> ## 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_union

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

Use the `set_union` function in APL to combine two dynamic arrays into one, returning a new array that includes all distinct elements from both. The order of elements in the result isn’t guaranteed and may differ from the original input arrays.

You can use `set_union` when you need to merge two arrays and eliminate duplicates. It’s especially useful in scenarios where you need to perform set-based logic, such as comparing user activity across multiple sources, correlating IPs from different datasets, or combining traces or log attributes from different events.

## 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">
    APL’s `set_union` works similarly to using `mvappend` followed by `mvdedup` in SPL. While SPL stores multivalue fields and uses field-based manipulation, APL focuses on dynamic arrays. You need to explicitly apply set logic in APL using functions like `set_union`.

    <CodeGroup>
      ```sql Splunk example theme={null}
      | eval result=mvappend(array1, array2)
      | eval result=mvdedup(result)
      ```

      ```kusto APL equivalent theme={null}
      extend result = set_union(array1, array2)
      ```
    </CodeGroup>
  </Accordion>

  <Accordion title="ANSI SQL users">
    Standard SQL doesn’t support arrays as first-class types or set functions like `set_union`. However, conceptually, `set_union` behaves like applying `UNION` between two subqueries that return one column each, followed by a `DISTINCT`.

    <CodeGroup>
      ```sql SQL example theme={null}
      SELECT value FROM (
        SELECT value FROM table1
        UNION
        SELECT value FROM table2
      )
      ```

      ```kusto APL equivalent theme={null}
      extend result = set_union(array1, array2)
      ```
    </CodeGroup>
  </Accordion>
</AccordionGroup>

## Usage

### Syntax

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

### Parameters

| Name   | Type    | Description                |
| ------ | ------- | -------------------------- |
| Array1 | dynamic | The first array to merge.  |
| Array2 | dynamic | The second array to merge. |

### Returns

A dynamic array that contains the distinct elements of both input arrays.

## Example

Use `set_union` to return the union of two arrays.

**Query**

```kusto theme={null}
['sample-http-logs']
| extend together = set_union(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%20together%20%3D%20set_union\(dynamic\(%5B1%2C%202%2C%203%5D\)%2C%20dynamic\(%5B2%2C%203%2C%204%2C%205%5D\)\)%22%7D)

**Output**

| \_time           | together          |
| ---------------- | ----------------- |
| May 22, 11:42:52 | \[1, 2, 3, 4, 5 ] |

## 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.