Entity Specialization in Databases

Specialization and Generalization

• Specialization involves breaking down a higher-level entity into lower-level entities based on distinguishing characteristics.
• Generalization is the opposite process, where similar lower-level entities are combined into a higher-level entity set.
• Both concepts can be represented through E-R diagrams and are interchangeable terminologies.

Representing Specialization via Schemas

• Method 1:

  • Create a schema for the higher-level entity and individual schemas for lower-level entities, linking them via primary keys.
  • Example schemas:
    • Person: ID, name, street, city
    • Student: ID, tot_cred
    • Employee: ID, salary
  • Downside: Requires accessing multiple relations to gather information about an employee.

• Method 2:

  • Schema for each entity includes all local and inherited attributes.
  • Example schemas:
    • Person: ID, name, street, city
    • Student: ID, name, street, city, tot_cred
    • Employee: ID, name, street, city, salary
  • Downside: Redundant storage of common attributes like name, street, and city for overlapping entities.

Design Constraints

• Completeness constraints define if an entity must belong to at least one lower-level entity:
  • Total: Entity must belong to a lower-level entity.
  • Partial: Entity may not belong to any lower level.
• Default is partial generalization, with total explicitly indicated in E-R diagrams.

Non-binary Relationships

• Most relationships are binary, but non-binary relationships may be more efficient for certain data structures.
• Example: Ternary relationships can visually complicate E-R diagrams.

Cardinality Constraints

• One arrow can represent cardinality constraints in ternary (or higher) relationships.
• Ambiguity arises with multiple arrows:
  • Either each A entity uniquely connects to one from B and C, or pairs from A, B, and C are uniquely associated.
• To maintain clarity, only one arrow is permitted per ternary relationship.

E-R Notation Symbols

• Specialization is depicted by a triangle labeled "ISA," indicating lower-level sets inherit attributes.
• Attribute inheritance allows lower-level entities to utilize attributes of their higher-level entities.

Alternative E-R Notation Forms

• Different notations like Chen and IDE1FX (Crow's feet) exist for drawing E-R diagrams, reflecting relationships differently.

UML Class Diagrams

• Unified Modeling Language (UML) Class Diagrams correspond to E-R diagrams but differ in cardinality representation.
• Binary relationships in UML are simply lines connecting entities with the relationship name alongside.
• Role names in relationships can also be labeled adjacent to corresponding entity sets, enhancing clarity in complex models.