Entity-Relationship Model

An Entity-Relationship model has three main concepts:

• Entities: and their types / sets

  Entity

  Entities are specific objects or things in the mini-world that are represented in the database. They may be either physical or non-physical objects.

  For example:

  • an EMPLOYEE
  • the research DEPARTMENT
  • the product PROJECT

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• Attributes: simple, composite, single-valued, multi-valued

  Attribute

  Attributes are properties used to describe an entity, a specific entity will have a value for each of its attributes. Each attribute has a value set (data type) associated with it, e.g. integer, string, date, etc.

  Types of attributes:

  • Simple attribute

    Simple attribute: each entity has an atomic value for this attribute. Example: ID

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  • Composite attribute

    Composite attribute: the attribute may be composed of several components, it may form a hierarchy where some components are themselves composite. Example: Name(First, Middle, Last)

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  • Single-valued attribute

    Single-valued attribute: each entity can have a single value for this attribute. Example: Age attribute for each Person.

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  • Multi-valued attribute

    Multi-valued attribute: an entity may have multiple values for this attribute. Example: Current Courses of a STUDENT.

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    An attribute may be both composite and multi-valued.

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• Relationships: and their types / sets