> For the complete documentation index, see [llms.txt](https://php-fhir-tools.ardenexal.net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://php-fhir-tools.ardenexal.net/fhirpath/functions/filtering.md).

# Filtering & Subsetting

Functions that select a subset of a collection or combine collections together.

## Filtering & subsetting

| Function             | Description                                                               | Example                                         |
| -------------------- | ------------------------------------------------------------------------- | ----------------------------------------------- |
| `where(criteria)`    | Returns only items for which the criteria expression evaluates to true.   | `Patient.name.where(use = 'official')`          |
| `select(projection)` | Projects each item through an expression and flattens the results.        | `Patient.name.select(given)`                    |
| `first()`            | Returns the first item (empty if the input is empty).                     | `Patient.name.first()`                          |
| `last()`             | Returns the last item.                                                    | `Patient.name.last()`                           |
| `tail()`             | Returns all items except the first.                                       | `Patient.name.tail()`                           |
| `skip(num)`          | Returns all items after the first `num` items.                            | `Patient.name.skip(1)`                          |
| `take(num)`          | Returns the first `num` items.                                            | `Patient.name.take(2)`                          |
| `single()`           | Returns the single item; errors if the collection has more than one item. | `Patient.name.where(use = 'official').single()` |
| `distinct()`         | Returns the collection with duplicate items removed.                      | `Patient.name.given.distinct()`                 |

## Combining

| Function           | Description                                                          | Example                           |
| ------------------ | -------------------------------------------------------------------- | --------------------------------- |
| `union(other)`     | Merges two collections, removing duplicates (function form of `\|`). | `name.given.union(name.family)`   |
| `intersect(other)` | Returns items present in both collections.                           | `a.intersect(b)`                  |
| `exclude(other)`   | Returns input items that are not present in `other`.                 | `a.exclude(b)`                    |
| `combine(other)`   | Merges two collections keeping duplicates.                           | `name.given.combine(name.family)` |

{% hint style="info" %}
`sort()` and the conditional `iif()` are documented under [Tree Navigation & Utility](/fhirpath/functions/navigation.md).
{% endhint %}


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