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MongoDB Aggregation Pipeline Stages | All About

In the world of MongoDB, the aggregation pipeline is a powerful tool. It lets developers transform and process documents flexibly and efficiently.

Today, we are going to take a look at the concepts of MongoDB aggregation pipelines, exploring stages, expressions, and a few examples.

Aggregation Pipeline Basics

At its core, an aggregation pipeline comprises one or more stages, each tasked with processing documents. These stages act as building blocks and run operations like document filtering, grouping, and value calculations.

In short, the documents generated by one stage become the input for the next, and so on.

A key benefit of the aggregation pipeline is its ability to return results for groups of documents. In other words, it offers a comprehensive data summary.

Furthermore, the diversity of stages available is another key point. While some stages generate new documents or filter out existing ones, others may not. Additionally, the same stage can appear multiple times in the pipeline, with exceptions for $out, $merge, and $geoNear.

Aggregation Expressions

To perform advanced calculations within a stage, MongoDB introduces aggregation expressions. These expressions use aggregation operators to calculate averages, sums, and more.

Key Aggregation Pipeline Stages

Here are some stages of a MongoDB aggregation pipeline:

• `$project`: Shape and transform documents by specifying the fields to include or exclude.
• `$match`: Filter documents based on specified criteria.
• `$group`: Organize documents into groups using a “group key” and output one document for each unique group.
• . `$sort`: Arrange documents in a specific order.
• `$skip` & `$limit`: Control the number of documents in the output.
• `$first` & `$last`: Retrieve the first or last document in a group.
• `$unwind`: Unravel arrays within documents for further processing.

The interplay between stages is crucial. For instance, the $match, $group, and $sort stages collectively form an aggregation pipeline. The output from $match flows into $group, and subsequently, the output from $group feeds into $sort.

Some other aggregation pipeline stages include $listLocalSessions, $currentOp, and $documents.

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Conclusion

In brief, our Support Experts introduced us to MongoDB Aggregation Pipeline Stages.