SECTION 3: Aggregation Framework & Pipelines

1. What is aggregation in MongoDB?
Aggregation is a way to process data records and return computed results. It involves operations like filtering, grouping, projecting, and sorting using a pipeline of stages.

2. What is an aggregation pipeline?
An aggregation pipeline is a framework for data aggregation modeled on the concept of data processing pipelines. It consists of multiple stages such as $match, $group, $sort, $project, etc.

3. How do you use the $match stage?

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

Explanation: Filters documents to only include those with status equal to "shipped".

4. How do you group data using $group?

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

Explanation: Groups documents by customerId and calculates the total amount spent.

5. What does $project do in aggregation?

db.users.aggregate([
  { $project: { name: 1, email: 1, _id: 0 } }
])

Explanation: Specifies which fields to include or exclude in the output.

6. How do you sort documents in an aggregation pipeline?

db.users.aggregate([
  { $sort: { age: -1 } }
])

Explanation: Sorts documents by age in descending order.

7. How do you limit the number of results in a pipeline?

db.users.aggregate([
  { $limit: 5 }
])

8. How do you skip documents in an aggregation?

db.users.aggregate([
  { $skip: 5 }
])

9. How do you calculate average using $avg in $group?

db.orders.aggregate([
  { $group: { _id: "$productId", avgAmount: { $avg: "$amount" } } }
])

10. How do you use multiple stages in a pipeline?

db.users.aggregate([
  { $match: { age: { $gt: 25 } } },
  { $sort: { age: 1 } },
  { $project: { name: 1, age: 1 } }
])

11. What is $unwind and how is it used?

db.users.aggregate([
  { $unwind: "$hobbies" }
])

Explanation: Deconstructs an array field from the input documents to output a document for each element.

12. How do you use $lookup for joining collections?

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

Explanation: Performs a left outer join with the customers collection.

13. How do you filter documents after a $lookup?
Use $unwind and $match after $lookup to filter joined results:

db.orders.aggregate([
  { $lookup: { ... } },
  { $unwind: "$customerDetails" },
  { $match: { "customerDetails.status": "active" } }
])

14. How do you count documents using aggregation?

db.users.aggregate([
  { $count: "totalUsers" }
])

15. How do you use $addFields?

db.users.aggregate([
  { $addFields: { fullName: { $concat: ["$firstName", " ", "$lastName"] } } }
])

Explanation: Adds a new field or modifies an existing one.

16. How do you calculate the maximum value in a group?

db.orders.aggregate([
  { $group: { _id: null, maxAmount: { $max: "$amount" } } }
])

17. How do you calculate the minimum value in a group?

db.orders.aggregate([
  { $group: { _id: null, minAmount: { $min: "$amount" } } }
])

18. How do you use conditional logic in aggregation?

db.users.aggregate([
  {
    $project: {
      name: 1,
      ageGroup: {
        $cond: { if: { $gte: ["$age", 18] }, then: "Adult", else: "Minor" }
      }
    }
  }
])

19. How do you combine aggregation and update?
Use aggregation to build a pipeline and apply it in $merge:

db.sales.aggregate([
  { $group: { _id: "$region", total: { $sum: "$amount" } } },
  { $merge: "regional_totals" }
])

Explanation: Aggregates data and merges the result into regional_totals.

20. How do you use variables inside aggregation pipelines?
Use the $let operator inside $project:

db.users.aggregate([
  {
    $project: {
      fullName: {
        $let: {
          vars: { full: { $concat: ["$firstName", " ", "$lastName"] } },
          in: "$full"
        }
      }
    }
  }
])