The logical ER model contains more detail than the conceptual ER model. Logical data model A logical ER model does not require a conceptual ER model, especially if the scope of the logical ER model includes only the development of a distinct information system. The conceptual data model may be used to form commonality relationships between ER models as a basis for data model integration. The purpose of the conceptual ER model is then to establish structural metadata commonality for the master data entities between the set of logical ER models. A conceptual ER model may be used as the foundation for one or more logical data models (see below).
Developing an enterprise-wide conceptual ER model is useful to support documenting the data architecture for an organization. The conceptual ER model normally defines master reference data entities that are commonly used by the organization.
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Note that the conceptual-logical-physical hierarchy below is used in other kinds of specification, and is different from the three schema approach to software engineering.Ĭonceptual data model This is the highest level ER model in that it contains the least granular detail but establishes the overall scope of what is to be included within the model set. There is a tradition for ER/data models to be built at two or three levels of abstraction. In a relational database a relationship between entities is implemented by storing the primary key of one entity as a pointer or "foreign key" in the table of another entity. In a simple relational database implementation, each row of a table represents one instance of an entity type, and each field in a table represents an attribute type. Diagrams created to represent attributes as well as entities and relationships may be called entity-attribute-relationship diagrams, rather than entity–relationship models.Īn ER model is typically implemented as a database. An ER model can also be expressed in a verbal form, for example: one building may be divided into zero or more apartments, but one apartment can only be located in one building.Įntities may be characterized not only by relationships, but also by additional properties ( attributes), which include identifiers called "primary keys". It is usually drawn in a graphical form as boxes ( entities) that are connected by lines ( relationships) which express the associations and dependencies between entities. It does not define the business processes it only presents a business data schema in graphical form. 3 Entity–relationships and semantic modelingĪn E-R model is usually the result of systematic analysis to define and describe what is important to processes in an area of a business.2.2 Relationships, roles and cardinalities.
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