Aggregation in a document database refers to the process of combining, analyzing, and transforming data stored across multiple documents to produce summarized or computed results. Unlike relational databases, which use tables and rows, document databases like MongoDB store data in flexible, JSON-like structures. Aggregation allows developers to perform operations such as grouping, filtering, sorting, and calculating values (sums, averages, etc.) across these documents. This is particularly useful when working with large datasets, as it avoids the need to manually process data in application code, improving efficiency and reducing complexity.
A common example of aggregation is using a pipeline-based approach, where data flows through a series of processing stages. For instance, in MongoDB, an aggregation pipeline might start by filtering documents with a $match stage, group related data using $group, and then sort results with $sort. Suppose you have a collection of sales orders. You could aggregate total sales per region by first matching orders within a date range, grouping by the region field to sum the amount values, and then sorting regions by total sales. Another example involves nested data: if documents contain arrays (e.g., a list of product tags), you could use $unwind to break arrays into individual documents, then count how often each tag appears across all products.
Aggregation is essential for generating reports, analytics, or preparing data for APIs. It enables operations that would otherwise require multiple queries or complex application logic. For example, calculating monthly revenue trends, identifying top-selling products, or merging data from related documents (like user profiles and their orders) becomes straightforward. While powerful, aggregation pipelines can become complex, so it’s important to optimize stages (e.g., filtering early to reduce processing) and use indexes where possible. Tools like MongoDB Compass provide visual builders to simplify pipeline creation, making aggregation accessible even for developers less familiar with its syntax.