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An entity-type is a category. An entity, strictly speaking, is an instance of a given entity-type. There are usually many instances of an entity-type. Because the term entity-type is somewhat cumbersome, most people tend to use the term entity as a synonym. Entities can be thought of as nouns. [7]
The identity of an entity is defined by the entity type it is an instance of; in that sense an entity type defines the class an entity belongs to and also defines what properties an entity will have. Properties describe some aspect of the entity by giving it a name and a type.
Each instance of the category entity is simultaneously an instance of the generic entity. Non-specific relationships A relationship in which an instance of either entity can be related to any number of instances of the other. View levels Three levels of view are defined in IDEF1X: entity relationship (ER), key-based (KB), and fully attributed (FA).
An entity only consists of an ID for accessing components. It is a common practice to use a unique ID for each entity. This is not a requirement, but it has several advantages: The entity can be referred using the ID instead of a pointer. This is more robust, as it would allow for the entity to be destroyed without leaving dangling pointers.
An entity instance can be constructed for either a single entity (if not abstract) or for a complex combination of entities in such a sub-supertype graph. For the big graphs the number of possible combinations is likely to grow in astronomic ranges.
The instance name is also used to reference other entity instances through attribute values or aggregate members. The referenced instance may be defined before or after the current instance. Instances of single entity data types are represented by writing the name of the entity in capital letters and then followed by the attribute values in the ...
The rows represent instances of that type of entity (such as "Lee" or "chair") and the columns represent values attributed to that instance (such as address or price). For example, each row of a class table corresponds to a class, and a class corresponds to multiple students, so the relationship between the class table and the student table is ...
Individuals (instances) are the basic, "ground level" components of an ontology. The individuals in an ontology may include concrete objects such as people, animals, tables, automobiles, molecules, and planets, as well as abstract individuals such as numbers and words (although there are differences of opinion as to whether numbers and words are classes or individuals).