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4 months ago
What are supertype and subtype entities? Include an example.
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Database Processing: Fundamentals, Design, and Implementation
Edition: 15th
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When instances of an entity can be categorized into different types with each type sharing some common characteristics while having certain characteristics that are unique to each type, the common attributes can be separated from the unique ones. This is represented in E-R diagrams through the use of supertype and subtype entities. The supertype entity class contains the attributes that are common to all of the subtypes. The subtype entity classes represent the different types, or categories, of the supertype. Only the attributes that are unique to a given subtype tend to be listed as the attributes of that subtype. Each subtype has a relationship with the supertype. All subtypes should have the same identifier as the supertype since they represent different perspectives of the same thing. As an example, consider VEHICLE (VIN, year, make) as a supertype, with subtypes CAR (VIN, model, seatingCapacity) and MOTORCYCLE (VIN, horsepower). All vehicles have VINs, a year, and a make. CARs additionally have a model and a seating capacity and MOTORCYCLES additionally have a horsepower rating.
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4 months ago
Smart ... Thanks!
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