MongoDB Interview Questions

110 questions with detailed answers

Question:
What is MongoDB?
Answer:
MongoDB is a NoSQL document database that stores data in flexible, JSON-like documents.

Question:
What is a document in MongoDB?
Answer:
A document is a record in MongoDB, similar to a row in relational databases, stored in BSON format.

Question:
What is a collection in MongoDB?
Answer:
A collection is a group of MongoDB documents, equivalent to a table in relational databases.

Question:
What is BSON?
Answer:
BSON (Binary JSON) is a binary representation of JSON documents used by MongoDB for storage.

Question:
How do you create a database in MongoDB?
Answer:
Use the "use database_name" command. The database is created when you first store data in it.

Question:
What is the default port for MongoDB?
Answer:
The default port for MongoDB is 27017.

Question:
How do you insert a document in MongoDB?
Answer:
Use insertOne() for single document or insertMany() for multiple documents.

Question:
What is the _id field in MongoDB?
Answer:
The _id field is a unique identifier for each document, automatically created if not specified.

Question:
How do you find all documents in a collection?
Answer:
Use the find() method without any parameters: db.collection.find()

Question:
What is MongoDB Atlas?
Answer:
MongoDB Atlas is a cloud-based database service provided by MongoDB Inc.

Question:
How do you update a document in MongoDB?
Answer:
Use updateOne(), updateMany(), or replaceOne() methods with update operators like $set.

Question:
What is the difference between find() and findOne()?
Answer:
find() returns a cursor to all matching documents, findOne() returns only the first matching document.

Question:
How do you delete documents in MongoDB?
Answer:
Use deleteOne() to delete a single document or deleteMany() to delete multiple documents.

Question:
What is an index in MongoDB?
Answer:
An index is a data structure that improves query performance by creating shortcuts to data.

Question:
How do you create an index in MongoDB?
Answer:
Use the createIndex() method: db.collection.createIndex({field: 1})

Question:
What is the $set operator?
Answer:
$set is an update operator that sets the value of a field in a document.

Question:
What is the $unset operator?
Answer:
$unset removes a field from a document.

Question:
How do you sort results in MongoDB?
Answer:
Use the sort() method: db.collection.find().sort({field: 1}) for ascending, -1 for descending.

Question:
What is the limit() method?
Answer:
The limit() method restricts the number of documents returned by a query.

Question:
What is the skip() method?
Answer:
The skip() method skips a specified number of documents in the result set.

Question:
What is the $inc operator?
Answer:
$inc increments the value of a field by a specified amount.

Question:
What is the $push operator?
Answer:
$push adds an element to an array field.

Question:
What is the $pull operator?
Answer:
$pull removes all instances of a value from an array.

Question:
How do you query for documents with a specific field value?
Answer:
Use db.collection.find({field: "value"}) to find documents where field equals value.

Question:
What is the $gt operator?
Answer:
$gt is a comparison operator that matches values greater than a specified value.

Question:
What is the $lt operator?
Answer:
$lt is a comparison operator that matches values less than a specified value.

Question:
What is the $in operator?
Answer:
$in matches any of the values specified in an array.

Question:
How do you count documents in a collection?
Answer:
Use countDocuments() method: db.collection.countDocuments({})

Question:
What is the difference between MongoDB and SQL databases?
Answer:
MongoDB is NoSQL (document-based), while SQL databases are relational with fixed schemas.

Question:
What is the mongo shell?
Answer:
The mongo shell is an interactive JavaScript interface to MongoDB for database operations.

Question:
How do you show all databases in MongoDB?
Answer:
Use the "show dbs" command in the mongo shell.

Question:
How do you show all collections in a database?
Answer:
Use the "show collections" command in the mongo shell.

Question:
What is the $exists operator?
Answer:
$exists matches documents that have or do not have a specified field.

Question:
What is the difference between $and and $or operators?
Answer:
$and requires all conditions to be true, $or requires at least one condition to be true.

Question:
What is a compound index?
Answer:
A compound index is an index on multiple fields: db.collection.createIndex({field1: 1, field2: -1})

Question:
What is a replica set in MongoDB?
Answer:
A replica set is a group of MongoDB servers that maintain the same data set for high availability.

Question:
What is sharding in MongoDB?
Answer:
Sharding is a method for distributing data across multiple machines for horizontal scaling.

Question:
What is the aggregation framework in MongoDB?
Answer:
The aggregation framework is a pipeline-based data processing framework for complex queries and transformations.

Question:
Explain the $match stage in aggregation pipeline.
Answer:
$match filters documents to pass only those that match specified conditions to the next stage.

Question:
What is the $group stage in aggregation?
Answer:
$group groups documents by specified fields and performs accumulator operations.

Question:
What is the $project stage in aggregation?
Answer:
$project reshapes documents by including, excluding, or adding new fields.

