# Overview

## Aggregation framework

Aggregation framework in MongoDB is a powerful feature that allows you to build complex queries that you normally cannot build with the common operators and functions. The framework achieves this by putting your documents into a multi-stage pipeline that transforms the documents in the desired result.

### Pipeline

The pipeline consists of a series of [stages](https://docs.mongodb.com/manual/reference/operator/aggregation-pipeline/#aggregation-pipeline-operator-reference) where each stage receives the output of the previous stage. The section contains samples for the following stages:

| Stage                          | Description                                                                                      |
| ------------------------------ | ------------------------------------------------------------------------------------------------ |
| **Project**                    | Adds new fields or removes existing ones                                                         |
| **Match**                      | Filters the documents                                                                            |
| **Group**                      | Groups documents by an identifier expression providing accumulator results                       |
| **Unwind**                     | Deconstructs an array field                                                                      |
| **Pagination** *(limit, skip)* | Skips the first *n* documents and limits the total results                                       |
| **Bucket**                     | Produces a distribution among documents based on a specified expression and specified boundaries |

![Aggregation framework](https://2076499567-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-M1LyPqYq4CEFXnjdYHF%2F-M5BC-Y16dPn7J-ljSDb%2F-M5BIZjkEpRjQ1g6_Cgk%2Faggregation.png?alt=media\&token=de541f21-68ae-4e05-9c48-e4f180851edd)