22%3A%22%5B'sample-http-logs'%5D%20%7C%20extend%20user_agent%20%3D%20ensure_field\('user_agent'%2C%20typeof\(string\)\)%20%7C%20extend%20referer%20%3D%20ensure_field\('referer'%2C%20typeof\(string\)\)%20%7C%20where%20isnotnull\(user_agent\)%20or%20isnotnull\(referer\)%20%7C%20project%20_time%2C%20%5B'uri'%5D%2C%20user_agent%2C%20referer%22%7D)
**Output**
| \_time | uri | user\_agent | referer |
| ---------------- | ---------- | ----------- | ------------------------------------------ |
| Jun 24, 09:28:10 | /api/users | Mozilla/5.0 | [https://example.com](https://example.com) |
This example safely accesses optional fields that may not exist in all log records, allowing the query to run successfully even when some fields are missing.
Access optional trace attributes that might not be present in all spans.
**Query**
```kusto theme={null}
['otel-demo-traces']
| extend http_method = ensure_field('http.method', typeof(string))
| extend http_path = ensure_field('http.path', typeof(string))
| where ['kind'] == 'server'
| project _time, ['trace_id'], ['service.name'], http_method, http_path
```
[Run in Playground](https://play.axiom.co/axiom-play-qf1k/query?initForm=%7B%22apl%22%3A%22%5B'otel-demo-traces'%5D%20%7C%20extend%20http_method%20%3D%20ensure_field\('http.method'%2C%20typeof\(string\)\)%20%7C%20extend%20http_path%20%3D%20ensure_field\('http.path'%2C%20typeof\(string\)\)%20%7C%20where%20%5B'kind'%5D%20%3D%3D%20'server'%20%7C%20project%20_time%2C%20%5B'trace_id'%5D%2C%20%5B'service.name'%5D%2C%20http_method%2C%20http_path%22%7D)
**Output**
| \_time | trace\_id | service.name | http\_method | http\_path |
| ---------------- | --------- | ------------ | ------------ | ---------- |
| Jun 24, 09:28:10 | abc123 | frontend | GET | /api/users |
This example safely accesses optional HTTP attributes in trace data, ensuring the query works even when these attributes are not present in all spans.
## List of related functions
* [isnull](/apl/scalar-functions/string-functions/isnull): Checks if a value is null. Use `isnull` to test the result of `ensure_field` to determine if a field exists.
* [isnotnull](/apl/scalar-functions/string-functions/isnotnull): Checks if a value is not null. Use `isnotnull` to verify that `ensure_field` successfully retrieved a field value.
* [coalesce](/apl/scalar-functions/string-functions/coalesce): Returns the first non-null value from a list. Use `coalesce` with `ensure_field` to provide default values when fields are missing.