Aggregation Pipeline: $match, $group, $project, $sort, $lookup

find() retrieves documents, but it does not summarize them inside the database. Questions like “What is the total revenue per category?” or “How many orders does each user have?” require grouping documents, calculating values, and reshaping the result, which is the role of the aggregation pipeline. An aggregation pipeline is an ordered list of stages. Each stage receives the documents from the previous stage, transforms them, and passes the result to the next stage, like an assembly line for data. This lesson covers five core stages: $match, $project, $group, $sort, and $lookup.

Note: The examples are runnable mongo shell aggregations against the seeded ecommerce collections. Each aggregate([...]) call lists its stages in the order they run.

To make the pipeline concrete, the next visual should show documents flowing through the five stages in sequence:

That starts with the stage that narrows the input. ... ...