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In GraphQL, data types are like blueprints for your application's information. They define the shape and kind of data that can be queried or modified.

While GraphQL provides some basic types, you'll often need to define your own to perfectly match your application's unique data structure.

GraphQL has built-in scalar types like String, Int, Boolean, Float, and ID. These are fundamental, single-value types.

But real-world data is complex! This is where Object Types come in. They let you define objects with multiple fields, each having its own type.

You define an object type using the type keyword in GraphQL's Schema Definition Language (SDL). Think of it as creating a custom class or struct.

Each field within an object type has a name and a type. Fields can be other object types, scalar types, or even lists of types.

Let's imagine we're building an API for a library. We'd need a way to represent a book. Here's how you might define a Book object type:

type Book {
  id: ID!
  title: String!
  author: String
  publicationYear: Int
}

The ! after a type means the field is non-nullable – it must always have a value.

Sometimes, built-in scalars aren't specific enough. For example, a Date field could be a String, but what if you need to ensure a specific format?

Custom Scalar Types allow you to define your own scalar types with specific serialization, parsing, and validation rules. This ensures data consistency.

Defining a custom scalar in your schema is straightforward. You use the scalar keyword, followed by the name of your new scalar type.

Later, your Spring Boot application will need to provide the actual logic for handling this custom scalar's data transformations.

scalar Date

type Event {
  id: ID!
  name: String!
  eventDate: Date!
}

Enum Types (short for 'enumeration') are special scalar types that represent a finite set of possible values. They're great for fields where you want to restrict choices to a predefined list.

Using enums makes your schema more self-documenting and helps prevent invalid data from being sent to your API.

For our library API, a book might have a specific status. We can define an enum for this:

enum BookStatus {
  AVAILABLE
  CHECKED_OUT
  LOST
  REPAIR
}

Now, any field using BookStatus can only have one of these four values.

Let's update our Book type to use our new BookStatus enum and a custom Date scalar.

scalar Date

enum BookStatus {
  AVAILABLE
  CHECKED_OUT
  LOST
}

type Book {
  id: ID!
  title: String!
  author: String
  publicationYear: Int
  status: BookStatus!
  lastCheckedOut: Date
}

This shows how custom types build a richer, more precise schema.

You've learned how to define custom data types in GraphQL using SDL!

• Object Types (type) structure complex data.
• Custom Scalar Types (scalar) handle specific data formats.
• Enum Types (enum) restrict field values to a finite set.

These tools are crucial for building a precise and robust GraphQL schema that accurately represents your application's domain.