Backend interview prep

Start

MongoDB CRUD Interview Questions

Master MongoDB CRUD Operations with interview-focused questions covering Create, Read, Update, Delete, Query Operators, Projection, Sorting, Pagination, Bulk Operations, Upsert, and Spring Boot examples.

Introduction

CRUD stands for

  • Create
  • Read
  • Update
  • Delete

Every MongoDB application performs CRUD operations thousands or even millions of times daily.

Examples include

  • User Registration
  • Product Search
  • Banking Transactions
  • Shopping Cart
  • Order Management
  • Social Media Posts

Understanding CRUD operations is essential for every Backend Developer and is one of the most frequently asked MongoDB interview topics.

MongoDB CRUD Architecture

flowchart LR

Application --> MongoDBDriver --> MongoDB

MongoDB --> Insert

MongoDB --> Find

MongoDB --> Update

MongoDB --> Delete

1. What is CRUD?

Answer

CRUD represents the four basic database operations.

Operation MongoDB Method
Create insertOne(), insertMany()
Read find(), findOne()
Update updateOne(), updateMany()
Delete deleteOne(), deleteMany()

2. What is insertOne()?

Used to insert one document.

Example

db.customers.insertOne({
    name:"John",
    city:"Austin"
})

3. What is insertMany()?

Inserts multiple documents.

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

Create Flow

flowchart LR

Application --> insertOne() --> Collection --> DocumentStored

4. Difference between insertOne() and insertMany()?

insertOne() insertMany()
One Document Multiple Documents
Single Insert Batch Insert
Lower Throughput Better Performance

5. What is find()?

Returns multiple documents.

db.customers.find()

6. What is findOne()?

Returns only one matching document.

db.customers.findOne({
    city:"Austin"
})

7. What is Projection?

Projection returns only selected fields.

db.customers.find(
 {},
 {name:1,city:1}
)

Projection

flowchart LR

Document --> Projection --> SelectedFields

8. What is updateOne()?

Updates the first matching document.

db.customers.updateOne(
 {name:"John"},
 {$set:{city:"Dallas"}}
)

9. What is updateMany()?

Updates all matching documents.

db.customers.updateMany(
 {city:"Austin"},
 {$set:{state:"Texas"}}
)

10. What is replaceOne()?

Replaces the entire document.

db.customers.replaceOne(
 {_id:1},
 {
   name:"Alex",
   city:"Houston"
 }
)

11. Difference between updateOne() and replaceOne()?

updateOne() replaceOne()
Updates Fields Replaces Entire Document
Keeps Existing Fields Removes Missing Fields

12. What is deleteOne()?

Deletes the first matching document.

db.customers.deleteOne({
    name:"John"
})

13. What is deleteMany()?

Deletes multiple documents.

db.customers.deleteMany({
    city:"Austin"
})

Delete Flow

flowchart LR

DeleteRequest --> MongoDB --> DocumentRemoved

14. What is Upsert?

Upsert means

Update

OR

Insert

If document exists

↓

Update

Otherwise

↓

Insert

db.customers.updateOne(
 {name:"John"},
 {$set:{city:"Dallas"}},
 {upsert:true}
)

15. What is Bulk Write?

Performs multiple operations in one request.

Supports

  • Insert
  • Update
  • Delete

Improves performance.

16. What is bulkWrite()?

db.customers.bulkWrite([
 {
   insertOne:{
      document:{name:"John"}
   }
 },
 {
   updateOne:{
      filter:{name:"John"},
      update:{$set:{city:"Austin"}}
   }
 }
])

17. What are Query Operators?

MongoDB provides operators such as

  • $eq
  • $gt
  • $gte
  • $lt
  • $lte
  • $ne
  • $in
  • $nin

18. Example of Comparison Operators

db.products.find({
    price:{
       $gt:500
    }
})

19. What are Logical Operators?

Supported

  • $and
  • $or
  • $not
  • $nor

Example

db.employee.find({
 $or:[
   {city:"Austin"},
   {city:"Dallas"}
 ]
})

20. What are Array Operators?

Examples

  • $all
  • $elemMatch
  • $size
db.students.find({
 skills:"Java"
})

21. What is $set?

Updates specific fields.

