GraphQL is a query language for APIs and a runtime for fulfilling those queries with your existing data. GraphQL provides a complete and understandable description of the data in your API, gives clients the power to ask for exactly what they need and nothing more, makes it easier to evolve APIs over time, and enables powerful developer tools.
Tutorial: How to build a GraphQL server – Apollo GraphQL
[H]ow to build a GraphQL server that connects to multiple backends: A SQL database, a MongoDB database and a REST endpoint. We’ll be combining all of them to build a very basic blog with authors, posts and views.