---
type: "fhirpath-function"
title: "FHIRPath Function: coalesce"
function: "coalesce"
category: "Filtering and projection"
section: "5.2.8"
source: "fhirpath/functions.json"
---
# FHIRPath Function: coalesce
{:.stu}
> **Note:** The contents of this section are Standard for Trial Use (STU)
{: .stu-note }
The `coalesce` function takes a variable number of arguments, each of which is a collection. It returns the first non-empty collection from the arguments. If all arguments are empty collections, the result is an empty collection.
{:.stu}

Note that short-circuit behaviour is expected in this function. In other words, arguments after the first non-empty argument are not evaluated. For implementations, this means delaying evaluation of the arguments (as is done with `iif`).
{:.stu}

```fhirpath
Patient.coalesce(name.where(use='official'), name.where(use='usual'), name.first()).text // preferentially select name via use
Patient.name.select(coalesce(family & ' ' & given.join(', '), text, 'unknown')) // select is required to process each name separately
coalesce(Patient.identifier.where(system = 'http://example.org/identifier').value.first(), 'unknown')
```
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Note that this function is useful for providing fallback options, and is more concise than using `iif` to check each collection in turn.
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Such as selecting specific telecom elements based on their use, and falling back to the first available telecom element if none of the specific uses are present:
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```fhirpath
iif( telecom.where(use='mobile').exists(), telecom.where(use='mobile'),
    iif( telecom.where(use='home').exists(), telecom.where(use='home'),
        iif( telecom.where(use='work').exists(), telecom.where(use='work'),
          telecom))).first()
// could equivalently be written as:
coalesce(telecom.where(use='mobile'), telecom.where(use='home'), telecom.where(use='work'), telecom).first()
```
{:.stu}

Another common case is to select a specific coding in a CodeableConcept if it is available, otherwise whatever coding is available.
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``` fhirpath
iif( code.coding.where(system='http://snomed.info/sct').exists(),
        code.coding.where(system='http://snomed.info/sct')),
            code.coding)
    .first().code
// could equivalently be written as:
coalesce(code.coding.where(system='http://snomed.info/sct'), code.coding).first().code
```
{:.stu}
## Summary
- **Category**: Filtering and projection
- **Section**: `5.2.8`
- **Return Type**: `collection`
- **Empty Input Result**: `empty`
- **Errors on Multiple Input**: `true`
## Description
The coalesce function takes a variable number of arguments, each of which is a collection. It returns the first non-empty collection from the arguments. If all arguments are empty collections, the result is an empty collection.
## Arguments
- `value`: `collection` - Collection to check for non-empty values
## Type Mapping
- `Any-Any`
## Example
```fhirpath
Patient.coalesce(name.where(use='official'), name.where(use='usual'), name.first()).text // preferentially select name via use
Patient.name.select(coalesce(family & ' ' & given.join(', '), text, 'unknown')) // select is required to process each name separately
coalesce(Patient.identifier.where(system = 'http://example.org/identifier').value.first(), 'unknown')
```