# UNION
URL: https://docs.valsight.ai/union/
Description: The UNION function combines the result sets of two or more input nodes into one output. All inputs must have the same levels and must not have overlapping rows.
**Category:** [Assumptions & result sets](/assumptions-result-sets/)

## Overview

The **UNION** function **combines the result sets of two or more input nodes** into one output.

Use this function when you want to append non-overlapping result slices from nodes that share the same dimensional structure. UNION is a stricter version of [ADDITION (+)](/addition/) and can serve as an extra guardrail because the inputs are required to have the same levels.

## Syntax

`UNION('Node1', 'Node2' [, 'Node3', ...])`

**Example usage:** `UNION('Salary Q1', 'Salary Q2')`

## Parameters

| Parameter            | Description                                     | Type           | Required |
| -------------------- | ----------------------------------------------- | -------------- | -------- |
| **Node1**            | First input node to combine                     | Node reference | Yes      |
| **Node2**            | Second input node to combine                    | Node reference | Yes      |
| **Additional nodes** | Any number of additional input nodes to combine | Node reference | No       |

## Output shape

| Aspect         | Behavior                                                                   |
| -------------- | -------------------------------------------------------------------------- |
| Dimensionality | Same as all inputs (they must all have the same levels).                   |
| Level values   | Union of all level values from all inputs.                                 |
| Row count      | Sum of all input row counts. No aggregation or deduplication is performed. |

## Watch out

* All inputs must have the **same levels**.
* Inputs must **not have overlapping rows**. If any row key appears in more than one input, the calculation fails at runtime.
* The same node **cannot appear more than once** in the parameter list.
* All inputs must have the **same unit**.

## Example

### Appending non-overlapping slices

This example shows UNION combining two disjoint salary slices with the same level structure. Because the rows do not overlap, the result appends both slices into one output.

**Input node:** `'Node1'`

| Year | Employee | Salary |
| ---- | -------- | ------ |
| 2025 | E1       | 50,000 |
| 2025 | E2       | 60,000 |

**Input node:** `'Node2'`

| Year | Employee | Salary |
| ---- | -------- | ------ |
| 2026 | E1       | 55,000 |
| 2026 | E2       | 66,000 |

**Formula:** `UNION('Node1', 'Node2')`

| Year | Employee | UNION Result |
| ---- | -------- | ------------ |
| 2025 | E1       | 50,000       |
| 2025 | E2       | 60,000       |
| 2026 | E1       | 55,000       |
| 2026 | E2       | 66,000       |

## Related

| Function                   | When to use instead                                                                                                      |
| -------------------------- | ------------------------------------------------------------------------------------------------------------------------ |
| [ADDITION (+)](/addition/) | When overlap is allowed or expected, or when the inputs do not need UNION's same-level and no-overlap constraints.       |
| [ROLLUP](/rollup/)         | When you need to change granularity or aggregate a node to different levels instead of combining disjoint result slices. |