I compared the relationships between primary and foreign keys and the relationships that arise between tables as a result of a join, with roads and paths between populated areas. With the development of each of the databases, and the addition of connections between tables, when viewed from a bird’s eye view, the database looks like a graph model, and each of the connections – like an edge of the graph, as an opportunity to navigate from table as a vertex to another one.

A few words about this in the following article:

Relational Databases — Navigation in the relational data model, Graph model