What are Entity Relationship Diagrams?

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Entity Relationship Diagrams (ERDs) illustrate the logical structure of databases.

An ER Diagram

Entity Relationship Diagram Notations

Peter Chen developed ERDs in 1976. Since then Charles Bachman and James Martin have added some sligh refinements to the basic ERD principles.

Entity

An entity is an object or concept about which you want to store information.

Weak Entity

A weak entity is an entity that must defined by a foreign key relationship with another entity as it cannot be uniquely identified by its own attributes alone.

Key attribute

A key attribute is the unique, distinguishing characteristic of the entity. For example, an employee's social security number might be the employee's key attribute.

Multivalued attribute

A multivalued attribute can have more than one value. For example, an employee entity can have multiple skill values.

Derived attribute

A derived attribute is based on another attribute. For example, an employee's monthly salary is based on the employee's annual salary.

Relationships

Relationships illustrate how two entities share information in the database structure.

Cardinality

Cardinality specifies how many instances of an entity relate to one instance of another entity.
Ordinality is also closely linked to cardinality. While cardinality specifies the occurences of a relationship, ordinality describes the relationship as either mandatory or optional. In other words, cardinality specifies the maximum number of relationships and ordinality specifies the absolute minimum number of relationships.

Recursive relationship

In some cases, entities can be self-linked. For example, employees can supervise other employees.