aggregation.txt

.. _aggregation:

======================
Aggregation Operations
======================

.. default-domain:: mongodb

.. facet::
   :name: genre
   :values: reference

.. meta:: 
   :keywords: sample dataset
   :description: Aggregation operations process multiple documents and return computed results.

.. contents:: On this page
   :local:
   :backlinks: none
   :depth: 1
   :class: singlecol

Aggregation operations process multiple documents and return computed
results. You can use aggregation operations to:

- Group values from multiple documents together.

- Perform operations on the grouped data to return a single result.

- Analyze data changes over time.

To perform aggregation operations, you can use:

- :ref:`Aggregation pipelines <aggregation-pipeline-intro>`, which are
  the preferred method for performing aggregations.

- :ref:`Single purpose aggregation methods
  <single-purpose-agg-methods>`, which are simple but lack the
  capabilities of an aggregation pipeline.

.. |page-topic| replace:: :atlas:`run aggregation pipelines in the UI </atlas-ui/agg-pipeline>`

.. cta-banner::
   :url: https://www.mongodb.com/docs/atlas/atlas-ui/agg-pipeline
   :icon: Cloud

   .. include:: /includes/fact-atlas-compatible.rst

.. _aggregation-pipeline-intro:

Aggregation Pipelines
---------------------

.. include:: /includes/aggregation-pipeline-introduction.rst

Aggregation Pipeline Example
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. include:: /includes/aggregation-pipeline-example.rst

For runnable examples containing sample input documents, see
:ref:`Complete Aggregation Pipeline Examples
<aggregation-pipeline-examples>`. 

Learn More About Aggregation Pipelines
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To learn more about aggregation pipelines, see
:ref:`aggregation-pipeline`.

.. _single-purpose-agg-methods:

Single Purpose Aggregation Methods
----------------------------------

The single purpose aggregation methods aggregate documents from a single
collection. The methods are simple but lack the capabilities of an
aggregation pipeline.

.. list-table::
   :header-rows: 1
   :widths: 50 50
   
   * - Method
     - Description

   * - :method:`db.collection.estimatedDocumentCount()`
     - Returns an approximate count of the documents in a collection or
       a view.

   * - :method:`db.collection.count()`
     - Returns a count of the number of documents in a collection or a
       view.

   * - :method:`db.collection.distinct()`
     - Returns an array of documents that have distinct values for the
       specified field.

.. toctree::
   :titlesonly:
   :hidden:

   Aggregation Pipeline </core/aggregation-pipeline>
   Reference </reference/aggregation>
   Map-Reduce </core/map-reduce>