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The data modeling technique can be used to describe any ontology (i.e. The first stage of information system design uses these models during the requirements analysis to describe information needs or the type of information that is to be stored in a database. The ER model is also normally used to design modifications to the relational database objects and to maintain the structural metadata of the database. The physical model is normally instantiated in the structural metadata of a database management system as relational database objects such as database tables, database indexes such as unique key indexes, and database constraints such as a foreign key constraint or a commonality constraint. Therefore, each physical ER model must contain enough detail to produce a database and each physical ER model is technology dependent since each database management system is somewhat different. The physical ER model is normally developed to be instantiated as a database. Physical data model One or more physical ER models may be developed from each logical ER model. The logical ER model is however developed independently of the specific database management system into which it can be implemented. The details of each data entity are developed and the relationships between these data entities are established. In addition to master data entities, operational and transactional data entities are now defined. 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.
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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.
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