Relational model

The relational data model is simple and elegant. It has a solid mathematic foundation based on sets theory and predicate calculus and is the most used data model for databases today.
One of the drivers for Codd's research was the fact that IMS programmers were spending large amounts of time doing maintenance on IMS applications when logical or physical changes occurred; therefore, his goal was to deliver a model that provided better data independence. His proposal was threefold:  Store the data in a simple data structure (tables)  Access it through a high level set-at-a-time Data Manipulation Language (DML)  Be independent from physical storage With a simple data structure, one has a better chance of providing logical data independence. With a high-level language, one can provide a high degree of physical data independence. Therefore, this model allows also for physical storage independence. This was not possible in either IMS or CODASYL.
Figure 1.4 illustrates an example showing an Entity-Relationship (E-R) diagram that represents entities (tables) and their relationships for a sample relational model. We discuss more about E-R diagrams in the next section.