Eed Diagrams

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An entity-relationship diagram, or ERD, shows the overall design structure of a database. The basic unit of an ERD is the entity. Each entity represents a particular kind of data. For instance, an employee training database might have an entity for employee data, and entity for department information, and an entity for the classes. One way to get a handle on entities is to remember that in a relational database there is generally one table for each entity.
Recognizing which entities you need in a database is a skill that develops with practice. Nevertheless, some tips can get you started.
When you were writing themes in junior high, your English teacher probably told you to have one paragraph for each topic or each point you wanted to make. In database design you have one entity for each data topic. In the example quoted above, you can see that employee information is different than class information. So often when you start detailing the specific pieces of information that need to be stored, those data topics naturally emerge.
Another tip is to ask yourself what the purpose of the database is. For instance, an employee database will almost always have an employee entity. A parts inventory database will certainly have a parts entity. Start with that entity. Principles of good database design (chapter 5) will help you see what pieces of information don’t fit in that entity and so must belong to other entities.
An Entity-Relationship Diagram shows the relationships among the various entities in a database. Each entity is shown as a rectangle. Where entities are related to other entities, the relationship is shown with a line. A diamond shape on the line is used to hold a word that describes the relationship.


For instance, the ERD in Figure 3-1 shows three entities: Employee, Class, and Department. The diamonds describe the relationships between the entities: An employee belongs to a department; an employee takes a class. Note...