Entities, attributes, and keys

The ER diagram represents entities, attributes, and relationships. An entity is a representation of a real-world object such as a car or a user. An attribute is a property of an object and describes it. A relationship represents an association between two or more entities.

The attributes might be composite or simple (atomic). Composite attributes can be divided into smaller subparts. A subpart of a composite attribute provides incomplete information that is semantically not useful by itself. For example, the address is composed of a street name, building number, and postal code. Any one of them isn't useful alone, without its counterparts.

Attributes could also be single-valued or multi-valued. The color of ...

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