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Top-down design process; we designate subgroupings within an entity set that are distinctive from other entities in the set.

  • These subgroupings become lower-level entity sets that have attributes or participate in relationships that do not apply to the higher-level entity set.
  • Depicted by a triangle component labeled ISA (E.g. customer “is a” person).
  • Attribute inheritance – a lower-level entity set inherits all the attributes and relationship participation of the higher-level entity set to which it is linked.


  • A bottom-up design process – combine a number of entity sets that share the same features into a higher-level entity set.
  • Specialization and generalization are simple inversions of each other; they are represented in an E-R diagram in the same way.
  • The terms specialization and generalization are used interchangeably.

Specialization and Generalization

  • Can have multiple specializations of an entity set based on different features.

E.g. permanent-employee vs. temporary-employee, in addition to officer vs. secretary vs. teller

Each particular employee would be a member of one of permanent-employee or temporary-employee,and also a member of one of officer, secretary, or teller

The ISA relationship also referred to as superclass - subclass relationship.



Consider the ternary relationship works-on, which we saw earlier

Suppose we want to record managers for tasks performed by an employee at a branch.

E-R Diagram with Aggregation

  • Relationship sets works-on and manages represent overlapping information
  • Every manages relationship corresponds to a works-on relationship
  • However, some works-on relationships may not correspond to any manages relationships
  • So we can’t discard the works-on relationship
  • Eliminate this redundancy via aggregation
  • Treat relationship as an abstract entity
  • Allows relationships between relationships
  • Abstraction of relationship into new entity
  • Without introducing redundancy, the following diagram represents:
  • An employee works on a particular job at a particular branch
  • An employee, branch, job combination may have an associated manager

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