If the backend is running on same machine (even in same app?) then why do you need a SQL server?!
I never said I want to use SQL, sorry. I don't want local storage either. And there's only one app/project. The back-end part in cpp in the app is for more computations- there might be more buttons and work on the app to do.
I think I don't need any SQL, MySQL, SQLite etc.
It looks that is hard for me to convey my thoughts.