[Notes] CSCI 585 DB Advanced Data Modeling

28 Jan 2020 | CSCI585, English/英文, Note

Credit to: Prof. Saty Raghavachary, CSCI 585, Spring 2020

outline

• Extended Entity Relationship (EER) model

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Entity Supertypes and Subtypes

• Entity supertype: Generic entity type related to one or more entity subtypes
  • Contains common characteristics
• Entity subtype: Contains unique characteristics of each entity subtype
• Criteria to determine the usage
  • There must be different, identifiable kinds of the entity in the user’s environment
  • The different kinds of instances should each have one or more attributes that are unique to that kind of instance

Specialization Hierarchy

• Depicts arrangement of higher-level entity supertypes and lower-level entity subtypes
• Relationships are described in terms of “is-a” relationships
• Subtype exists within the context of a supertype
• Every subtype has one supertype to which it is directly related
• Supertype can have many subtypes
• Provides the means to:
  • Support attribute inheritance
  • Define a special supertype attribute known as the subtype discriminator
  • Define disjoint/overlapping constraints and complete/partial constraints

Inheritance

• Enables an entity subtype to inherit attributes and relationships of the supertype
• All entity subtypes inherit their primary key attribute from their supertype
• At the implementation level, supertype and its subtype(s) maintain a 1:1 relationship
• Entity subtypes inherit all relationships in which supertype entity participates
• Lower-level subtypes inherit all attributes and relationships from its upper-level supertypes

Subtype Discriminator

• Attribute in the supertype entity that determines to which entity subtype the supertype occurrence is related
• Default comparison condition is the equality comparison

Disjoint and Overlapping Constraints

• Disjoint subtypes: Contain a unique subset of the supertype entity set
  • Known as nonoverlapping subtypes
  • Implementation is based on the value of the subtype discriminator attribute in the supertype
• Overlapping subtypes: Contain nonunique subsets of the supertype entity set
  • Implementation requires the use of one discriminator attribute for each subtype

Completeness Constraint

• Specifies whether each supertype occurrence must also be a member of at least one subtype
• Types
  • Partial completeness: Not every supertype occurrence is a member of a subtype
  • Total completeness: Every supertype occurrence must be a member of any