Basic aggregation

aggregation.md

Mastering MongoDB aggregation requires understanding its powerful operators, pipelines, and stages. Below are the most commonly used aggregation stages and operators with examples:

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1. $match

Filters documents to pass only those that match specified conditions.

db.orders.aggregate([
  { $match: { status: "shipped" } }
]);

2. $group

Groups documents by a specified field and performs operations like sum, average, or count.

db.orders.aggregate([
  { $group: { _id: "$customerId", totalAmount: { $sum: "$amount" } } }
]);

3. $project

Shapes the output documents by including, excluding, or transforming fields.

db.orders.aggregate([
  { $project: { orderId: 1, totalCost: { $multiply: ["$price", "$quantity"] } } }
]);

4. $sort

Sorts documents in ascending (1) or descending (-1) order.

db.orders.aggregate([
  { $sort: { createdAt: -1 } }
]);

5. $limit and $skip

Limits the number of documents and skips the specified number of documents.

db.orders.aggregate([
  { $sort: { createdAt: -1 } },
  { $limit: 5 },
  { $skip: 10 }
]);

6. $unwind

Deconstructs an array field to output a document for each element.

db.orders.aggregate([
  { $unwind: "$items" }
]);

7. $lookup

Performs a left outer join with another collection.

db.orders.aggregate([
  {
    $lookup: {
      from: "customers",
      localField: "customerId",
      foreignField: "_id",
      as: "customerDetails"
    }
  }
]);

8. $addFields

Adds or modifies fields in the output documents.

db.orders.aggregate([
  { $addFields: { totalCost: { $multiply: ["$price", "$quantity"] } } }
]);

9. $facet

Processes multiple pipelines and outputs multiple results in one stage.

db.orders.aggregate([
  {
    $facet: {
      totalSales: [{ $group: { _id: null, total: { $sum: "$amount" } } }],
      orderCount: [{ $count: "orderCount" }]
    }
  }
]);

10. $bucket

Categorizes documents into groups, or "buckets," based on a field’s values.

db.orders.aggregate([
  {
    $bucket: {
      groupBy: "$amount",
      boundaries: [0, 50, 100, 200],
      default: "Other",
      output: { count: { $sum: 1 } }
    }
  }
]);

11. $replaceRoot

Replaces the input document with a specified field.

db.orders.aggregate([
  { $replaceRoot: { newRoot: "$customerDetails" } }
]);

12. $out

Writes the results of the aggregation pipeline to a new collection.

db.orders.aggregate([
  { $match: { status: "shipped" } },
  { $out: "shippedOrders" }
]);

---

Common Operators in Aggregation

• Arithmetic Operators: $add, $subtract, $multiply, $divide
• Array Operators: $size, $arrayElemAt, $push
• Comparison Operators: $eq, $ne, $lt, $lte, $gt, $gte
• Conditional Operators: $cond, $ifNull
• String Operators: $concat, $substr, $toUpper, $toLower
• Date Operators: $dateToString, $dayOfYear, $month, $year

---

Advanced Example: Comprehensive Aggregation Pipeline

A pipeline to calculate total sales, average order amount, and group by customer.

db.orders.aggregate([
  { $match: { status: "shipped" } },
  {
    $group: {
      _id: "$customerId",
      totalSpent: { $sum: "$amount" },
      averageOrderValue: { $avg: "$amount" },
      orderCount: { $sum: 1 }
    }
  },
  { $sort: { totalSpent: -1 } },
  {
    $lookup: {
      from: "customers",
      localField: "_id",
      foreignField: "_id",
      as: "customerDetails"
    }
  },
  { $project: { _id: 0, customer: { $arrayElemAt: ["$customerDetails.name", 0] }, totalSpent: 1, averageOrderValue: 1, orderCount: 1 } }
]);

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Would you like to dive into any specific stage or operator in more detail?