MongoDB is a NoSQL, document-oriented database that stores data as JSON-like documents in BSON format. It is schema-less, horizontally scalable, and built for high performance. Documents are stored in collections instead of tables, and each document can have a different structure.

What is the difference between SQL and MongoDB?

SQL uses tables, rows, and columns with a fixed schema. MongoDB uses collections, documents, and fields with a flexible schema. SQL uses JOIN for relationships while MongoDB uses $lookup or embedded documents. SQL scales vertically while MongoDB scales horizontally via sharding.

What is BSON in MongoDB?

BSON stands for Binary JSON. MongoDB stores documents internally in BSON format, which is faster to parse than plain JSON and supports additional data types like ObjectId, Date, Int32, Int64, Decimal128, and Binary that are not available in standard JSON.

What is an aggregation pipeline in MongoDB?

The aggregation pipeline in MongoDB processes documents through a series of stages. Each stage transforms the data. Common stages include $match (filter), $group (aggregate), $sort, $limit, $project, $lookup (join), and $unwind. It is used for complex data analysis and reporting.

What is indexing in MongoDB?

An index in MongoDB is a special data structure that stores a small portion of the collection's data in an easy-to-traverse form, dramatically speeding up queries. Types include single field, compound, multikey, text, TTL, hashed, and wildcard indexes. The _id field is indexed by default.

What is a replica set in MongoDB?

A replica set is a group of MongoDB servers that maintain the same dataset for high availability. It consists of one primary node (receives all writes) and one or more secondary nodes (replicate from primary). If the primary goes down, secondaries automatically elect a new primary.

What is sharding in MongoDB?

Sharding is MongoDB's approach to horizontal scaling — distributing data across multiple machines called shards. A shard key determines how data is distributed. The mongos router directs queries to the correct shard. Config servers store cluster metadata. Sharding is used when data exceeds single server capacity.

What is the default port for MongoDB?

The default port for MongoDB is 27017.

What is upsert in MongoDB?

Upsert is a combination of update and insert. When upsert is set to true in an update operation, MongoDB updates the document if it exists, or inserts a new document if no match is found. This avoids the need to check for document existence before updating.

What is the difference between embedded documents and references in MongoDB?

Embedded documents store related data inside the same document (denormalization) — faster for reads but can cause data duplication. References store the _id of related documents (normalization) — avoids duplication but requires multiple queries or $lookup. Use embedding for data read together, references for frequently changing or independently accessed data.

MongoDB Interview Cheatsheet 2026 — 30 Most Asked Questions & Answers

MongoDB Interview Cheatsheet 2025 — 30 Most Asked Questions & Answers

Preparing for a MongoDB interview? This cheatsheet covers the 30 most important MongoDB interview questions and answers — from basics like documents, collections, and BSON, to advanced topics like the aggregation pipeline, indexing strategies, replication, and sharding.

Each topic includes a concise explanation, comparison tables, code examples with mongosh syntax, and key points to remember. Whether you’re a beginner or experienced developer, this MongoDB quick reference will help you revise faster and walk into your interview confident.

Topics covered: MongoDB vs SQL, CRUD operations, Query operators, Aggregation pipeline, $lookup joins, Indexing types, Embedded vs References, Replication & Replica Sets, Sharding, Transactions, ObjectId, BSON, Schema design, Atlas, Mongoose, and more.

MongoDB Interview NoSQL Aggregation Pipeline MongoDB Indexing Replica Set Sharding CRUD Operations mongosh MongoDB 2025

🍃 MongoDB Interview Cheatsheet

30 essential topics — click any row to expand the answer, mark as revised, and track your progress.

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About This MongoDB Interview Cheatsheet

This cheatsheet is designed for developers preparing for MongoDB interviews at all levels — junior, mid-level, and senior. It covers both theoretical concepts and practical mongosh query examples.

What makes a good MongoDB interview answer? Always explain the concept, give a real-world use case, and mention trade-offs. For example, when asked about indexing, don’t just define it — explain when to use compound indexes vs single-field indexes, and how to verify index usage with explain().

Most frequently asked MongoDB interview topics:

MongoDB vs SQL — document model vs relational model
CRUD operations — insertOne, find, updateOne with $set, deleteOne
Aggregation pipeline — $match, $group, $lookup, $project stages
Indexing — types of indexes, when to use compound indexes, TTL indexes
Schema design — embedded documents vs references, one-to-many patterns
Replication — replica sets, primary/secondary, automatic failover
Sharding — horizontal scaling, shard keys, mongos router
Transactions — ACID compliance, multi-document transactions since v4.0

Bookmark this page and use the Mark Done button on each topic to track your revision progress before your interview.