MongoDB Interview Questions & Answers

MongoDB interview questions usually test whether you understand the document model, basic commands, query performance, and production concepts such as indexes, replication, sharding, and backups. This updated list covers beginner, intermediate, and advanced MongoDB questions with direct answers.

Use these MongoDB interview questions to revise the basics first, then move to CRUD operations, aggregation, indexing, schema design, and MongoDB versus relational database comparisons.

MongoDB interview questions on basics and document model

What is MongoDB?

MongoDB is a document-oriented NoSQL database. It stores data in flexible JSON-like documents and internally uses BSON, which extends JSON with extra data types such as ObjectId, Date, and binary data.

Is MongoDB a database or a DBMS?

MongoDB is commonly called a database, but technically it is a database management system for document databases. A MongoDB deployment can contain many databases, and each database can contain many collections.

Why is MongoDB called a NoSQL database?

MongoDB is called a NoSQL database because it does not store data mainly as relational tables. It stores data as documents inside collections, so the data model is document-based rather than table-based.

Could you mention some NoSQL databases other than MongoDB?

Other NoSQL databases include Cassandra, HBase, CouchDB, Redis, Neo4j, Amazon DynamoDB, and Azure Cosmos DB. These databases are not all the same type; for example, Redis is a key-value store and Neo4j is a graph database.

What are the equivalents of tables and rows in MongoDB?

A MongoDB collection is similar to a table in an SQL database, and a MongoDB document is similar to a row. A document field is similar to a column, but documents in the same collection can have different fields unless validation rules are configured.

SQL term	MongoDB term
Database	Database
Table	Collection
Row	Document
Column	Field
Primary key	_id

What specification does a MongoDB document follow?

MongoDB documents follow the BSON specification. BSON stands for Binary JSON and supports JSON-like documents with additional data types.

What data types are allowed in MongoDB documents?

MongoDB supports BSON data types such as string, integer, double, decimal, boolean, object, array, ObjectId, date, null, binary data, and regular expression.

Is there something like a primary key in MongoDB?

MongoDB uses the _id field as the unique identifier for each document in a collection. If you do not provide an _id, MongoDB creates one automatically.

What is a MongoDB database?

In MongoDB, Database is a collection of MongoDB Collections.

Tutorial – MongoDB Database.

What is a MongoDB collection?

A MongoDB collection is a group of documents. It is similar to a table, but it can store flexible document structures.

Can MongoDB collections be restricted in size?

Yes. MongoDB supports capped collections, which have a fixed maximum size. When a capped collection reaches its size limit, older documents are overwritten by newer documents.

What is a document in MongoDB? Give an example.

Document is an entity in which zero or more ordered field-value pairs are stored.

Following is a sample MongoDB Document :

{
    name: "Robin",
    age: 23,
    place: "New York",
    hobbies: ["Singing", "Reading Books"]
}

Can MongoDB documents be nested?

Yes. MongoDB documents can contain embedded documents and arrays. Nested documents are useful when related data is usually read together with the parent document.

MongoDB interview questions on shell commands and collections

How do you start a MongoDB shell?

Mongo Daemon should be started in prior.

To start Mongo Daemon, run the following command :

 ~$ sudo service mongod start

Once Mongo Daemon is up and running, we can start a Mongo Shell using mongo command in terminal.

 ~$ mongo

In current MongoDB tooling, mongosh is commonly used instead of the older mongo shell command.

mongosh

What is the default port on which MongoDB starts?

By default, MongoDB listens on port 27017, unless the port is changed in the MongoDB configuration.

How do you connect to a MongoDB instance running on another computer in the network?

Host and Port options of mongo command can be used to connect to mongod instance running on different node connected to the network.

 ~$ <span class="crayon-v">mongo</span> <span class="crayon-o">--</span><span class="crayon-v">host</span> <span class="crayon-o"><</span><span class="crayon-v">host</span><span class="crayon-o">></span> <span class="crayon-o">--</span><span class="crayon-v">port</span> <span class="crayon-o"><</span><span class="crayon-v">port_number</span><span class="crayon-o">></span>

Can you run multiple MongoDB instances on the same computer?

