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Entity relationship diagram with square shaped organization entities named like student, class, and course, connected by lines with elliptical shaped attributes named like course_id, class_start_date, and class_ref_num. Entities are connected with diamond shaped relationships named like attends, and teaches.

An Entity-Relationship (E-R) Diagram visually depicts an organization's entities, the entities' relationships to each other, and the business rules (i.e., cardinality and dependency) associated with the relationships. It describes the data or information aspects of a business domain or its process requirements, in an abstract way.

There are three basic elements in an ER Diagram: entity, attribute, relationship. There are more elements which are based on the main elements. They are weak entity, multivalued attribute, derived attribute, weak relationship and recursive relationship. Cardinality is also used in ER diagrams to further define relationships. ERDs are usually drawn in top-to-bottom and left-to-right fashion.