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An entity-relationship model (ERM) is a theoretical and conceptual way of showing data relationships in software development. ERM is a database modeling technique that generates an abstract diagram or visual representation of a system’s data that can be helpful in designing a relational database. These diagrams are known as entity-relationship diagrams, ER diagrams or ERDs.
Entity-relationship patterns were first proposed by Peter Pin-Shan Chen of the Massachusetts Institute of Technology (MIT) in 1976.
The first step in information system design dictates that the requirements analysis models illustrate the type of data or information that needs to be collected. The data modeling method may be used to illustrate a specific interest area’s ontology. Like the relational model, abstract data is converted to a logical data model when the design of an information system is built on a database. Likewise, this is converted to a physical model when it is physically designed.
The building blocks of an ERD are entities, relationships and attributes. Entities have entity types, which are known as instances of the corresponding entities. Each entity type can exist independently of another; for example, the entity "vehicle" can have the entity types "car" and "bus." Relationship is the property that links the entity types together. For example, the entity type husband is related to the entity type wife by a relationship known as "is-married-to." Attributes are properties that belong to the entity types as well as to the relationships.
There are a number of ER diagramming tools available on the market. The most common ones are MySQL Workbench and OpenModelSphere.