A demo project for GraphQL taken from here:
https://www.pluralsight.com/guides/building-a-graphql-server-with-spring-boot

Just to learn about GraphQL. I didn't implement the error handling yet, as I want to focus only on the minimal basics of GraphQL for now.

You can use GraphiQL to send queries
http://localhost:8080/graphiql

Try these samples to query the api:

findAllBooks {
   id
   title
 }

{
  findAllBooks {
    id
    isbn
    title
    pageCount
    author {
      firstName
      lastName
    }
  }
}

To create, update or delete entries use mutation

mutation {
  newAuthor(
    firstName: "Demo"
    lastName: "Author"
  ) {
    id firstName lastName
  }
}

mutation {
  newBook(
    title: "Demo Book"
    isbn: "0123456789"
    pageCount: 123
    author: 8
  ) {
    id title author {
      firstName lastName
    }
  }
}

H2 is used as a in-memory database. Here you can query the DB directly:
http://localhost:8080/h2-console/login.jsp
Use this JDBC URL: jdbc:h2:mem:testdb
And this username: sa