Introduction to the Aggregation Pipeline

The Aggregation Pipeline

The aggregation pipeline is MongoDB's answer to SQL GROUP BY, JOIN, and computed columns. Documents flow through a series of stages — each stage transforms the stream and passes it to the next.

Common stages: $match (filter), $group (aggregate), $sort, $project (reshape), $limit, $skip, $unwind (flatten arrays), and $lookup (join another collection).

$match and $group

// Count students per course, only enrolled ones
db.students.aggregate([
  { $match: { enrolled: true } },
  {
    $group: {
      _id: "$course",
      count: { $sum: 1 },
      avgAge: { $avg: "$age" }
    }
  },
  { $sort: { count: -1 } }
]);