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Aconceptual schema orconceptual data model is a high-level description of informational needs underlying the design of adatabase.[1][2] It typically includes only the core concepts and the main relationships among them. This is a high-level model with insufficient detail to build a complete, functional database.[3] It describes the structure of the whole database for a group of users. The conceptual model is also known as thedata model that can be used to describe the conceptual schema when a database system is implemented.[citation needed] It hides the internal details of physical storage and targets the description of entities, datatypes, relationships and constraints.
A conceptual schema is a map ofconcepts and theirrelationships used fordatabases. This describes thesemantics of an organization and represents a series ofassertions about its nature. Specifically, it describes the things of significance to anorganization (entity classes), about which it is inclined to collect information, and their characteristics (attributes) and the associations between pairs of those things of significance (relationships).
Because a conceptual schema represents the semantics of an organization, and not adatabase design, it may exist on various levels of abstraction. The originalANSI four-schema architecture began with the set ofexternal schemata that each represents one person's view of the world around him or her. These are consolidated into a singleconceptual 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 calledinheritance inobject oriented terms. The set ofinstances of an entity class may be subdivided into entity classes in their own right. Thus, each instance of asub-type entity class is also an instance of the entity class'ssuper-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 beexclusive or not. A methodology may require that each instance of a super-type mayonly be an instance ofone sub-type. Similarly, a super-type/sub-type relationship may beexhaustive or not. It is exhaustive if the methodology requires that each instance of a super-typemust be an instance of a sub-type. A sub-type named "Other" is often necessary.
Adata structure diagram (DSD) is a data model or diagram used to describe conceptual data models by providing graphical notations which document entities and their relationships, and the constraints that bind them.