Types of Entities
There are several different schemes for categorizing entities based on qualitatively different criteria. For purposes of this manual, the following two schemes are defined:
The following table provides definitions for these types:
|
Entity Type |
Definition |
Example |
|
Major |
An entity with relatively large cardinality and degree that is updated frequently. |
Order table |
|
Minor |
An entity with small cardinality and degree that is rarely updated. Minor entities are typically used in a single, 1:M association, and their primary key is often nonnumeric. |
Nation Code table |
|
Supertype |
A generic entity that is a superclass of one or more subtype entities. Supertype and subtype entities model the same real world entity at a high level. Supertypes must, by definition, have one or more reciprocal subtypes. |
Publications table |
|
Subtype |
A specific entity that is a disjoint subclass of one and only one supertype entity. Subtype and supertype entities model the same real world entity at a high level. Subtype entities typically have a higher degree than their supertypes, with the additional attributes describing detailed characteristics of the subtype that distinguish it from the other subtype entities of a mutual supertype. |
(all as subtypes of the supertype Publications) |