If you are preparing for a MongoDB interview, having a clear understanding of the most frequently asked questions can save time and improve your confidence. This list covers 35+ important MongoDB interview questions with concise, accurate answers.
It is designed to help you quickly review core concepts, understand practical use cases, and clarify topics that often come up during technical discussions. Whether you are applying for a developer, DBA, or data engineer role, these questions can help you assess your readiness and focus on what interviewers are likely to ask.
Fun Fact – MongoDB is a widely used NoSQL database with over 54,000 customers and a growing global community. More than 175,000 developers join its user base each month.
Note – These MongoDB interview questions are categorized into basic, freshers, experienced, and advanced levels to help you prepare according to your experience and role.
MongoDB Basic Interview Questions
Here is a list of basic MongoDB interview questions and answers.
- What is MongoDB, and how is it different from a relational database?
MongoDB is a NoSQL document database. Unlike relational databases that use rows and tables, MongoDB stores data in JSON-like documents, making it flexible and schema-less. This structure is ideal for applications that deal with changing or nested data.
- Explain the concept of a document and a collection in MongoDB.
A document is a key-value pair data structure, similar to a JSON object. A collection is a group of these documents, similar to a table in SQL. Each document in a collection can have different fields.
- What data types are supported in MongoDB documents?
MongoDB supports strings, numbers (int, long, double, decimal), arrays, booleans, dates, null, embedded documents, ObjectId, binary data, and regular expressions.
- How does MongoDB store data internally?
MongoDB stores data in BSON (Binary JSON) format, which is optimized for speed and traversing. Collections are stored in databases as files on disk using the WiredTiger storage engine.
- What is a primary key in MongoDB, and how is it defined?
Every document in MongoDB must have a unique _id field, which serves as the primary key. If not manually set, MongoDB generates a unique ObjectId for it.
MongoDB Interview Questions for Freshers
Here are some commonly asked interview questions on MongoDB for freshers.
- What is the role of _id in MongoDB documents?
_id uniquely identifies each document within a collection. It acts as the default primary key and helps in indexing and quick retrieval.
- How do you insert data into a collection?
You can use the insertOne() or insertMany() methods. Example:
db.users.insertOne({ name: “John”, age: 28 })
- What command is used to retrieve all documents in a collection?
Use the find() method without any filter:
db.collection.find()
It returns a cursor to all documents in the collection.
- Can MongoDB handle unstructured data? Explain.
Yes. MongoDB supports unstructured and semi-structured data. It allows storing documents with varying fields and structures, making it ideal for dynamic content.
- What is the difference between find() and findOne() in MongoDB?
find() returns a cursor pointing to multiple documents that match the query. findOne() returns only the first matching document. Use find() when you expect multiple results.
MongoDB Interview Questions for Experienced
Let’s go through some important MongoDB interview questions and answers for experienced professionals.
- How do indexes work in MongoDB, and when should you use them?
Indexes store parts of data in an efficient format for faster access. When a query is executed, indexes reduce the number of documents scanned. Use them on frequently queried fields or fields used in sorting.
- What is replication in MongoDB, and how does it work?
Replication involves copying data across multiple servers (replica set). One node acts as primary (accepts writes), while others are secondaries (read-only unless configured otherwise). If the primary fails, an automatic election picks a new one.
- Explain the difference between sharding and replication.
Replication copies data for fault tolerance. Sharding splits data across servers for horizontal scalability. Use replication for high availability and sharding when data size exceeds single-server limits.
- How do you handle schema changes in MongoDB collections?
“Since MongoDB is schema-less, documents can be updated individually using $set or $unset. For bulk updates or versioned changes, I write migration scripts to update all affected documents gradually.”
- What strategies can improve MongoDB query performance?
Use appropriate indexes, avoid large document sizes, project only required fields, and use the aggregation pipeline for heavy transformations. Also, analyze queries using explain() to check execution plans.
MongoDB Interview Questions for 5 Years Experienced
- What is the most challenging MongoDB issue you have faced in production?
