the API is literally a query language for the DB
The API is the contract you define for how people with interact with your application. While some people may choose to model this after their underlying database, conceptually, they are very different.
On small applications, maybe you can get away with treating them as one and the same, but for longer-lived applications which need to integrate and co-exist with other systems/services, that choice can come back to bite you.
I'm not saying you shouldn't/can't treat them as the same thing, just be sure you know the trade-offs and long-term repercussions.
True.
Would be nice to have some kind of graphical tool, that would allow to model these things.
Add databases
Extract the schemas from these databases automatically, which should be trivial with SQL databases
Create GraphQL types manually
link the types up with the databases (the tricky part, especially with multiple different databases)
If that's not a concern, and you want speed/ease of development, take a look at TypeGraphQL [1]. They have a couple of examples [2][3] of how nicely it plays with TypeORM. You can create one set of domain object classes, annotate them with decorators for both TypeGraphQL and TypeORM, and then expose a GraphQL schema that matches the underlying database with a minimal amount of code (you could probably even get clever with some generic fucntions to cut down the code you'd need to write for the resolvers).
[1] https://typegraphql.com/ [2] https://github.com/MichalLytek/type-graphql/tree/v0.17.5/exa... [3] https://github.com/MichalLytek/type-graphql/tree/v0.17.5/exa...