Database

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Entity: an object in the real world that is distinguishable from other objects.
Entity Set: a collection of similar entities.

Attribute:
Key: a minimal set of attributes whose values uniquely identify an entity in the set.
Candidate key: there may be more than one attribute able to identify an entity.
Primary Key: we designate one of them as the primary key.

Relationship: an association among two or more entities.
Descriptive attributes: a relationship can have descriptive attributes.