- Describe a situation where you had to optimize a slow MongoDB query.
- How do you manage version upgrades in a production MongoDB environment?
MongoDB Interview Questions for 10 Years Experienced
- How have you scaled MongoDB in a high-traffic application?
- Tell me about a time you designed a complex data model using MongoDB.
- Describe how you ensured data consistency across a sharded cluster.
MongoDB Advanced Interview Questions
These are some advanced interview questions MongoDB professionals often face.
- How does the WiredTiger storage engine differ from MMAPv1?
WiredTiger is MongoDB’s default storage engine since version 3.2. It supports document-level locking, compression, and better concurrency. MMAPv1 used collection-level locking and was less efficient under heavy write workloads. WiredTiger also uses write-ahead journaling and compression to save disk space.
- What is a compound index, and when would you use it?
A compound index includes multiple fields in a defined order. It is useful when queries filter or sort using more than one field. For example, an index on { age: 1, city: 1 } helps with queries filtering both age and city or sorting by age within city.
- Explain write concern and read concern in MongoDB.
Write concern defines the level of acknowledgment requested from MongoDB for write operations. For example, w: “majority” waits for confirmation from most replica set members.
Read concern controls the consistency level of read operations. For instance, majority read concern guarantees that the read reflects data acknowledged by most nodes.
- How does MongoDB handle concurrency and locking?
MongoDB uses document-level locking with the WiredTiger engine. This allows multiple write operations on different documents in the same collection without blocking. It improves concurrency and throughput, especially in high-traffic systems.
MongoDB Scenario Based Interview Questions
Let’s cover some important scenario based MongoDB interview questions and answers.
- If a node in a replica set fails, what steps does MongoDB take automatically?
MongoDB automatically triggers an election to select a new primary if the existing one goes down. During this time, writes pause until a new primary is elected. The failed node, when back online, syncs from another member to catch up.
- You need to migrate data from a relational database to MongoDB. What steps would you follow?
“First, I analyze the relational schema and map it to a document model. Then I export data using tools like mysqldump or queries, transform it into JSON or BSON format, and import using mongoimport or a custom script. Finally, I validate data types and test queries.”
- How would you troubleshoot a MongoDB deployment with high memory usage?
“I start by checking metrics via mongostat or monitoring tools like Atlas or Ops Manager. I examine slow queries using db.currentOp() and analyze indexes. Unindexed queries, large working sets, or large documents often cause memory spikes. I also check for proper index usage with explain().”
- A query is timing out in production. How do you diagnose and fix it?
“I use explain(“executionStats”) to inspect the query plan. If it’s scanning many documents, I check if the right index exists. I also review server load, active connections, and whether projection is used properly. Fixing it may involve rewriting the query or adding indexes.”
MongoDB Query Interview Questions
Here are some commonly asked MongoDB Query questions and answers.
- How do you update a nested field in a MongoDB document?
You use dot notation to update nested fields.
Example:
db.users.updateOne({ _id: 1 }, { $set: { “address.city”: “London” } })
- What is the difference between $push and $addToSet operators?
$push adds a value to an array, even if it already exists.
$addToSet adds a value only if it doesn’t already exist in the array, avoiding duplicates.
- How would you perform a text search in MongoDB?
First, create a text index on the fields you want to search:
db.articles.createIndex({ title: “text”, body: “text” })
Then, run a search using:
db.articles.find({ $text: { $search: “climate change” } })
Other Important MongoDB Interview Questions
MongoDB DBA Interview Questions
- How do you back up and restore a MongoDB database?
- What monitoring tools do you use for MongoDB health checks?
- How would you handle replica set failover manually?
MongoDB Aggregation Interview Questions
- What is the aggregation pipeline in MongoDB?
- How do $group and $match stages work together in aggregation?
- How would you calculate average order value using aggregation?
Node.js MongoDB Interview Questions
- How do you connect a Node.js app to MongoDB using Mongoose?
- What is a schema in Mongoose, and how is it defined?
- How do you handle validation errors in a Node.js + MongoDB application?
