The Entity Relationship Diagram
At the very highest level is the entity relationship diagram, or ERD. The ERD identifies the major subject areas of the corporation. The different subjects connect through relationships. The relationships describe the business logic binding one subject to another.
The ERD does not purport to show any great level of detail. Instead, the ERD shows major categories of data and how those categories relate to each other. A simple ERD is depicted in Figure 12.1.