Definition: An entity-relationship (ER) diagram is a specialized graphic that illustrates the relationships between entities in a database. ER diagrams often use symbols to represent three different types of information. Boxes are commonly used to represent entities. Diamonds are normally used to represent relationships and ovals are used to represent attributes.
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Also Known As: ER Diagram, E-R Diagram, entity-relationship model
Examples: Consider the example of a database that contains information on the residents of a city. The ER digram shown in the image above contains two entities -- people and cities. There is a single "Lives In" relationship. Each person lives in only one city, but each city can house many people.