Schema Definition Language (SDL)

◕ GraphQLLesson 2Beginner

The Schema Definition Language (SDL) is how you define the shape of your GraphQL API. It is the contract between client and server — describing every type, field, and operation available. All GraphQL servers start with a schema.

1 Defining Object Types

GraphQL SDL — Type Definitions

# Object Type — the building block of GraphQL schemas
type User {
  id:        ID!          # ID scalar, non-null (! = required)
  name:      String!      # Non-null string
  email:     String!      # Non-null string
  age:       Int          # Nullable integer
  isActive:  Boolean!     # Non-null boolean
  role:      Role!        # Enum type
  posts:     [Post!]!     # Non-null list of non-null Posts
  createdAt: String!
}

type Post {
  id:       ID!
  title:    String!
  body:     String!
  author:   User!
  tags:     [String!]
  comments: [Comment!]!
}

type Comment {
  id:     ID!
  text:   String!
  author: User!
  post:   Post!
}

enum Role {
  USER
  EDITOR
  ADMIN
}

2 Root Operation Types

GraphQL SDL — Query, Mutation, Subscription

# Every schema has up to 3 root types

type Query {
  # Read operations
  user(id: ID!): User
  users(role: Role, limit: Int, offset: Int): [User!]!
  post(id: ID!): Post
  posts(authorId: ID): [Post!]!
  me: User                        # Current authenticated user
}

type Mutation {
  # Write operations
  createUser(input: CreateUserInput!): User!
  updateUser(id: ID!, input: UpdateUserInput!): User!
  deleteUser(id: ID!): Boolean!
  createPost(input: CreatePostInput!): Post!
  addComment(postId: ID!, text: String!): Comment!
}

type Subscription {
  # Real-time events
  commentAdded(postId: ID!): Comment!
  userOnline: User!
}

# Input types (used for mutations)
input CreateUserInput {
  name:     String!
  email:    String!
  password: String!
  role:     Role
}

input UpdateUserInput {
  name:  String
  email: String
}

3 Nullability Rules

Type Syntax	Meaning	Example Value
`String`	Nullable string	"hello" or null
`String!`	Non-null string	"hello" (never null)
`[String]`	Nullable list of nullable strings	["a", null] or null
`[String!]`	Nullable list of non-null strings	["a", "b"] or null
`[String!]!`	Non-null list of non-null strings	["a", "b"] (never null)

4 Code Challenge

Challenge: Design a GraphQL schema for an e-commerce platform. Define types for Product, Order, OrderItem, and Customer. Add appropriate queries to list/get products and orders, and mutations to place an order and update product stock.