MongoDB Aggregation Pipeline: The Basics with Examples

Understand $match, $group, $sort, $project and $lookup with simple examples — a frequent topic in MEAN Stack interviews.

The aggregation pipeline processes documents through a sequence of stages, where each stage transforms the documents and passes them to the next — similar to a Unix pipe.

Total sales per category

db.orders.aggregate([
  { $match: { status: 'DELIVERED' } },
  { $group: { _id: '$category', total: { $sum: '$amount' } } },
  { $sort: { total: -1 } },
  { $project: { _id: 0, category: '$_id', total: 1 } }
]);

Common stages

• $match — filters documents (place it early to use indexes)
• $group — groups by a key and calculates totals, averages and counts
• $project — reshapes documents, includes or excludes fields
• $sort, $limit, $skip — ordering and pagination
• $lookup — joins documents from another collection
• $unwind — deconstructs an array into separate documents

• MongoDB
• Aggregation
• Queries