Question:
What is the $unwind stage in aggregation?
Answer:
$unwind deconstructs an array field to output a document for each element.

Question:
What is the $sort stage in aggregation?
Answer:
$sort reorders documents by specified fields in ascending or descending order.

Question:
What is the $limit stage in aggregation?
Answer:
$limit passes only the first n documents to the next stage.

Question:
What is the $skip stage in aggregation?
Answer:
$skip skips the first n documents and passes the remaining documents to the next stage.

Question:
What is a text index in MongoDB?
Answer:
A text index supports text search queries on string content with language-specific stemming.

Question:
How do you perform text search in MongoDB?
Answer:
Use the $text operator with a text index: db.collection.find({$text: {$search: "term"}})

Question:
What is the $addFields stage in aggregation?
Answer:
$addFields adds new fields to documents, equivalent to $project with all existing fields included.

Question:
What is the $out stage in aggregation?
Answer:
$out writes the results of the aggregation pipeline to a specified collection.

Question:
What is a sparse index in MongoDB?
Answer:
A sparse index only includes documents that have the indexed field, skipping documents without it.

Question:
What is a TTL index in MongoDB?
Answer:
A TTL (Time To Live) index automatically removes documents after a specified time period.

Question:
What is the difference between embedded documents and references?
Answer:
Embedded documents store related data within the same document, references store ObjectIds pointing to other documents.

Question:
What is the $elemMatch operator?
Answer:
$elemMatch matches documents containing an array field with at least one element matching all criteria.

Question:
What is the $size operator?
Answer:
$size matches documents where an array field has exactly the specified number of elements.

Question:
What is the $all operator?
Answer:
$all matches arrays that contain all specified elements, regardless of order.

Question:
What is the $type operator?
Answer:
$type matches documents where a field is of a specified BSON type.

Question:
What is the $regex operator?
Answer:
$regex provides regular expression capabilities for pattern matching in queries.

Question:
What is the $nor operator?
Answer:
$nor matches documents that fail all query expressions in the array.

Question:
What is the $not operator?
Answer:
$not performs logical NOT operation on the specified expression.

Question:
What is a capped collection in MongoDB?
Answer:
A capped collection is a fixed-size collection that maintains insertion order and overwrites oldest documents.

Question:
What is the $currentDate operator?
Answer:
$currentDate sets the value of a field to the current date or timestamp.

Question:
What is the $min operator in updates?
Answer:
$min updates the field value only if the specified value is less than the current field value.

Question:
What is the $max operator in updates?
Answer:
$max updates the field value only if the specified value is greater than the current field value.

Question:
What is the $mul operator?
Answer:
$mul multiplies the value of a field by a specified number.

Question:
What is the $rename operator?
Answer:
$rename renames a field in a document.

Question:
What is the $addToSet operator?
Answer:
$addToSet adds elements to an array only if they do not already exist in the set.

Question:
What is the $pop operator?
Answer:
$pop removes the first or last element of an array.

Question:
What is the $pullAll operator?
Answer:
$pullAll removes all instances of specified values from an array.

Question:
What is the $lookup stage in aggregation?
Answer:
$lookup performs a left outer join to another collection in the same database.

Question:
What is a geospatial index in MongoDB?
Answer:
A geospatial index supports queries on location data using 2d or 2dsphere indexes.

Question:
What is the $near operator?
Answer:
$near returns documents in order of proximity to a specified point (requires geospatial index).

Question:
What is the $geoWithin operator?
Answer:
$geoWithin selects documents with geospatial data within a specified shape.

Question:
What is the difference between $lookup and $graphLookup?
Answer:
$lookup performs simple joins, $graphLookup performs recursive searches through connected documents.

Question:
What is the $facet stage in aggregation?
Answer:
$facet processes multiple aggregation pipelines within a single stage on the same input.

Question:
What is the $bucket stage in aggregation?
Answer:
$bucket categorizes documents into groups based on specified boundaries.

Question:
What is the $bucketAuto stage in aggregation?
Answer:
$bucketAuto automatically determines boundaries to evenly distribute documents into buckets.

Question:
What is the $replaceRoot stage in aggregation?
Answer:
$replaceRoot replaces the input document with the specified document.

Question:
What is the $merge stage in aggregation?
Answer:
$merge writes results to a collection, with options for handling existing documents.

Question:
What is a partial index in MongoDB?
Answer:
A partial index only indexes documents that meet a specified filter expression.

Question:
What is the $where operator?
Answer:
$where passes a JavaScript expression or function to query for documents.

Question:
What is the $expr operator?
Answer:
$expr allows use of aggregation expressions within the query language.

Question:
What is MongoDB GridFS?
Answer:
GridFS is a specification for storing and retrieving files larger than 16MB in MongoDB.

Question:
What is the oplog in MongoDB?
Answer:
The oplog (operations log) is a capped collection that keeps a rolling record of all operations.

