Graphql Inner and Left Join

We are using Apollo Router + Netflix DGS to build federated graphql services. Suppose that we have a common relational database right now for books and authors, but want to move towards a microservice architecture, where books and authors are stored separately in their own database.

Suppose that I have the following graphql schema:

type Author {
  name: String
  books: [Book]
}

type Book {
  title: String!
}

type Query {
  getAuthors(name: String!, title: String) {
    authors: [Authors]
  }
}

Here are two query scenarios / results:

Scenario 1 – returns multiple authors named Lewis and list of books (0:N) associated with each author named Lewis

query {
  getAuthors("Lewis") {
    name
    books
  }
}

Scenario 2

query {
  getAuthors(name: "Lewis", title: "Cronicles of Narnia") {
    name
    books
  }
}

In a single relational database where books and authors are stored together, my query could be:

Single Relational DB Query

select * from authors a inner join books b on a.id = b.author_id where name = "Lewis" and title = "Cronicles of Narnia";

select * from authors a left join books b where a.id = b.author_id and name = "Lewis" and title = "Cronicles of Narnia";

In my future, microservice, multi-database where books and authors are stored seperately, my query could be:

Microservice / Multi-database Query:

select * from authors where name = "Lewis";

select * from books where title = "Cronicles of Narnia";

• Then allow Apollo or Netflix DSG to join at the application level.

Question 1

What should my Scenario 2 graphql query return? I actually need to support both options below in different scenarios, so is there a different way to write the query for each?

• Option 1) just one author named Lewis and just one book named "Cronicles of Narnia"? (this is the INNER JOIN)

• Option 2) a list of authors named Lewis and, where linked to a specific "Lewis", append the book(s) named "Cronicles of Narnia"? (this is the LEFT JOIN)

Question 2

In my future, multi-database scenario, how would graphql (Apollo, Netflix DSG) behave? Is it more like Option 1 or Option 2? As mentioned, would need to support both.

Question 3

I’ve also seen a similar query written like this. Do these queries have different meanings? Is this widely supported?

query {
  getAuthors(name: "Lewis") {
    name
    books (title: "Cronicles of Narnia")
  }
}

Thanks!

Justin