GraphQL is not about
real database access, it is a specification concerned with communicating with an API and be definition it is data source agnostic. It is not is a "complete package" to set up a full web application.
On top of an existing MVC based API, it is simply another Controller and it backing service which connect to the existing Models. It would be up to you to define the relationship between the Graph query and it's associated Model.
Redgate has an article for setting up a project using GraphQL with a Net Core API base:
Getting Started with GraphQL in ASP.NET - Simple Talk[
^]
I know it does not go over setting it up with a database but it does go over the underpinnings, you just need to add in your database and your desired method of talking to it; it could be through a framework (Entity, nHibernate) or just plain old ADO or OLE.
If you need something a little more,
NReco has a package out that includes a database layer to use ADO connected databases.
NReco GraphQL API for SQL database (C#/.NET)[
^]
Submit an article or tip