Question 1

What is MongoDB?

Accepted Answer

MongoDB is an open-source, document-oriented NoSQL database program. Instead of using tables and rows as in traditional relational databases, MongoDB uses JSON-like documents with optional schemas (BSON). It is designed for scalability, high performance, and high availability.

Question 2

What is a Document in MongoDB?

Accepted Answer

A document is the basic unit of data in MongoDB, analogous to a row in an RDBMS. Documents are composed of field-and-value pairs and are stored in BSON (Binary JSON) format.
Code Example:
{
  "name": "John Doe",
  "age": 30,
  "interests": ["coding", "reading"]
}

Question 3

What is a Collection in MongoDB?

Accepted Answer

A collection is a grouping of MongoDB documents. It is equivalent to a table in a relational database. Collections do not enforce a strict schema, although schema validation is possible.

Question 4

What is BSON?

Accepted Answer

BSON stands for **Binary JSON**. It is a binary-encoded serialization of JSON-like documents. BSON extends the JSON model to provide additional data types (like Date and BinData) and to be more efficient for storage and scanning.

Question 5

What are the primary advantages of MongoDB?

Accepted Answer

Key advantages include:
1. **Flexible Schema**: Documents in a collection can have different fields.
2. **Scalability**: Seamless horizontal scaling via sharding.
3. **High Performance**: Optimized for fast reads and writes.
4. **Rich Query Language**: Supports aggregation, geospatial queries, and text search.
5. **High Availability**: Built-in replication via Replica Sets.

Question 6

What is the '_id' field in MongoDB?

Accepted Answer

The `_id` field is a unique identifier required for every document in a collection. If you don't provide one, MongoDB automatically generates an `ObjectId` for it. It acts as the primary key for the document.

Question 7

What is an ObjectId in MongoDB?

Accepted Answer

An ObjectId is a 12-byte BSON type that is unique within a collection. It consists of:
- A 4-byte timestamp (seconds since Unix epoch).
- A 5-byte random value.
- A 3-byte incrementing counter.

Question 8

How do you create a database in MongoDB?

Accepted Answer

In the MongoDB Shell, you use the `use` command. MongoDB will not actually create the database until you insert data into it.
Code Example:
use interview_db

Question 9

How do you insert a document into a collection?

Accepted Answer

You use `insertOne()` for a single document or `insertMany()` for multiple documents.
Code Example:
db.users.insertOne({ name: "Alice", age: 25 });

Question 10

How do you find all documents in a collection?

Accepted Answer

You use the `find()` method. To make the output readable, you can append `.pretty()` (in older shells) or just run it.
Code Example:
db.users.find();

Question 11

What is the difference between MongoDB and RDBMS?

Accepted Answer

- **RDBMS**: Structured (Tables/Rows), uses SQL, strict schema, joins are common, vertically scalable.
- **MongoDB**: Unstructured (Collections/Documents), uses MQL (MongoDB Query Language), dynamic schema, embedding/linking instead of joins, horizontally scalable.

Question 12

What is Namespace in MongoDB?

Accepted Answer

A namespace is the concatenation of the database name and the collection name (e.g., `dbName.collectionName`). Together they uniquely identify a collection.

Question 13

How do you update a document in MongoDB?

Accepted Answer

You use `updateOne()`, `updateMany()`, or `replaceOne()`. Use the `$set` operator to update specific fields.
Code Example:
db.users.updateOne(
  { name: "Alice" }, 
  { $set: { age: 26 } }
);

Question 14

How do you delete a document?

Accepted Answer

You use `deleteOne()` or `deleteMany()`.
Code Example:
db.users.deleteOne({ name: "Alice" });

Question 15

What is the purpose of the '$set' operator?

Accepted Answer

The `$set` operator replaces the value of a field with the specified value. If the field does not exist, `$set` adds a new field with the specified value.

Question 16

What is the difference between findOne() and find()?

Accepted Answer

- **find()**: Returns a cursor to all documents that match the query criteria.
- **findOne()**: Returns only the first document that matches the query criteria.

Question 17

How do you limit the number of results in MongoDB?

Accepted Answer

You use the `.limit()` method on the cursor.
Code Example:
db.users.find().limit(5);

Question 18

How do you sort results in MongoDB?

Accepted Answer

You use the `.sort()` method. Use **1** for ascending and **-1** for descending.
Code Example:
db.users.find().sort({ age: 1 }); // Youngest first

Question 19

What is Projection in MongoDB?

Accepted Answer

Projection is the process of selecting only the necessary fields from a document rather than the entire document. You pass a second argument to `find()`.
Code Example:
db.users.find({}, { name: 1, _id: 0 }); // Show only name

Question 20

What is the use of the '$in' operator?

Accepted Answer

The `$in` operator selects the documents where the value of a field equals any value in the specified array.
Code Example:
db.users.find({ age: { $in: [20, 25, 30] } });

Question 21

How do you count the number of documents in a collection?

Accepted Answer

You use `countDocuments()` or `estimatedDocumentCount()`.
Code Example:
db.users.countDocuments({ age: { $gt: 18 } });

Question 22

What is an Index in MongoDB?

