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Questions and Answers
What is a disjointness constraint in database design?
A constraint that addresses whether an instance of a supertype may simultaneously be a member of two (or more) subtypes.
What does the disjoint rule specify?
Overlap rule allows an entity instance to be a member of two (or more) subtypes at the same time.
True
Subtype discriminators are attributes of a supertype whose values determine the target __________.
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What is a supertype/subtype hierarchy?
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What is the term used to identify the model that has resulted from extending the original E-R model with new modeling constructs?
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What is the main modeling construct incorporated in the EER model to enable modeling a general entity type and subdivide it into specialized entity types?
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Generalization is the process of defining a more specialized entity type from a set of more general entity types.
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Specialization is the process of defining one or more __________ of the supertype.
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Match the following attributes with their respective employee type:
- Hourly employees
- Salaried employees
- Contract consultants
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Study Notes
Introduction to Enhanced E-R Model
- The Enhanced Entity Relationship (EER) model is an extension of the original E-R model that includes new modeling constructs to better represent complex data.
- The EER model allows for the modeling of supertype/subtype relationships, which enables us to model a general entity type (supertype) and subdivide it into several specialized entity types (subtypes).
Supertypes and Subtypes
- A subtype is a subgrouping of entities in an entity type that is meaningful to the organization and shares common attributes or relationships distinct from other subgroupings.
- A supertype is a generic entity type that has a relationship with one or more subtypes.
- A supertype inherits attributes from its subtypes, and a subtype may have special attributes and be involved in relationships of its own.
Notation for Supertype/Subtype Relationships
- The notation used for supertype/subtype relationships involves a circle connected to the supertype with a line, and each subtype connected to the circle with a line.
- A U-shaped symbol on each line connecting a subtype to the circle indicates that the subtype is a subset of the supertype.
Attributes and Relationships
- Attributes that are shared by all entities are associated with the supertype.
- Attributes that are unique to a particular subtype are associated with that subtype.
- 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.
Attribute Inheritance
- Attribute inheritance is a property where subtype entities inherit values of all attributes and instances of all relationships of their supertype.
- This property makes it unnecessary to include supertype attributes or relationships redundantly with the subtypes.
Supertype/Subtype Relationships in a Hospital
- A hospital entity type PATIENT has two subtypes: OUTPATIENT and RESIDENT PATIENT.
- Each subtype has attributes that are unique to that subtype.
Representing Specialization and Generalization
- Generalization is the process of defining a more general entity type from a set of more specialized entity types.
- Specialization is the process of defining one or more subtypes of the supertype and forming supertype/subtype relationships.
Completeness Constraints
- A completeness constraint addresses the question of whether an instance of a supertype must also be a member of at least one subtype.
- There are two possible rules: total specialization and partial specialization.
Total Specialization Rule
- A rule that specifies that each entity instance of a supertype must be a member of some subtype in the relationship.
- It is indicated by a double line.
Partial Specialization Rule
- A rule that specifies that an entity instance of a supertype is allowed not to belong to any subtype.
- It is specified by a single line.
Disjointness Constraints
- A disjointness constraint addresses whether an instance of a supertype may simultaneously be a member of two or more subtypes.
- There are two possible rules: the disjoint rule and the overlap rule.
Disjoint Rule
- A rule that specifies that if an entity instance (of the supertype) is a member of one subtype, it cannot simultaneously be a member of any other subtype.
- It is specified by the letter "d" in the circle joining the supertype and its subtypes.
Overlap Rule
- A rule that specifies that an entity instance can simultaneously be a member of two or more subtypes.
- It is specified by placing the letter "o" in the circle.
Subtype Discriminators
- A subtype discriminator is an attribute of a supertype whose values determine the target subtype or subtypes.
Subtype Hierarchy
- A hierarchical arrangement of supertypes and subtypes in which each subtype has only one supertype.
Example of Supertype/Subtype Hierarchy
- A person might belong to more than one subtype (e.g., ALUMNUS and EMPLOYEE), so the overlap rule is used.
- Note that overlap allows for any overlap.
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Description
This quiz covers the enhancements made to the E-R model to better represent complex business relationships and data. Learn about the changes and advancements made to the model.
