---
type: "fhirpath-function"
title: "FHIRPath Function: combine"
function: "combine"
category: "Combining"
section: "5.4.2"
source: "fhirpath/functions.json"
---
# FHIRPath Function: combine
Merge the input and other collections into a single collection without eliminating duplicate values. Combining an empty collection with a non-empty collection will return the non-empty collection.

> **Note:** The contents of this section are Standard for Trial Use (STU)
{: .stu-note }
When `preserveOrder` is `false`, or not supplied, there is no expectation of order. When `preserveOrder` is `true`, the items of the other collection are appended to the items of the input collection, preserving the order of items in both collections.
{: .stu}

For example, considering the same two lists of integers used in the union example `A: 1, 1, 2, 3` and `B: 2, 3`:
{: .stu}
```fhirpath
A.combine(B) // 1, 1, 2, 2, 3, 3 - order is not guaranteed to be preserved (could be in any order)
A.combine(B, true) // 1, 1, 2, 3, 2, 3 - The order is preserved using the `preserveOrder` argument
A.combine( {} ) // 1, 1, 2, 3 - combining an empty collection with a non-empty collection returns the non-empty collection
```
{: .stu}
Note that the duplicate `1`s are not removed from the collection using combine, where using `union` or `|` they would have been.
{: .stu}
## Summary
- **Category**: Combining
- **Section**: `5.4.2`
- **Return Type**: `collection`
- **Empty Input Result**: `empty`
- **Errors on Multiple Input**: `false`
## Description
Merge the input and other collections into a single collection without eliminating duplicate values. Combining an empty collection with a non-empty collection will return the non-empty collection. When `preserveOrder` is `false`, or not supplied, there is no expectation of order. When `preserveOrder` is `true`, the elements of the other collection are appended to the elements of the input collection, preserving the order of elements in both collections.
## Arguments
- `other`: `collection` - Collection to combine with
- `preserveOrder` (optional): `Boolean` - Optional parameter to preserve order of elements
## Type Mapping
- `Any-Any`
## Example
```fhirpath
A.combine(B) // 1, 1, 2, 2, 3, 3 - order is not guaranteed to be preserved
A.combine(B, true) // 1, 1, 2, 3, 2, 3 - The order is preserved using the `preserveOrder` argument
A.combine( {} ) // 1, 1, 2, 3 - combining an empty collection with a non-empty collection returns the non-empty collection
```