Schema Definition Language (SDL) in GraphQL

# Scalar types
scalar Date
scalar JSON

# Enums
enum Role {
  ADMIN
  USER
  GUEST
}

# Object types
type User {
  id: ID!                    # Non-nullable ID
  name: String!              # Required string
  email: String              # Optional string
  age: Int
  isActive: Boolean!
  role: Role!
  createdAt: Date!
  metadata: JSON
  posts: [Post!]!            # Non-null list of non-null Posts
  friends: [User]            # Nullable list of nullable Users
}

type Post {
  id: ID!
  title: String!
  content: String!
  published: Boolean!
  author: User!              # Relationship
  comments: [Comment!]!
  tags: [String!]!
}

# Input types for mutations
input CreateUserInput {
  name: String!
  email: String!
  password: String!
  role: Role = USER          # Default value
}

input UpdateUserInput {
  name: String
  email: String
  isActive: Boolean
}

# Interfaces
interface Node {
  id: ID!
}

# Union types
union SearchResult = User | Post | Comment

# Root types
type Query {
  # User queries
  users(limit: Int = 10, offset: Int = 0): [User!]!
  user(id: ID!): User
  currentUser: User
  
  # Post queries
  posts(published: Boolean): [Post!]!
  post(id: ID!): Post
  
  # Search
  search(query: String!): [SearchResult!]!
}

type Mutation {
  # User mutations
  createUser(input: CreateUserInput!): User!
  updateUser(id: ID!, input: UpdateUserInput!): User
  deleteUser(id: ID!): Boolean!
  
  # Post mutations
  createPost(title: String!, content: String!): Post!
  publishPost(id: ID!): Post
}

type Subscription {
  # Real-time updates
  userAdded: User!
  postPublished: Post!
  commentAdded(postId: ID!): Comment!
}