Definition

entity-relationship diagram (ERD or ER diagram)

An entity-relationship diagram is a data modeling technique that creates a graphical representation of the entities, and the relationships between entities, within an information system.(View diagram.)

The three main components of an ERD are:

  • The entity is a person, object, place or event for which data is collected. For example, if you consider the information system for a business, entities would include not only customers, but the customer's address, and orders as well. The entity is represented by a rectangle and labelled with a singular noun.
  • The relationship is the interaction between the entities. In the example above, the customer places an order, so the word "places" defines the relationship between that instance of a customer and the order or orders that they place. A relationship may be represented by a diamond shape, or more simply, by the line connecting the entities. In either case, verbs are used to label the relationships.
  • The cardinality defines the relationship between the entities in terms of numbers. An entity may be optional: for example, a sales rep could have no customers or could have one or many customers; or mandatory: for example, there must be at least one product listed in an order. There are several different types of cardinality notation; crow's foot notation, used here, is a common one. In crow's foot notation, a single bar indicates one, a double bar indicates one and only one (for example, a single instance of a product can only be stored in one warehouse), a circle indicates zero, and a crow's foot indicates many. The three main cardinal relationships are: one-to-one, expressed as 1:1; one-to-many, expressed as 1:M; and many-to-many, expressed as M:N.

The steps involved in creating an ERD are:

  • Identify the entities.
  • Determine all significant interactions.
  • Analyze the nature of the interactions.
  • Draw the ERD.

A number of CASE tools, such as Visible Analyst and Data Architect, can be used to generate ERDs.

Related glossary terms: Hancock, cooked data
This was last updated in March 2007
Posted by: Margaret Rouse

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