Aggregation Operations

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Aggregation Pipelines

Single Purpose Aggregation Methods

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:

Aggregation pipelines, which are the preferred method for performing aggregations.
Single purpose aggregation methods, which are simple but lack the capabilities of an aggregation pipeline.

Aggregation Pipelines

An aggregation pipeline consists of one or more stages that process documents:

Each stage performs an operation on the input documents. For example, a stage can filter documents, group documents, and calculate values.
The documents that are output from a stage are passed to the next stage.
An aggregation pipeline can return results for groups of documents. For example, return the total, average, maximum, and minimum values.

Starting in MongoDB 4.2, you can update documents with an aggregation pipeline if you use the stages shown in Updates with Aggregation Pipeline.

Note

Aggregation pipelines run with the db.collection.aggregate() method do not modify documents in a collection, unless the pipeline contains a $merge or $out stage.

Aggregation Pipeline Example

The following aggregation pipeline example contains two stages and returns the total order quantity of medium size pizzas grouped by pizza name:

db.orders.aggregate( [
   // Stage 1: Filter pizza order documents by pizza size
   {
      $match: { size: "medium" }
   },
   // Stage 2: Group remaining documents by pizza name and calculate total quantity
   {
      $group: { _id: "$name", totalQuantity: { $sum: "$quantity" } }
   }
] )

The $match stage:

Filters the pizza order documents to pizzas with a size of medium.
Passes the remaining documents to the $group stage.

The $group stage:

Groups the remaining documents by pizza name.
Uses $sum to calculate the total order quantity for each pizza name. The total is stored in the totalQuantity field returned by the aggregation pipeline.

For runnable examples containing sample input documents, see Complete Aggregation Pipeline Examples.

Learn More About Aggregation Pipelines

To learn more about aggregation pipelines, see Aggregation Pipeline.

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.

Method	Description
`db.collection.estimatedDocumentCount()`	Returns an approximate count of the documents in a collection or a view.
`db.collection.count()`	Returns a count of the number of documents in a collection or a view.
`db.collection.distinct()`	Returns an array of documents that have distinct values for the specified field.

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Aggregation Pipeline