Conceptual schema

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A conceptual schema or conceptual data model is a map of concepts and their relationships. This describes the semantics of an organization and represents a series of assertions about its nature. Specifically, it describes the things of significance to an organization (entity classes), about which it is inclined to collect information, and characteristics of (attributes) and associations between pairs of those things of significance (relationships).

Because it represents the semantics of an organization, and not a database design, it may exist on various levels of abstraction. The original ANSI four-schema architecture began with the set of external schemas that each represent one person's view of the world around him or her. These are consolidated into a single conceptual schema that is the superset of all of those external views. A data model can be as concrete as each person's perspective, but this tends to make it inflexible. If that person's world changes, the model must change. Conceptual data models take a more abstract perspective, identifying the fundamental things, of which the things an individual deals with are just examples.

The model does allow for what is called inheritance in object oriented terms. The set of instances of an entity class may be subdivided into entity classes in their own right. Thus, each instance of a sub-type entity class is also an instance of the entity class's super-type. Each instance of the super-type entity class, then is also an instance of one of the sub-type entity classes.

Super-type/sub-type relationships may be exclusive or not. A methodology may require that each instance of a super-type may only be an instance of one sub-type. Similarly, a super-type/sub-type relationship may be exhaustive or not. It is exhaustive if the methodology requires that each instance of a super-type must be an instance of a sub-type.

[edit] Sample Relationships

Each PERSON may be the vendor in one or more ORDERS.
Each ORDER must be from one and only one PERSON.
PERSON is a sub-type of PARTY. (Meaning that every instance of PERSON is also an instance of PARTY.)
Each Employee may have the supervisor within Employee.