4.5 Supertype and subtype entities

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4 Terms

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subtype entity / supertype entity

  • A subtype entity is a subset of another entity type, called the supertype entity.

  • e.g. GasVehicle is a subset of Vehicle, so GasVehicle is a subtype entity of the Vehicle supertype entity.

  • On ER diagrams, subtype entities are drawn within the supertype.

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IsA relationship

A supertype entity identifies its subtype entities. The identifying relationship is called an IsA relationship.

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partition

A partition of a supertype entity is a group of mutually exclusive(belong to only one subtype entity) subtype entities.

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Database Design

  • are determined, the database designer looks for supertype and subtype entities.

  • once Analysis id done, logical design converts an entity-relationship model to tables, columns, and keys for a specific database system.

  • Creating supertype and subtype entities is the last of four analysis steps:

    1. Discover entities, relationships, and attributes

    2. Determine cardinality

    3. Distinguish strong and weak entities

    4. Create supertype and subtype entities