Question:
What is read concern in MongoDB?
Answer:
Read concern controls the consistency and isolation properties of read operations.

Question:
What is write concern in MongoDB?
Answer:
Write concern describes the level of acknowledgment requested from MongoDB for write operations.

Question:
What are the different read concern levels?
Answer:
Read concern levels include: local, available, majority, linearizable, and snapshot.

Question:
How would you design a MongoDB schema for a social media application?
Answer:
Use embedded documents for user profiles, separate collections for posts/comments, implement follower relationships with arrays or separate collections based on scale.

Question:
Explain MongoDB transactions and when to use them.
Answer:
MongoDB supports multi-document ACID transactions. Use for operations requiring atomicity across multiple documents or collections.

Question:
How do you optimize MongoDB queries for better performance?
Answer:
Create appropriate indexes, use projection to limit fields, optimize aggregation pipelines, use explain() to analyze query performance.

Question:
What are the best practices for MongoDB schema design?
Answer:
Embed for one-to-one and one-to-few relationships, reference for one-to-many and many-to-many, consider query patterns and data access patterns.

Question:
How do you handle data consistency in a distributed MongoDB setup?
Answer:
Use replica sets for high availability, configure appropriate read/write concerns, implement application-level consistency checks.

Question:
Explain the MongoDB sharding architecture.
Answer:
Sharding distributes data across multiple shards using a shard key, with config servers storing metadata and mongos routing queries.

Question:
How do you choose an appropriate shard key?
Answer:
Choose a shard key with high cardinality, good distribution, and aligns with query patterns to avoid hotspots.

Question:
What are the limitations of MongoDB?
Answer:
Limited joins, no foreign key constraints, memory usage for indexes, 16MB document size limit, eventual consistency in some scenarios.

Question:
How do you implement full-text search in MongoDB?
Answer:
Create text indexes on string fields, use $text operator with $search, configure language-specific stemming and stop words.

Question:
Explain MongoDB change streams and their use cases.
Answer:
Change streams provide real-time notifications of data changes, useful for triggering actions, maintaining caches, or syncing data.

Question:
How do you backup and restore MongoDB data?
Answer:
Use mongodump/mongorestore for logical backups, filesystem snapshots for physical backups, or MongoDB Atlas automated backups.

Question:
What is the difference between MongoDB and other NoSQL databases?
Answer:
MongoDB is document-oriented vs key-value (Redis), column-family (Cassandra), or graph (Neo4j) databases, each optimized for different use cases.

Question:
How do you monitor MongoDB performance?
Answer:
Use MongoDB Compass, db.stats(), db.serverStatus(), profiler, and monitoring tools like MongoDB Atlas monitoring or third-party solutions.

Question:
Explain MongoDB security best practices.
Answer:
Enable authentication, use role-based access control, encrypt data in transit and at rest, network security, regular security updates.

Question:
How do you handle large datasets in MongoDB?
Answer:
Implement sharding for horizontal scaling, use appropriate indexes, optimize queries, consider data archiving strategies.

Question:
What is the aggregation pipeline optimization in MongoDB?
Answer:
MongoDB optimizes pipelines by reordering stages, combining stages, and using indexes when possible. Use $match early to reduce data.

Question:
How do you implement pagination in MongoDB?
Answer:
Use skip() and limit() for simple pagination, or cursor-based pagination with _id or timestamp fields for better performance.

Question:
Explain MongoDB connection pooling and its importance.
Answer:
Connection pooling reuses database connections to reduce overhead. Configure pool size based on application needs and server capacity.

Question:
How do you handle schema evolution in MongoDB?
Answer:
Use flexible schema design, implement versioning strategies, gradual migration approaches, and maintain backward compatibility.

Question:
What are MongoDB drivers and how do they work?
Answer:
MongoDB drivers are language-specific libraries that provide APIs for connecting to and interacting with MongoDB databases.

Question:
How do you implement data validation in MongoDB?
Answer:
Use JSON Schema validation rules at the collection level, implement application-level validation, and use required fields and data types.

Question:
Explain MongoDB replica set elections and failover.
Answer:
Replica sets automatically elect a new primary when the current primary becomes unavailable, ensuring high availability through consensus.

Question:
How do you optimize MongoDB for write-heavy workloads?
Answer:
Use appropriate write concerns, optimize indexes, consider sharding, use bulk operations, and tune server configuration.

Question:
What is the role of the primary and secondary nodes in a replica set?
Answer:
Primary handles all write operations and can serve reads, secondaries replicate data from primary and can serve reads with appropriate read preference.
Study Tips
  • Read each question carefully
  • Try to answer before viewing the solution
  • Practice explaining concepts out loud
  • Review regularly to reinforce learning
Share & Practice

Found this helpful? Share with others!

Feedback