Entity-relationship model

The Entity-relationship model or Entity-relationship diagram (ERD) is a data model or diagram for high-level descriptions of conceptual data models, and it provides a graphical notation for representing such data models in the form of entity-relationship diagrams. Such data models are typically used in the first stage of information-system design; they are used, for example, to describe information needs and/or the type of information that is to be stored in the database during the requirements analysis. The data modelling technique, however, can be used to describe any ontology (i.e. an overview and classification of used terms and their relationships) for a certain universe of discourse (i.e. area of interest). In the case of the design of an information system that is based on a database, the conceptual data model is, at a later stage (usually called logical design), mapped to a logical data model, such as the relational model; this in turn is mapped to a physical model during physical design. Note that sometimes, both of these phases are referred to as "physical design".

There are a number of conventions for entity-relationship diagram (ERDs). The classical notation is described in the remainder of this article, and mainly relates to conceptual modelling. There are a range of notations more typically employed in logical and physical database design, including Information Engineering, IDEF1x (ICAM DEFinition Language) and Dimensional Modelling.


Common Symbols

Missing image
An example of two related entities

An entity represents a discrete object. Entities can be thought of (roughly) as nouns. Examples: a computer, an employee, a song, a mathematical theorem. A relationship captures how two or more entities are related to one another. Relationships can be thought of (again, roughly) as verbs. Examples: an owns relation between a company and a computer, a supervises relation between an employee and a department, a performs relation between an artist and a song, a proved relation between a mathematician and a theorem. Entities are drawn as rectangles, relationships as diamonds.

Entities and relationships can both have attributes. Examples: an employee entity might have a social security number attribute (in the US); the proved relation may have a date attribute. Attributes are drawn as ovals connected to their owning entity sets by a line.

Every entity (unless it is a weak entity) must have a minimal set of uniquely identifying attributes. This set is called the entity's primary key.

Entity-relationship diagrams don't show single entities or single instances of relations. Rather, they show entity sets and relationship sets (displayed as rectangles and diamonds respectively). Example: a particular song is an entity. The collection of all songs in a database is an entity set. The proved relationship between Andrew Wiles and Fermat's last theorem is a single relationship. The set of all such mathematician-theorem relationships in a database is a relationship set.

Lines are drawn between entity sets and the relationship sets they are involved in. If all entities in an entity set must participate in a relation in the relationship set, a thick line is drawn. This is called a participation constraint. If each entity of the entity set can participate in at most one relationship in the relationship set, an arrow is drawn from the entity set to the relationship set. This is called a key constraint. To indicate that each entity in the entity set is involved in exactly one relationship, a thick arrow is drawn.

Associative entity is used to solve the problem of two entities with a many-to-many relationship [1] (http://infocom.cqu.edu.au/Courses/spr2000/95169/Extra_Examples/ERD.htm).

Unary Relationships (http://www.utexas.edu/courses/mis325/faq/faq034.htm) - a unary relationship is a relationship between the rows of a single table.

Less Common Symbols

A weak entity is an entity that can't be uniquely identified by its own attributes alone, and therefore must use as its primary key both its own attributes and the primary key of an entity it is related to. A weak entity set is indicated by a bold rectangle (the entity) connected by a bold arrow to a bold diamond (the relationship).

Sometimes two entities are more specific subtypes of a more general type of entity. For example, programmers and marketers might both be types of employees at a software company. To indicate this, a triangle with "ISA" on the inside is drawn. The superclass is connected to the point on top and the two (or more) subclasses are connected to the base.

Finally, through aggregation a relation and all its participating entity sets can be treated as a single entity set for the purpose of taking part in another relation. This is indicated by drawing a dotted rectangle around all aggregated entities and relationships.


Entity relationship models can be classified in BERMs (Binary Entity Relation Model) and GERMs (General Entity Relationship Model) according to if only binary relationships are allowed.

See also


  • Peter P. Chen, The Entity-Relationship Model - Toward a Unified View of Data. ACM Transactions on Database Systems (TODS) Volume 1 Number 1: pp 9-36 (1976) [2] (http://www.cobase.cs.ucla.edu/pub/dblp/html/db/journals/tods/tods1.html)

This paper is one of the most cited papers in the computer field. It was selected as one of the most influential papers in computer science in a recent survey of over 1,000 computer science professors. The citation is listed, for example, in DBLP: http://dblp.uni-trier.de/ [3] (http://dblp.uni-trier.de/)

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