Yes. Using different port number for each Mongo Instance, multiple instances can be started.

Among the multiple MongoDB instances, how do you connect to a specific MongoDB instance?

As Mongo Instances could be differentiated by port number, we can use the same information to start a mongo shell with the --port option.

 ~$ <span class="crayon-v">mongo </span><span class="crayon-o">--</span><span class="crayon-v">port</span> <span class="crayon-o"><</span><span class="crayon-v">port_number</span><span class="crayon-o">></span>

How do you start working on a specific MongoDB database through mongo shell?

use <database_name> command in mongo shell can be used to connect to a specific MongoDB Database.

What command lists available databases in MongoDB?

To get the list of databases, open Mongo Shell and run the show dbs command.

How do you create a MongoDB database?

MongoDB Database can be created through Mongo Shell using use <database_name> command. If the database_name provided to the use command does not exist, a new database is created. Technically, a database is not created until a document is inserted to a collection in the database.

How do you explicitly create a MongoDB collection?

db.createCollection(<collection_name>, [options]) command can be used to create a collection explicitly with the configuration options provided.

Which command is used to show all collections in a MongoDB database?

First we need to select the database. Then we can use show collections in Mongo Shell to list all MongoDB Collections present in a MongoDB Database.

How do you delete a MongoDB collection?

drop() function can be called on the collection to delete it. Following is the syntax :

 db.<collection_name>.drop()

Upon successful deletion, the command echoes back true to the prompt.

What happens when you try to delete a collection that does not exist?

<span class="crayon-v">db</span>.<span class="crayon-o"><</span><span class="crayon-v">collection_name</span><span class="crayon-o">></span>.<span class="crayon-e">drop</span>() command returns false.

MongoDB interview questions on CRUD commands and aggregation

How do you insert documents in MongoDB?

Use insertOne() to insert one document and insertMany() to insert multiple documents into a collection.

db.users.insertOne({ name: "Asha", age: 25 })

db.users.insertMany([
  { name: "Kiran", age: 28 },
  { name: "Meera", age: 31 }
])

How do you display documents in MongoDB?

Use find() to display matching documents and findOne() to return a single matching document.

db.users.find({ age: { $gte: 25 } })

db.users.findOne({ name: "Asha" })

How do you update a MongoDB document?

Use updateOne() or updateMany() with update operators such as $set, $inc, and $unset.

db.users.updateOne(
  { name: "Asha" },
  { $set: { city: "Hyderabad" } }
)

What is an upsert in MongoDB?

An upsert updates a matching document if one exists, or inserts a new document if no document matches the filter. It is enabled with upsert: true.

What is the MongoDB aggregation pipeline?

The aggregation pipeline processes documents through stages such as $match, $group, $project, $sort, $lookup, and $unwind.

db.orders.aggregate([
  { $match: { status: "PAID" } },
  { $group: { _id: "$customerId", totalAmount: { $sum: "$amount" } } },
  { $sort: { totalAmount: -1 } }
])

What is MapReduce in MongoDB?

MapReduce is a programming model for processing and summarizing large datasets using map and reduce functions. MongoDB has supported db.collection.mapReduce(), but for most modern MongoDB work, the aggregation pipeline is preferred.

MongoDB interview questions on indexes, replication, and sharding

What is an index in MongoDB?

An index is a data structure that helps MongoDB find documents faster. Indexes improve read performance, but they also use storage and add write overhead because the indexes must be updated when documents change.

db.users.createIndex({ email: 1 })

What is a compound index in MongoDB?

A compound index is an index on more than one field. It is useful when queries commonly filter or sort by the same group of fields.

db.orders.createIndex({ customerId: 1, createdAt: -1 })

How do you check whether a query uses an index?

Use explain("executionStats") to inspect the query plan, scanned documents, index usage, and execution statistics.

What is replication in MongoDB?

Replication is the process of keeping copies of data on multiple MongoDB servers. MongoDB implements replication with replica sets. A replica set improves availability by allowing another member to take over if the primary fails.

