What is the MongoDB Aggregation Pipeline? | MongoDB Interview Questions

What You'll Learn

What aggregation is
Pipeline stages
Common examples

What is Aggregation?

Aggregation processes data records and returns computed results. It's like SQL GROUP BY with more power.

Pipeline Stages

code.jsJavaScript

db.collection.aggregate([
  { $match: { ... } },    // Filter documents
  { $group: { ... } },    // Group and aggregate
  { $sort: { ... } },     // Sort results
  { $project: { ... } },  // Shape output
  { $limit: 10 }          // Limit results
]);

Common Stages

$match - Filter

code.jsJavaScript

{ $match: { status: "active", age: { $gte: 18 } } }

$group - Aggregate

code.jsJavaScript

{
  $group: {
    _id: "$department",
    totalSalary: { $sum: "$salary" },
    avgSalary: { $avg: "$salary" },
    count: { $sum: 1 }
  }
}

$project - Shape Output

code.jsJavaScript

{
  $project: {
    name: 1,
    email: 1,
    fullName: { $concat: ["$firstName", " ", "$lastName"] }
  }
}

$sort and $limit

code.jsJavaScript

{ $sort: { totalSales: -1 } },
{ $limit: 10 }

Full Example

code.jsJavaScript

// Get top 5 departments by average salary
db.employees.aggregate([
  { $match: { status: "active" } },
  { $group: {
      _id: "$department",
      avgSalary: { $avg: "$salary" },
      count: { $sum: 1 }
  }},
  { $sort: { avgSalary: -1 } },
  { $limit: 5 },
  { $project: {
      department: "$_id",
      avgSalary: { $round: ["$avgSalary", 2] },
      employeeCount: "$count"
  }}
]);