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The Enhanced E-R Model and Business Rules Specialization and Generalization E-ER Diagram With Aggregation E-ER Design Decisions Supertypes and Subtypes Constraints in Supertype Entity Clusters Action Assertion Classifications
Typology: Study notes
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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.
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.
Design Constraints on a Design Constraints on a Specialization/Generalization Specialization/Generalization (^) Constraint on which entities can be members of a given lower-level entity set.
Disjoint An entity can belong to only one lower- level entity set. Noted in E-R diagram by writing disjoint next to the ISA triangle Overlapping An entity can belong to more than one lower- level entity set Design Constraints on a Specialization/Generalization Design Constraints on a Specialization/Generalization
Aggregation Aggregation (^) Consider the ternary relationship workson , which we saw earlier (^) Suppose we want to record managers for tasks performed by an employee at a branch
Relationship sets works-on and manages represent overlapping information
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.
The use of an attribute or entity set to represent an object. Whether a real-world concept is best expressed by an entity set or a relationship set. The use of a ternary relationship versus a pair of binary relationships.
E-R Diagram for a Banking Enterprise E-R Diagram for a Banking Enterprise
Subtype:Subtype: A subgrouping of the entities in an entity type which has attributes that are distinct from those in other subgroupings Supertype:Supertype: An generic entity type that has a relationship with one or more subtypes Inheritance:Inheritance:
Employee supertype with three subtypes All employee subtypes will have emp nbr, name, address, and date-hired Each employee subtype will also have its own attributes
Relationships at the supertype level indicate that all subtypes will participate in the relationship The instances of a subtype may participate in a relationship unique to that subtype. In this situation, the relationship is shown at the subtype level