Is MongoDB fault tolerant?

MongoDB can be configured for fault tolerance using replica sets. If the primary node becomes unavailable, eligible secondary nodes can elect a new primary. Write concern controls how many members must acknowledge a write.

What is sharding in MongoDB?

Sharding is MongoDB’s horizontal scaling method. It distributes data across multiple shards so that large datasets and high-throughput workloads can be handled by more than one server or replica set.

What should be done for large datasets and high throughput in MongoDB?

For large datasets and high throughput, use proper indexes, monitor slow queries, configure replica sets for availability, and use sharding when one replica set is not enough. A good shard key is important for balanced distribution.

MongoDB interview questions on schema design and MySQL comparison

How is MongoDB different from relational databases?

MongoDB stores data as documents inside collections, while relational databases store data in tables with rows and columns. MongoDB often embeds related data when it is read together, while relational databases usually normalize data and join tables.

Is MongoDB harder than MySQL?

MongoDB is not necessarily harder than MySQL. Developers familiar with JSON may find MongoDB easy to start with. Developers who need complex joins, strict relational constraints, and SQL reporting may find MySQL more familiar.

When should you embed documents in MongoDB?

Embed documents when related data is usually read together, has a clear parent-child relationship, and does not grow without limit.

When should you reference documents in MongoDB?

Reference documents when related data is large, reused by many documents, changes independently, or would make the parent document grow too much.

What requirements suggest MongoDB for an application?

MongoDB is a reasonable choice when the application works naturally with JSON-like objects, needs flexible schema changes, stores nested or semi-structured data, and does not depend heavily on complex joins across many tables.

MongoDB interview questions on transactions, import, and security

Does MongoDB support transactions?

Yes. MongoDB supports multi-document transactions. However, good MongoDB schema design still tries to keep data that is updated together in the same document when that model is suitable.

What is write concern in MongoDB?

Write concern defines the level of acknowledgment requested from MongoDB for write operations. A stronger write concern can require acknowledgment from a majority of replica set members.

How do you import documents into MongoDB?

For JSON, CSV, or TSV files, use mongoimport. It is designed for importing data files into MongoDB collections.

mongoimport --db companydb --collection users --file users.json --jsonArray

How do you secure a MongoDB deployment?

Important MongoDB security practices include enabling authentication, using role-based access control, limiting network exposure, using TLS where needed, keeping MongoDB updated, and maintaining tested backups.

MongoDB interview preparation checklist

• Basics: Revise database, collection, document, field, BSON, and _id.
• CRUD: Know insert, find, update, delete, and upsert commands.
• Indexes: Explain single-field, compound, unique, and query-plan analysis.
• Aggregation: Practice $match, $group, $project, $sort, $lookup, and $unwind.
• Availability: Understand replica sets, primary, secondary, election, and write concern.
• Scalability: Explain sharding and shard key selection.
• Design: Be ready to justify embedding versus referencing with an example.

Reference links for MongoDB interview preparation

• MongoDB Basics
• MongoDB Manual
• MongoDB Indexes
• MongoDB Aggregation
• IBM: What is MongoDB?

FAQs on MongoDB interview questions and answers

Editorial QA checklist for MongoDB interview questions page

• The page covers beginner, intermediate, and advanced MongoDB interview questions.
• The answer for MongoDB default port is corrected to 27017.
• The article addresses search intent around MongoDB basics, MongoDB as a DBMS, MongoDB versus MySQL, and common interview preparation topics.
• The FAQ section contains MongoDB-specific questions and does not exceed five FAQs.
• New code blocks use PrismJS-compatible classes such as language-javascript, language-bash, and syntax.
• Existing image and internal TutorialKart links are preserved, and official MongoDB reference links are included.

Summary of MongoDB interview questions preparation

For MongoDB interviews, prepare more than definitions. You should be able to explain documents, collections, BSON, _id, CRUD commands, indexes, aggregation, replication, sharding, transactions, and schema design trade-offs. A strong answer usually connects each MongoDB feature to a practical use case.