Schema

The schema defines the capabilities of a GraphQL server. It defines the possible queries, mutations, subscriptions, and additional types and directives. While the schema can be written in a programming language, it is often written in SDL (the GraphQL Schema Definition Language). Here is the most basic schema, written in SDL:

schema {
  query: Query
}

type Query {
  hello: String
}

It has a single root query field, hello, of type String (when we send a hello query, the server will return a string value). We can omit the schema declaration when we use operation types named Query, Mutation, and Subscription, so the above is equivalent to:

type Query {
  hello: String
}

With this schema, the client can make the below query:

query {
  hello
}

and receive this response:

{
  "data": {
    "hello": "world!"
  }
}

The root fields—those listed under type Query { ... }, type Mutation { ... }, and type Subscription { ... }—are the entry points to our schema. They’re the fields that can be selected by the client at the root level of an operation.