GraphQL — flexible queries with one endpoint, type system, resolvers, and N+1 dataloader patterns. When GraphQL beats REST.

What is GraphQL?

GraphQL is a query language for APIs that allows clients to request exactly the data they need. Unlike REST, clients specify the structure of the response.

GraphQL vs REST

Aspect	GraphQL	REST
Endpoints	Single `/graphql`	Multiple per resource
Data fetching	Client specifies	Server determines
Over-fetching	No	Common
Under-fetching	No	Common (N+1)
Versioning	Evolve schema	Version URLs

Schema Definition

type User {
  id: ID!
  name: String!
  email: String!
  posts: [Post!]!
}

type Post {
  id: ID!
  title: String!
  author: User!
}

type Query {
  user(id: ID!): User
  users: [User!]!
}

type Mutation {
  createUser(name: String!, email: String!): User!
}

Query Example

Request

query {
  user(id: "123") {
    name
    email
    posts {
      title
    }
  }
}

Response

{
  "data": {
    "user": {
      "name": "John",
      "email": "john@example.com",
      "posts": [
        { "title": "First Post" },
        { "title": "Second Post" }
      ]
    }
  }
}

Operations

Operation	Purpose	Example
Query	Read data	`query { users { name } }`
Mutation	Write data	`mutation { createUser(...) }`
Subscription	Real-time updates	`subscription { newPost { title } }`

Benefits

Benefit	Description
No over-fetching	Get only what you need
No under-fetching	Get related data in one request
Strong typing	Schema defines all types
Introspection	Self-documenting API
Versioning	Deprecate fields, not endpoints

Challenges

Challenge	Mitigation
N+1 queries	DataLoader pattern
Complexity	Query cost analysis
Caching	More complex than REST
File uploads	Separate endpoint or multipart

Interview Tips

Explain over-fetching and under-fetching problems
Know schema, query, mutation, subscription
Discuss N+1 problem and DataLoader
Compare with REST for different use cases
Mention when GraphQL is overkill

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GraphQL

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