2.SDL | Notion

Introduction to GraphQL SDL

Definition: SDL is the syntax used to define the structure of data within a GraphQL schema. This is how you specify the types and relationships between those types in your GraphQL server.
Purpose: Allows developers to declare how client applications can query the data exposed by a GraphQL server.

Types: The building blocks of a GraphQL schema.
- Scalar Types: Basic data types (e.g., Int, Float, String, Boolean, ID).
- Object Types: Represent a kind of object you can fetch from your service, and what fields it has.
  - Example:
```
type User {
  id: ID!
  name: String!
  email: String!
  posts: [Post]
}
```
- Interface Types: Abstract types that let you define fields that multiple types must include.
  - Example:
```
interface Character {
  id: ID!
  name: String!
  friends: [Character]
  appearsIn: [Episode]!
}
```
- Union Types: Define a type that could be one of many types.
  - Example:
```
union SearchResult = Book | Author | Magazine
```

Non-Nullable Fields: Defined with ! that indicates the field cannot return null.
List Fields: Defined with [ ] that indicates the field is a list of types.
- Example:
```
type User {
  hobbies: [String!]
}
```

Relationships: Connections between types, important for defining how different types interact.
- Example:
```
type Post {
  id: ID!
  title: String!
  author: User!
}
```

Use of Directives: Special tokens in the schema language that can change the way the server interprets parts of the schema.
- Common Directives:
  - @deprecated: Indicates that a field is no longer supported.
  - @include(if: Boolean): Only includes this field in the result if the argument is true.
  - @skip(if: Boolean): Skips this field if the argument is true.
- Example:
```
type User {
  id: ID!
  oldEmail: String @deprecated(reason: "Use `email`.")
  email: String!
}
```

Input Types: Special kinds of object types that allow you to pass objects as arguments.
- Example:
```
input NewUserInput {
  name: String!
  email: String!
  password: String!
}
```
Enum Types: Allow you to restrict a field to have one of a predefined set of values.
- Example:
```
enum Role {
  ADMIN
  USER
  GUEST
}
```

Queries: Allow clients to fetch data. They are similar to GET requests in REST.
Mutations: Allow clients to modify data (create, update, delete). They are like POST, PUT, DELETE in REST.
Subscriptions: Enable real-time updates to the client through technologies like WebSockets.