GraphQL Fundamentals
Understand the core concepts of GraphQL, including queries, mutations, subscriptions, and schema definition language (SDL).
GraphQL Fundamentals is a free NestJS Enterprise Backend APIs lesson on CoddyKit — lesson 1 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the NestJS Enterprise Backend APIs learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.
What is GraphQL?
Welcome to GraphQL! It's a powerful query language for APIs and a runtime for fulfilling those queries with your existing data.
Think of it as asking for exactly what you need, nothing more, nothing less. This helps make your APIs efficient and flexible.
GraphQL vs. REST APIs
Traditionally, REST APIs often require multiple requests to fetch all necessary data, leading to "over-fetching" or "under-fetching".
- Over-fetching: Getting more data than you need.
- Under-fetching: Needing multiple requests to get all data.
GraphQL solves this by letting clients define the exact data structure they want in a single request.
The Core: GraphQL Schema
At the heart of every GraphQL API is its schema. This schema acts as a contract between the client and the server.
It defines all the data types and operations (queries, mutations, subscriptions) that clients can perform. It's like a blueprint for your API.
Defining Schemas with SDL
GraphQL schemas are written using the Schema Definition Language (SDL). It's a simple, human-readable syntax.
SDL allows you to specify your API's capabilities and data structures clearly. It's language-agnostic, meaning it works with any backend language.
schema {
query: Query
}
type Query {
hello: String
}Building Blocks: Object Types
The most fundamental component of a GraphQL schema is the Object Type. It represents a type of object you can fetch from your service.
Each object type has fields, which are specific pieces of data it can hold. Fields can also be other object types!
type User {
id: ID!
name: String!
email: String
posts: [Post!]
}
type Post {
id: ID!
title: String!
content: String
}Data Types: Scalars & Lists
Scalar types are the basic building blocks that resolve to a single value, like String, Int, Float, Boolean, and ID (a unique identifier).
List types allow a field to return a list of a certain type, indicated by []. The ! means a field is Non-Null, so it must always return a value.
type Product {
id: ID! # Non-null ID
name: String! # Non-null String
price: Float! # Non-null Float
tags: [String!] # List of non-null Strings
inStock: Boolean
}Fetching Data with Queries
Queries are how clients request data from a GraphQL server. You specify the exact fields you want to receive.
This ensures you only get the data you ask for, avoiding over-fetching common in REST APIs.
query GetProductNames {
products {
id
name
}
}Dynamic Queries with Arguments
Fields in GraphQL can take arguments, allowing you to pass parameters to customize the data fetched. This is useful for filtering or finding specific items.
Arguments are defined in the schema and used by the client when making a query.
query GetProductById {
product(id: "prod123") {
id
name
price
}
}Modifying Data with Mutations
While queries are for reading data, mutations are for writing, updating, or deleting data on the server.
Like queries, mutations also specify what data to return after the operation, ensuring immediate feedback on changes.
mutation CreateNewProduct {
createProduct(name: "New Gadget", price: 99.99) {
id
name
}
}Real-time Updates: Subscriptions
Subscriptions are a special type of operation that allows clients to receive real-time updates from the server when specific events occur.
This is perfect for features like live chat, notifications, or real-time dashboards. It typically uses WebSockets.
Schema Fundamentals Check
Consider the following GraphQL SDL snippet for an Author type:
type Author {
id: ID!
name: String!
books: [Book!]
}Which of the following statements are true about the books field?
GraphQL Fundamentals Recap
Great job! You've learned the core concepts of GraphQL:
- It's a query language for APIs, offering precise data fetching.
- The schema defines the API's capabilities using SDL.
- You use queries to read, mutations to write, and subscriptions for real-time data.
Next, we'll see how to integrate GraphQL into a NestJS application!
Frequently asked questions
Is the “GraphQL Fundamentals” lesson free?
Yes — the full text of “GraphQL Fundamentals” is free to read here on the web, and the NestJS Enterprise Backend APIs course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the NestJS Enterprise Backend APIs course, upgrade to CoddyKit PRO.
What will I learn in “GraphQL Fundamentals”?
Understand the core concepts of GraphQL, including queries, mutations, subscriptions, and schema definition language (SDL). You practise NestJS Enterprise Backend APIs with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start NestJS Enterprise Backend APIs?
No prior experience is required. NestJS Enterprise Backend APIs on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 3, so you can start here or from the beginning and move at your own pace.
How long does the “GraphQL Fundamentals” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this NestJS Enterprise Backend APIs lesson?
Yes. Every NestJS Enterprise Backend APIs lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- GraphQL Fundamentals
- NestJS GraphQL Setup
- Resolvers and Subscriptions