{
   $set:{
      city:"Dallas"
   }
}

22. What is $unset?

Removes fields.

{
   $unset:{
      phone:""
   }
}

23. What is $inc?

Increments numeric values.

{
   $inc:{
      balance:100
   }
}

Useful for

  • Wallet
  • Inventory
  • Likes

24. What is $push?

Adds an element to an array.

{
   $push:{
      skills:"MongoDB"
   }
}

25. What is $pull?

Removes an array element.

{
   $pull:{
      skills:"Java"
   }
}

26. What is Sorting?

Ascending

db.employee.find().sort({
 salary:1
})

Descending

db.employee.find().sort({
 salary:-1
})

27. What is Pagination?

Uses

  • skip()
  • limit()

Example

db.products.find()
.skip(20)
.limit(10)

Pagination Flow

flowchart LR

Query --> Skip --> Limit --> Result

28. What is Count?

db.employee.countDocuments({
 department:"IT"
})

Returns total matching documents.

29. Banking Example

Deposit

db.accounts.updateOne(
 {_id:101},
 {$inc:{balance:500}}
)

30. E-Commerce Example

Product Search

db.products.find({
 category:"Laptop"
})
.sort({
 price:1
})
.limit(20)

31. HR Example

Employee Update

db.employee.updateOne(
 {_id:100},
 {$set:{designation:"Manager"}}
)

32. Logging Example

Delete old logs

db.logs.deleteMany({
 year:2023
})

33. Spring Boot Example

@Autowired
private MongoTemplate mongoTemplate;

Employee employee =
mongoTemplate.findById(
    "100",
    Employee.class
);

Repository Example

public interface EmployeeRepository
extends MongoRepository<Employee,String>{

}

34. Common CRUD Mistakes

  • Missing Indexes
  • Updating Entire Documents
  • Using skip() on huge datasets
  • Missing Projections
  • Ignoring Bulk Operations

CRUD Workflow

flowchart LR

Create --> Read --> Update --> Delete

Enterprise Best Practices

  • Create indexes for frequently searched fields.
  • Use Projection to reduce network traffic.
  • Use Bulk Operations for batch processing.
  • Use Upsert where appropriate.
  • Avoid replacing entire documents unnecessarily.
  • Prefer pagination over returning large datasets.
  • Use indexes for sorting.
  • Validate user input.
  • Monitor slow queries.
  • Test CRUD operations with production-sized datasets.

Quick Revision

Topic MongoDB Method
Insert One insertOne()
Insert Many insertMany()
Find One findOne()
Find Many find()
Update One updateOne()
Update Many updateMany()
Replace replaceOne()
Delete One deleteOne()
Delete Many deleteMany()
Upsert updateOne(..., upsert:true)
Pagination skip() + limit()
Bulk Processing bulkWrite()

Interview Tips

Interviewers frequently ask

  • Difference between insertOne() and insertMany().
  • Difference between find() and findOne().
  • What is Projection?
  • What is Upsert?
  • Difference between updateOne() and replaceOne().
  • Explain $set and $unset.
  • Explain $push and $pull.
  • How is pagination implemented?
  • What are Bulk Operations?
  • Show CRUD using Spring Boot.

Always explain that MongoDB CRUD operations are optimized for document-based storage, and features such as projections, bulk operations, update operators, and indexes significantly improve application performance.

Summary

MongoDB CRUD operations provide a rich and efficient API for managing document data. From simple inserts and queries to bulk updates, projections, pagination, and advanced update operators, MongoDB enables developers to build high-performance, scalable applications with minimal complexity.

Understanding CRUD methods, query operators, update operators, bulk processing, and Spring Data MongoDB integration is essential for backend development and MongoDB interviews.