# GraphQL

GraphQL is a query language for APIs and a runtime for fulfilling those queries with your existing data. It provides a complete and understandable description of the data in your API, gives clients the power to ask for exactly what they need and nothing more.

## 1. GraphQL vs REST

While REST APIs rely on multiple endpoints (URLs) to return fixed data structures, GraphQL typically exposes a **single endpoint**.

*   **REST:** You might need to hit `/users/1` to get user details, then `/users/1/posts` to get their posts. This often leads to **Over-fetching** (getting too much data) or **Under-fetching** (not getting enough in one request).
*   **GraphQL:** You send a single query describing the nested data you want (User + Posts), and the server returns exactly that.

## 2. Core Concepts

### Schema
The schema defines the "shape" of your data graph. It specifies types and relationships.

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

type Post {
  id: ID!
  title: String!
}
```

### Query
A request to **read** or fetch data. It looks like JSON keys without values.

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

### Mutation
A request to **write** (create, update, delete) data.

```graphql
mutation {
  createPost(title: "Hello World", userId: "123") {
    id
    title
  }
}
```

### Resolvers
Resolvers are functions written on the server that are responsible for populating the data for a single field in your schema.

## 3. Example Response

If you send the query above, the server returns JSON that mirrors the query structure:

```json
{
  "data": {
    "user": {
      "name": "Alice",
      "posts": [
        { "title": "My First Post" },
        { "title": "GraphQL is Cool" }
      ]
    }
  }
}
```

[[programming/rest-apis]]
[[programming/database-basics]]