Aggregation Intro

Pipelines in stages

Aggregation processes documents through stages such as $match, $group, $sort, and $project.

Count by year

This pipeline filters, then groups books by year.

db.books.aggregate([
  { $match: { year: { $gte: 2020 } } },
  { $group: { _id: "$year", total: { $sum: 1 } } },
  { $sort: { _id: 1 } }
])

Match early

Put $match first when possible so later stages see fewer documents. Indexes can support early matches.

Grow skills gradually

Aggregation can reshape arrays, join collections with $lookup, and compute windows. Start with match/group/sort, then expand.