The physical data model (relational model) is created using an entity-relationship diagram, based on Codd's 1970 relational model. Each relationship groups real-world objects with shared characteristics, as per set theory. The diagram features tables (relations) and inter-relations between them. Additionally, data structures are low-level abstractions that group related data and define operations that can be performed on the data. Various types of data structures exist, including stacks, queues, trees, and lists.