What is Aggregation in MongoDB?

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Aggregation operations process data records and return computed results. Aggregation operations group values from multiple documents together and can perform a variety of operations on the grouped data to return a single result. The key element in aggregation is called the pipeline. It also helps us in performing a few operations like min, max, sum, etc. MongoDB provides three ways to perform aggregation: a)Aggregation Pipeline; b) Map-Reduce Function; c) Single Purpose Aggregation Methods

MongoDB’s aggregation framework is modeled on the concept of data processing pipelines. Documents enter a multi-stage pipeline that transforms the documents into an aggregated result.

A sequence of data aggregation operations or stages. The pipeline is an array.The Aggregation Stages are namely: $count; $group; $limit; $lookup; $match; $merge; $sort; $project; $unwind; $unset.

The pipeline definition is :

var pipeline = [

                      {….},

                      {….},

                      {….},

               ]

Example:

var pipeline = [

                      {$group:{_id:”$name”}},

                      {$sort:{“birth_year”:1}},

                      {$limit:5},

                ]

db.students.aggregate(pipeline)