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. 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. It 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 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. Typically, it represents records of entities and events monitored and directed by business processes, rather than the processes themselves. Some ER models show super and subtype entities connected by generalization-specialization relationships, and an ER model can be used also in the specification of domain-specific ontologies.Īn ER model is usually the result of systematic analysis to define and describe what data is created and needed by processes in an area of a business. Consequently, the ER model becomes an abstract data model, that defines a data or information structure which can be implemented in a database, typically a relational database.Įntity–relationship modeling was developed for database and design by Peter Chen and published in a 1976 paper, with variants of the idea existing previously, but today it is commonly used for teaching students the basics of data base structure. In software engineering, an ER model is commonly formed to represent things a business needs to remember in order to perform business processes. ![]() A basic ER model is composed of entity types (which classify the things of interest) and specifies relationships that can exist between entities (instances of those entity types). JSTOR ( November 2016) ( Learn how and when to remove this template message)Īn entity–attribute-relationship diagram for an MMORPG using Chen's notationĪn entity–relationship model (or ER model) describes interrelated things of interest in a specific domain of knowledge.Unsourced material may be challenged and removed.įind sources: "Entity–relationship model" – news Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |