> ## Documentation Index
> Fetch the complete documentation index at: https://axiom.co/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# set_difference

> This page explains how to use the set_difference function in APL.

Use the `set_difference` function in APL to compute the distinct elements in one array that aren’t present in another. This function helps you filter out shared values between two arrays, producing a new array that includes only the unique values from the first input array.

Use `set_difference` when you need to identify new or missing elements, such as:

* Users who visited today but not yesterday.
* Error codes that occurred in one region but not another.
* Service calls that appear in staging but not production.

## 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.

<AccordionGroup>
  <Accordion title="Splunk SPL users">
    In Splunk SPL, similar logic often uses the `setdiff` function from the `mv` (multivalue) function family. APL’s `set_difference` behaves similarly, returning values that are only in the first multivalue field.

    <CodeGroup>
      ```sql Splunk example theme={null}
      | eval a=mvappend("a", "b", "c"), b=mvappend("b", "c")
      | eval diff=mvfilter(NOT match(a, b))
      ```

      ```kusto APL equivalent theme={null}
      print a=dynamic(['a', 'b', 'c']), b=dynamic(['b', 'c'])
      | extend diff=set_difference(a, b)
      ```
    </CodeGroup>
  </Accordion>

  <Accordion title="ANSI SQL users">
    ANSI SQL doesn’t support array operations directly, but you can emulate set difference with `EXCEPT` when working with rows, not arrays. APL provides native array functions like `set_difference` for this purpose.

    <CodeGroup>
      ```sql SQL example theme={null}
      SELECT value FROM array1
      EXCEPT
      SELECT value FROM array2;
      ```

      ```kusto APL equivalent theme={null}
      print a=dynamic(['a', 'b', 'c']), b=dynamic(['b', 'c'])
      | extend diff=set_difference(a, b)
      ```
    </CodeGroup>
  </Accordion>
</AccordionGroup>

## Usage

### Syntax

```kusto theme={null}
set_difference(Array1, Array2)
```

### Parameters

| Name     | Type  | Description                                          |
| -------- | ----- | ---------------------------------------------------- |
| `Array1` | array | The array to subtract from.                          |
| `Array2` | array | The array containing values to remove from `Array1`. |

### Returns

An array that includes all values from `Array1` that aren’t present in `Array2`. The result doesn’t include duplicates.

## Example

Use `set_difference` to return the difference between two arrays.

**Query**

```kusto theme={null}
['sample-http-logs']
| extend difference = set_difference(dynamic([1, 2, 3]), dynamic([2, 3, 4, 5]))
```

[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%20difference%20%3D%20set_difference\(dynamic\(%5B1%2C%202%2C%203%5D\)%2C%20dynamic\(%5B2%2C%203%2C%204%2C%205%5D\)\)%22%7D)

**Output**

| \_time           | difference |
| ---------------- | ---------- |
| May 22, 11:42:52 | \[5, 1, 4] |

## List of related functions

* [set\_difference](apl/scalar-functions/mathematical-functions/set-difference): Returns elements in the first array that aren’t in the second. Use it to find exclusions.
* [set\_has\_element](/apl/scalar-functions/mathematical-functions/set-has-element): Tests whether a set contains a specific value. Prefer it when you only need a Boolean result.
* [set\_union](/apl/scalar-functions/mathematical-functions/set-union): Returns the union of two or more sets. Use it when you need any element that appears in at least one set instead of every set.