MongoDB — Aggregation

Basic Aggregation

db.users.aggregate([
  { $match: { active: true } },
  { $group: { _id: "$country", count: { $sum: 1 } } }
])

Pipeline Stages

Stage	Description
$match	Filter documents
$group	Group documents
$sort	Sort documents
$project	Reshape documents
$limit	Limit results
$skip	Skip documents
$lookup	Join collections
$unwind	Deconstruct arrays

Examples

// Group by country
db.users.aggregate([
  { $group: { _id: "$country", count: { $sum: 1 } } }
])

// Average age by country
db.users.aggregate([
  { $group: { _id: "$country", avgAge: { $avg: "$age" } } }
])

// Sort by count
db.users.aggregate([
  { $group: { _id: "$country", count: { $sum: 1 } } },
  { $sort: { count: -1 } }
])

Lookup (Join)

db.orders.aggregate([
  {
    $lookup: {
      from: "users",
      localField: "userId",
      foreignField: "_id",
      as: "user"
    }
  }
])

Mini Practice

1. Use $match and $group
2. Calculate aggregates
3. Sort results
4. Use $lookup

Up Next

Continue with Aggregation Pipeline — pipeline details.