Accepted Answer

Indexes are special data structures that store a small portion of the collection's data set in a form that is easy to traverse. They improve the efficiency of search queries significantly.

Question 23

What is a GridFS?

Accepted Answer

GridFS is a specification for storing and retrieving files that exceed the BSON-document size limit of 16 MB. It divides a file into chunks and stores each chunk as a separate document.

Question 24

What is the default port for MongoDB?

Accepted Answer

The default port for MongoDB is **27017**.

Question 25

What is the command to see all databases?

Accepted Answer

You use the `show dbs` command (or `show databases`).
Code Example:
show dbs

Question 26

How do you perform a simple count in MongoDB?

Accepted Answer

Use the `countDocuments()` method for an accurate count or `estimatedDocumentCount()` for a faster but potentially less accurate estimate.
Code Example:
db.users.countDocuments({ status: "active" });

Question 27

What is an Aggregation Pipeline in MongoDB?

Accepted Answer

The aggregation pipeline is a framework for data transformation. Documents enter a multi-stage pipeline that transforms them into aggregated results. Common stages include `$match`, `$group`, `$sort`, and `$project`.
Code Example:
db.orders.aggregate([
  { $match: { status: "A" } },
  { $group: { _id: "$cust_id", total: { $sum: "$amount" } } }
]);

Question 28

Explain the '$match' stage in Aggregation.

Accepted Answer

The `$match` stage filters the documents based on specific criteria. It is recommended to use `$match` early in the pipeline to reduce the number of documents processed in subsequent stages.

Question 29

Explain the '$group' stage in Aggregation.

Accepted Answer

The `$group` stage groups input documents by a specified identifier expression and applies accumulator expressions (like `$sum`, `$avg`, `$push`) to each group.
Code Example:
{ $group: { _id: "$category", totalStock: { $sum: "$stock" } } }

Question 30

What is the difference between Embedding and Referencing in Schema Design?

Accepted Answer

- **Embedding**: Storing related data in a single document. Good for high-read performance and 'one-to-few' relationships.
- **Referencing**: Storing references (like IDs) to related documents in other collections. Good for 'one-to-many' or 'many-to-many' relationships to avoid document size limits.

MongoDB NoSQL Interview Master

Difficulty Filters

Tools & Options

What is MongoDB?

What is a Document in MongoDB?

What is a Collection in MongoDB?

What is BSON?

What are the primary advantages of MongoDB?

What is the '_id' field in MongoDB?

What is an ObjectId in MongoDB?

How do you create a database in MongoDB?

How do you insert a document into a collection?

How do you find all documents in a collection?

What is the difference between MongoDB and RDBMS?

What is Namespace in MongoDB?

How do you update a document in MongoDB?

How do you delete a document?

What is the purpose of the '$set' operator?

What is the difference between findOne() and find()?

How do you limit the number of results in MongoDB?

How do you sort results in MongoDB?

What is Projection in MongoDB?

What is the use of the '$in' operator?

How do you count the number of documents in a collection?

What is an Index in MongoDB?

What is a GridFS?

What is the default port for MongoDB?

What is the command to see all databases?

How do you perform a simple count in MongoDB?

What is an Aggregation Pipeline in MongoDB?

Explain the '$match' stage in Aggregation.

Explain the '$group' stage in Aggregation.

What is the difference between Embedding and Referencing in Schema Design?

What is a TTL (Time-To-Live) Index?

What is a Capped Collection?

What is the use of the '$lookup' operator?

How do you handle Transactions in MongoDB?

What are Covered Queries?

Explain the Concept of Sharding.

What is a Shard Key?

What is a Replica Set?

What is a Write Concern in MongoDB?

What is a Read Preference?

What is the purpose of the '$unwind' stage?

Explain the '$facet' stage in aggregation.

What is the difference between '$addFields' and '$project'?

What is a Compound Index?

What is a Partial Index?

How do you view index usage in MongoDB?

What is the 'oplog' (Operations Log)?

What is an Arbiter in a Replica Set?

How to backup and restore MongoDB databases?

What is the use of the '$out' stage in aggregation?

Explain the difference between 2d and 2dsphere indexes.

How does MongoDB handle data consistency?

What is Joins in MongoDB?

Explain the Working of 'Election' in Replica Sets.

What are Change Streams?

Explain the use of 'Hint' in MongoDB.

What is the 'wiredTiger' storage engine?

What are Orphaned Documents?

What is the 'Journal' in MongoDB?

Explain the 'Working Set' concept.

What is a Sparse Index?

Explain 'Mongoose' in the context of MongoDB.

What is the 16MB Document Limit?

Explain 'Horizontal' vs 'Vertical' Scaling.

What is 'Read Concern' in MongoDB?

Explain 'Map-Reduce' in MongoDB.

What are 'Views' in MongoDB?

What is 'Causal Consistency'?

How to tune MongoDB performance?

What is 'Write Strategy' for Sharding?

Explain 'Balancing' in Sharded Clusters.

What are 'Sessions' in MongoDB?

How to monitor MongoDB?

What is 'Schema Validation'?

Explain 'Disk Compression' in WiredTiger.

Scenario: How to find top 5 most expensive orders using aggregation?

Scenario: How to calculate total revenue per category?