List of entities
- It is significant.
List only entities that are important to your database users and that are worth the trouble and expense of computer tabulation.
- It is generic.
List only types of things, not individual instances. For instance, symphony might be an entity, but Beethoven's Fifth would be an entity instance or entity occurrence.
- It is fundamental.
List only entities that exist independently and do not require anything else to explain them. Anything you might call a trait, a feature, or a description is not an entity. For example, a part number is a feature of the fundamental entity called part. Also, do not list things that you can derive from other entities; for example, avoid any sum, average, or other quantity that you can calculate in a SELECT expression.
- It is unitary.
Be sure that each entity you name represents a single class. It cannot be separated into subcategories, each with its own features. In the telephone directory example in Partial page from a telephone directory, the telephone number, an apparently simple entity, actually consists of three categories, each with different features.
These choices are neither simple nor automatic. To discover the best choice of entities, you must think carefully about the nature of the data you want to store. Of course, that is exactly the point of a formal data model. The following section describes the telephone directory example in detail.