00 - MongoDB Aggregation Pipeline

MongoDB Aggregation Pipeline

---

What is Aggregation?

When working with databases in real-world applications, you often need more than simple CRUD operations. You need to analyze, transform, and summarize data across documents and collections. This is where aggregation comes in.

MongoDB provides three mechanisms for aggregating data:

Mechanism	Scope	Complexity	Use Case
Query Methods	Single collection	Low	Simple counts, distinct values
Aggregation Framework	Multiple collections	Medium–High	Complex transformations, joins, analytics
MapReduce	Multiple collections	Very High	Deprecated since MongoDB 5.0 — use the Aggregation Framework instead

This guide focuses on the Aggregation Framework, which is the recommended approach for all aggregation tasks. Simple query methods are covered briefly as a starting point.

Note: MapReduce has been deprecated since MongoDB 5.0 and should no longer be used. The db.collection.group() method was removed in MongoDB 4.2. All aggregation tasks should use the Aggregation Pipeline instead.

---

How to Read This Guide

This scriptum is organized from simple to complex. Follow the chapters in order:

Part 1: Foundations

1. 01 - Query Methods — Simple aggregation with count(), distinct(), and group()
2. 02 - The Aggregation Pipeline — Core concept: how pipelines work
3. 03 - Sample Data — The example dataset used throughout this guide

Part 2: Pipeline Stages

4. 04 - Document Stages — Control which documents flow through: $match, $sort, $skip, $limit, $unwind, $out
5. 05 - Structure Stages — Control what each document looks like: $addFields, $project, $replaceRoot
6. 06 - Relationship Stages — Join data from other collections: $lookup
7. 07 - Aggregation Stages — Group and summarize: $group, $bucket

Part 3: Expressions

8. 08 - Expressions Overview — What expressions are and how they work
9. 09 - Arithmetic Operators — $add, $subtract, $multiply, $divide, $abs
10. 10 - Comparison Operators — $eq, $ne, $gt, $lt, $gte, $lte
11. 11 - Boolean and Control Flow Operators — $and, $or, $not, $cond, $switch, $cmp
12. 12 - Array Operators — $arrayElemAt, $concatArrays, $in, $map, $filter, $reduce, $isArray
13. 13 - Accumulator Operators — $sum, $min, $max, $avg, $addToSet, $push
14. 14 - Variable and Reference Expressions — $$CURRENT, $expr, $let
15. 15 - Set Operators — $setDifference, $setIntersection, $setUnion, $setIsSubset

---

Quick Reference: All Pipeline Stages

Stage	Purpose
$match	Filter documents (like find())
$sort	Sort documents
$skip	Skip first n documents
$limit	Limit to first n documents
$unwind	Deconstruct arrays into separate documents
$out	Write results to a collection (replaces entire collection)
$merge	Write results to a collection (can insert, update, or replace individual documents — more flexible than $out)
$addFields	Add new fields to documents
$project	Select/rename/compute fields
$replaceRoot	Replace the document root
$lookup	Join with another collection
$group	Group by key and aggregate
$bucket	Group by value ranges
$count	Count documents in the stream

---

Quick Reference: All Expression Operators

Category	Operators
Arithmetic	$add, $subtract, $multiply, $divide, $abs
Comparison	$eq, $ne, $gt, $gte, $lt, $lte, $cmp
Boolean	$and, $or, $not
Control Flow	$cond, $switch
Array	$arrayElemAt, $concatArrays, $in, $map, $filter, $reduce, $isArray
Accumulators	$sum, $min, $max, $avg, $addToSet, $push
Variables	$$CURRENT, $expr, $let
Set	$setDifference, $setIntersection, $setUnion, $setIsSubset