Entity Types

The beauty of data modeling is that you can take the same information and show it at different levels of detail depending on the audience. In this book we discuss three levels of detail: conceptual, logical, and physical. Entities exist at all three levels.

For an entity to exist at a conceptual level, it must be both basic and critical to the business. What is basic and critical depends very much on the concept of scope. At a universal level, there are certain concepts common to all companies, such as Customer, Product, and Employee. Making the scope slightly narrower, a given industry may have certain unique concepts. Phone Number, for example, will be a valid concept for a telecommunications company, but perhaps not for other industries, such as manufacturing. In the publishing data model, Author, Title, and Order are conceptual entities, shown as names within rectangles (see Figure 4.1).

Figure 4.1 Sample conceptual entities

Entities at a logical level represent the business at a more detailed level than at the conceptual level. Frequently, a conceptual entity represents many logical data model entities. Logical entities contain properties, called “attributes” which we will discuss in Chapter 6. Figure 4.2 shows the three corresponding logical entities, based upon the conceptual entities from Figure 4.1.

Figure 4.2 Sample logical entities

At a physical level, the entities correspond to technology-specific objects, often database tables. The physical level is the same as the logical level, with the exception of compromises that were needed to make up for deficiencies in technology, often related to performance or storage. Figure 4.3 shows the three corresponding physical entities, based upon the logical entities from Figure 4.2. The physical entities also contain database-specific information, such as the format and length of an attribute (Author Last Name is 50 characters), and whether the attribute is required to have a value (Author Tax Identifier is not null and therefore required to have a value and Author Birth Date is null and therefore not required to have a value).

Figure 4.3 Sample physical entities