Q1. What is the aggregation pipeline in MongoDB?
The aggregation pipeline is a framework for data aggregation modeled on data processing pipelines. Documents enter a multi-stage pipeline that transforms them into aggregated results. Stages like $match, $group, $sort, $project process documents sequentially. It's similar to SQL GROUP BY but more flexible.
Q2. How do you use $match and $group together?
$match filters documents, $group groups them:
db.orders.aggregate([
{ $match: { status: "completed" } },
{ $group: { _id: "$customerId", total: { $sum: "$amount" } } }
])
Q3. What is the $project stage?
$project reshapes documents by including, excluding, or adding fields:
db.users.aggregate([
{ $project: { fullName: { $concat: ["$firstName", " ", "$lastName"] }, age: 1 } }
])
Q4. How do you sort and limit in aggregation?
Use $sort and $limit stages:
db.products.aggregate([
{ $sort: { price: -1 } },
{ $limit: 5 }
])
Q5. What is $unwind used for?
$unwind deconstructs an array field, creating a separate document for each element:
db.orders.aggregate([
{ $unwind: "$items" },
{ $group: { _id: "$items.productId", totalSold: { $sum: "$items.quantity" } } }
])