Conceptual schema

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 

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.


Example 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.


Data structure diagram


A data 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 binds them.