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 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. 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. An entity–attribute-relationship diagram for an MMORPG using Chen's notation
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