MongoDB interview questions

1. What is MongoDB?

MongoDB is a document-oriented database. It stores data in documents that look like JSON. It uses collections to group documents.

Full lesson: Mongodb intro

2. What is a collection in MongoDB?

A collection is a group of documents inside a database. It is like a table in a relational database.

Full lesson: Mongodb intro

3. What is a document in MongoDB?

A document is one data record in a collection. It is made of key-value pairs and stored in BSON format. It is like a row in a relational database.

Full lesson: Mongodb intro

4. How do you insert one document into a collection?

Use the insertOne method on a collection. Pass the document as an object. MongoDB adds an _id field if you do not give one.

db.users.insertOne({ name: "Tom", age: 30 })

Full lesson: Mongodb insert

5. How do you find all documents in a collection?

Call find with no arguments. It returns all documents in the collection.

db.myCollection.find()

Full lesson: Mongodb query

6. How do you find documents that match a condition?

Pass a query object to the find method. MongoDB returns only the documents that match. This example finds users older than 25.

db.myCollection.find({ age: { $gt: 25 } })

Full lesson: Mongodb query

7. How do you update a document?

Use the updateOne method with a filter and an update operator. The $set operator changes a field value.

db.users.updateOne({ name: "Tom" }, { $set: { age: 31 } })

Full lesson: Mongodb update

8. How do you delete a document?

Use the deleteOne method with a filter. MongoDB removes the first document that matches the filter.

db.users.deleteOne({ name: "Tom" })

Full lesson: Mongodb remove

9. What is the MongoDB Shell?

The MongoDB Shell is an interactive command line tool. It uses JavaScript. You can run commands and scripts in it.

Full lesson: Mongodb shell

10. What is an index in MongoDB?

An index is a special data structure that speeds up queries. Without an index, MongoDB scans every document in the collection.

db.collection.createIndex({ age: 1 })

Full lesson: Mongodb indexing

11. What is an ObjectId?

An ObjectId is a 12-byte BSON value. It holds a timestamp, a machine code, a process ID, and a random number. MongoDB uses it as the default _id value.

newObjectId = ObjectId()

Full lesson: Mongodb objectid

12. How do you sort query results?

Add the sort method after find. Use 1 for ascending order and -1 for descending order.

db.users.find().sort({ age: -1 })

Full lesson: Mongodb sort

13. How do you find documents by field type?

Use the $type operator in the query. You can give a type name or a numeric code. MongoDB returns documents whose field has that type.

db.collection.find({ age: { $type: "int" } })

Full lesson: Mongodb operators type

14. Why can an index slow down write operations?

Each index needs storage space. MongoDB must also update the index on every insert, update, and delete. Too many indexes add overhead to writes.

Full lesson: Mongodb indexing limitations

15. What is a common mistake when you create an index on an array field?

MongoDB creates an index entry for each array value. A large array makes the index grow fast. Check the array size before you add the index.

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

Full lesson: Mongodb advanced indexing

16. What is NoSQL?

NoSQL means Not Only SQL. It is a database movement for data that does not fit well in relational tables.

Full lesson: Nosql

17. How do you install MongoDB on Windows?

Download the MongoDB Community Server package from the official website. Run the installer and follow the steps.

Full lesson: Mongodb window install

18. How do you start the MongoDB Shell?

Open a terminal and type mongosh. The shell connects to the default host and port.

mongosh

Full lesson: Mongodb shell

19. How do you create a user in MongoDB?

Use the db.createUser() method in the admin database. Provide a user name, a password, and roles.

db.createUser({ user: "admin", pwd: "pass", roles: ["root"] })

Full lesson: Mongodb user

20. How do you insert many documents at once?

Use the insertMany() method. Pass an array of documents to the method.

db.users.insertMany([{ name: "Alice" }, { name: "Bob" }])

Full lesson: Mongodb insert

21. How do you update many documents at once?

Use the updateMany() method. It updates all documents that match the filter.

db.users.updateMany({ age: { $lt: 30 } }, { $set: { status: "young" } })

Full lesson: Mongodb update

22. How do you delete many documents at once?

Use the deleteMany() method. It deletes all documents that match the filter.

db.users.deleteMany({ status: "inactive" })

Full lesson: Mongodb remove

23. How do you create an index on an array field?

Use the createIndex() method on the array field. MongoDB creates a separate index entry for each array element.

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

Full lesson: Mongodb advanced indexing

24. What is GridFS?

GridFS stores files that are larger than 16 MB. It splits a file into small chunks and stores the chunks in two collections.

Full lesson: Mongodb gridfs

25. How do you connect to MongoDB from PHP?

Use the MongoDB PHP driver. Create a MongoClient object to connect to the database.

<?php
$m = new MongoClient();
$db = $m->test;
?>

Full lesson: Mongodb php

26. How do you connect to MongoDB from Java?

Add the MongoDB Java driver jar to your classpath. Use the MongoClient class to connect.

MongoClient mongoClient = new MongoClient("localhost", 27017);

Full lesson: Mongodb java

27. What is a common mistake when you use an index on a field that you rarely read?

You add extra write overhead and use storage space for no benefit. Do not create an index on a field that you rarely query.

Full lesson: Mongodb indexing limitations

28. What is a common mistake when you use a regular expression in a query with an index?

You expect the index to speed up the query. MongoDB cannot use an index for regular expressions and operators such as $nin and $not.

Full lesson: Mongodb indexing limitations

29. What is a common mistake when you sort with the sort() method?

You forget that 1 means ascending order and -1 means descending order. Use the correct number for the order you want.

db.users.find().sort({ age: 1 })

Full lesson: Mongodb sort

30. What is a common mistake when you install MongoDB on Windows?

You try to install a 32-bit version on a modern system. The latest versions do not have 32-bit installation files.

Full lesson: Mongodb window install

31. How do you install MongoDB on Linux?

Use the package manager for your Linux distribution. Import the MongoDB public key and add the repository first. Then install the mongodb-org package.

sudo apt-get install -y mongodb-org

Full lesson: Mongodb linux install

32. How do you install MongoDB on macOS?

Use Homebrew to install MongoDB on macOS. Run the brew tap command first. Then run the brew install command.

brew tap mongodb/brew
brew install mongodb-community

Full lesson: Mongodb osx install

33. What is the difference between updateOne and replaceOne?

updateOne changes fields with update operators. replaceOne replaces the whole document with a new document. Use replaceOne when you want a full replacement.

db.users.replaceOne({ name: "Alice" }, { name: "Alice", age: 30 })

Full lesson: Mongodb update

34. How do you use the $set operator in an update?

The $set operator changes the value of a field. It adds the field if the field does not exist. Other fields stay the same.

db.users.updateOne({ name: "Alice" }, { $set: { age: 26 } })

Full lesson: Mongodb update

35. How do you use the upsert option in an update?

The upsert option creates a new document when no document matches the filter. Set upsert to true in the options object. The update then inserts a document.

db.users.updateOne({ name: "Bob" }, { $set: { age: 20 } }, { upsert: true })

Full lesson: Mongodb update

36. What is a compound index?

A compound index includes more than one field. It can speed up queries that filter on those fields. The field order in the index matters.

db.users.createIndex({ name: 1, age: -1 })

Full lesson: Mongodb advanced indexing

37. What is a unique index?

A unique index stops duplicate values in a field. MongoDB rejects an insert or update that makes a duplicate value. Create it with the unique option.

db.users.createIndex({ email: 1 }, { unique: true })

Full lesson: Mongodb indexing

38. What is a text index?

A text index supports text search on string fields. Create it with the text value. Then use the $text operator in a query.

db.articles.createIndex({ body: "text" })
db.articles.find({ $text: { $search: "mongodb" } })

Full lesson: Mongodb advanced indexing

39. What is a covered query?

A covered query reads all data from the index. The query does not read the documents. The index must include all fields in the filter and the projection.

db.users.find({ name: "Alice" }, { _id: 0, name: 1 }).hint({ name: 1 })

Full lesson: Mongodb advanced indexing

40. What is the findOneAndUpdate method?

The findOneAndUpdate method updates one document and returns it. It returns the document before or after the update. Use the returnDocument option to choose.

db.users.findOneAndUpdate({ name: "Alice" }, { $set: { age: 27 } }, { returnDocument: "after" })

Full lesson: Mongodb update

41. What is a common mistake when you use updateOne without a filter?

An empty filter matches the first document in the collection. The update changes a document that you did not choose. Always use a clear filter for updateOne.

db.users.updateOne({ _id: ObjectId("...") }, { $set: { age: 26 } })

Full lesson: Mongodb update

42. What is a common mistake when you delete with an empty filter?

An empty filter in deleteMany removes all documents. The data is gone and you cannot undo the delete. Use a clear filter or deleteOne for safety.

db.users.deleteMany({ status: "inactive" })

Full lesson: Mongodb remove

43. What is a common mistake when you use $set with a nested field?

A plain field name replaces the whole nested object. Use dot notation to change one nested field. The dot notation keeps the other nested fields.

db.users.updateOne({ name: "Alice" }, { $set: { "address.city": "Paris" } })

Full lesson: Mongodb update

44. What is a common mistake when you create a unique index on a field with duplicates?

The index creation fails if duplicate values exist. Clean the duplicate data first. Then create the unique index.

db.users.createIndex({ email: 1 }, { unique: true })

Full lesson: Mongodb indexing limitations

45. What is a common mistake when you use an index with a large number of fields?

A compound index has a limit on the number of fields. Too many fields make the index large and slow to update. Keep the index small and match your queries.

db.users.createIndex({ a: 1, b: 1, c: 1 })

Full lesson: Mongodb indexing limitations