MongoDB Atlas Interview Questions
- What is MongoDB Atlas, and how does it differ from self-hosted MongoDB?
- How do you set up automated backups in MongoDB Atlas?
- What security features are built into MongoDB Atlas?
Also Read - Top 35+ NodeJS Interview Questions and Answers
MongoDB Coding Interview Questions
Here are some common coding related MongoDB questions for interview.
- Write a MongoDB query to find all users who registered in the last 30 days.
To fetch users registered in the last 30 days, filter based on the registrationDate field:
db.users.find({
registrationDate: {
$gte: new Date(Date.now() – 30 * 24 * 60 * 60 * 1000)
}
})
This query compares each user’s registrationDate against a date 30 days ago from the current time.
- Write a query to update a user’s email based on their username.
To update a user’s email where the username matches:
db.users.updateOne(
{ username: “johndoe” },
{ $set: { email: “john.doe@example.com” } }
)
This will update the email of the user with the username “johndoe”.
- Write an aggregation pipeline to group orders by user and sum the total.
To calculate the total order value per user:
db.orders.aggregate([
{
$group: {
_id: “$userId”,
totalAmount: { $sum: “$amount” }
}
}
])
This groups orders by userId and adds up the amount field for each user.
- Write a query to delete all products with zero stock.
To remove products where the stock is 0:
db.products.deleteMany({ stock: 0 })
This deletes all documents from the products collection where the stock field is equal to zero.
MongoDB Viva Questions
These are some important MongoDB questions that are often asked during vivas.
- What are capped collections in MongoDB?
Capped collections are fixed-size collections that overwrite old data when the size limit is reached. They maintain insertion order.
- What is the default size limit of a document in MongoDB?
The default size limit is 16MB per document.
- Explain the use of GridFS in MongoDB.
GridFS is used to store and retrieve files larger than 16MB. It splits files into chunks and stores them across collections.
- Can you store files in MongoDB? If yes, how?
Yes, using GridFS. Files are broken into chunks and stored with metadata, making it possible to retrieve and stream large files.
- What is the role of the ObjectId?
ObjectId is a unique identifier automatically generated for each document. It includes a timestamp, machine ID, process ID, and counter.
Wrapping Up
These 35+ MongoDB interview questions cover everything from basic concepts to real-world scenarios. Reviewing them can help you feel better prepared for your next technical interview.
Looking for MongoDB job opportunities? Visit Hirist—an online job portal built for tech professionals. You can easily find the top MongoDB jobs in India and apply directly.
FAQs
How to prepare for a MongoDB interview?
Start by understanding MongoDB architecture, CRUD operations, indexing, aggregation, and replication. Practice queries. Review scenario-based questions and read recent documentation or changelogs.
What is the average salary for MongoDB developers in India?
Employees with MongoDB skills earn an average salary of ₹25.3 lakhs per year in India. Most salaries range between ₹17 lakhs and ₹103.4 lakhs, depending on experience and role.
Do I need to know SQL to learn MongoDB?
No. MongoDB uses a different syntax. But knowing SQL helps in understanding data structures and query logic, which can make learning MongoDB easier.
What roles require MongoDB knowledge?
MongoDB is used in roles like backend developer, full-stack developer, data engineer, and database administrator. Many cloud and DevOps roles also expect basic MongoDB knowledge.
Can I use MongoDB for large-scale applications?
Yes. MongoDB supports sharding, replication, and indexing. These features help it scale efficiently for high-traffic or large-data applications.
What’s the best way to practice MongoDB?
Use MongoDB Atlas for free cloud setups. Build small projects. Write queries using find(), aggregate(), and update operations. Practice real-world data use cases.
What are the 4 basic operations in MongoDB?
The four basic operations are Create, Read, Update, and Delete—commonly known as CRUD. These are performed using methods like insert(), find(), update(), and delete().
Why is MongoDB faster than SQL?
MongoDB is faster for large-scale, flexible data. It uses a document model, avoids joins, and supports indexing. This reduces query time in many real-time applications.
