Generalization

Posted By on October 3, 2014


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Reducing E-R Diagrams to Tables
Aggregation

Generalization

Consider extending the entity set account by classifying accounts as being either savings-account or chequing-account.

Each of these is described by the attributes of account plus additional attributes. (savings has interest-rate and chequing has overdraft-amount.)

We can express the similarities between the entity sets by generalization. This is the process of forming containment relationships between a higher-level entity set and one or more lower-level entity sets.

In E-R diagrams, generalization is shown by a triangle, as shown in Figure

  
Figure : Generalization

  • Generalization hides differences and emphasizes similarities.
  • Distinction made through attribute inheritance.
  • Attributes of higher-level entity are inherited by lower-level entities.
  • Two methods for conversion to a table form:
    • Create a table for the high-level entity, plus tables for the lower-level entities containing also their specific attributes.
    • Create only tables for the lower-level entities.
Reducing E-R Diagrams to Tables
Aggregation

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Posted by Akash Kurup

Founder and C.E.O, World4Engineers Educationist and Entrepreneur by passion. Orator and blogger by hobby

Website: http://world4engineers.com