From Oracle FAQ
A relationship refers to the association between two entities in an ERD. Each end of the relationship shows the degree of how the entities are related and the optionality.
The degree can be expressed as:
- 1:1 - One-to-One relationship in an ERD
- 1:N - One-to-many relationship in an ERD
- N:N - Many-to-many relationship in an ERD
For example: An employee can belong to one and only one department, but a department can have many employees (1:N).
- ARC relationship - indicate mutually exclusive relationships
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