Again and again hackers use SQL injection to get information from the backend database. Is it fair to say because a relational database by its nature has all the links within the data sets (e.g., 1-to-1, 1-to-* and so on), therefore it enables a user with any form of access to illegally access everything, either in theory or practice?
How data is related or not within the database has nothing to do with SQLi. Flat files, NoSQL, etc. will not affect the ability for a user to perform unauthorized access to data.
SQLi is about unsecured direct access to the database (in whatever form that is), resulting in a user manipulating that access in order to obtain data without the expected authorization.