COP 4710 Lecture 5: Databse week 5
25 Sep 2018
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Document Summary
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. 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. 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. Constraint on which entities can be members of a given lower-level entity set. Example: all customers over 65 years are members of senior- Constraint on whether or not entities may belong to more than one lower-level entity set within a single generalization. An entity can belong to only one lower-level entity set.
