CGS2545 Chapter 4

Terms for Chapter 4

Enhanced entity-relationship (EER) model: The model that has resulted from extending the original E-R model with new modeling constructs.

Subtype: A subgrouping of the entities in an entity type that is meaningful to the organization and that shares common attributes or relationships distinct from other subgroupings.

Supertype: A generic entity type that has a relationship with one or more subtypes.

Attribute inheritance: A property by which subtype entities inherit values of all attributes of the supertype.

Generalization: The process of defining a more general entity type from a set of specialized entity types.

Specialization: The process of defining one or more subtypes of the supertype and forming supertype/subtype relationships.

Completeness constraint: A type of constraint that addresses the question whether an instance of a supertype must also be a member of at least one subtype.

Total specialization rule: Specifies that each entity instance of the supertype must be a member of some subtype in the relationship.

Partial specialization rule: Specifies that an entity instance of the supertype is allowed not to belong to any subtype.

Disjointness constraint: A constraint that addresses the question whether an instance of a supertype may simultaneously be a member of two (or more) subtypes.

Disjoint rule: Specifies that an entity instance (of the supertype) is a member of one subtype, it cannot simultaneously be a member of any other subtype.

Overlap rule: Specifies that an entity instance can simultaneously be a member of two (or more) subtypes.

Subtype discriminator: An attribute of the supertype whose values determine the target subtype or subtypes.

Supertype/ subtype hierarchy: A hierarchical arrangement of supertypes and subtypes, where each subtype has only one supertype.

Entity cluster: A set of one or more entity types and associated relationships grouped into a single abstract entity type.

Derivation: A statement derived from other knowledge in the business.

Structural assertion: A statement that expresses some aspect of the static structure of the organization.

Action assertion: A statement of a constraint or control on the actions of the organization.

Derived fact: A fact that is derived from business rules using an algorithm or inference.

Anchor object: A business rule (a fact) on which actions are limited.

Action: An operation, such as create, delete, update, or read, which may be performed on data objects.

Corresponding object : A business rule (a fact) that influences the ability to perform an action on another business rule.

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Questions from Companion Website

Multiple Choice

1. Which might be a subtype under the supertype entity STUDENT?
Correct answer: IN_STATE
STUDENT IN STATE STUDENT would be a subtype.

2. In a data model, a supertype is connected to its subtypes by
Correct answer: two lines and a circle.
A supertype is connected to its subtypes by two lines with a circle between them.

3. ____ is the property by which subtype entities inherit values of all attributes of the subtype.
Correct answer: Attribute inheritance
Attribute inheritance reduces redundancy when describing subtypes.

4. The process of defining subtypes and forming supertype/subtype relationships is called
Correct answer: specialization.
Each subtype is formed based on some distinguishing characteristic.

5. The process of defining a more basic entity type from a set of specialized entity types is called
Correct answer: generalization.
Generalization is a bottom-up process.

6. In an EER data model, a double line between a supertype and its subtype indicates
Correct answer: total specialization.
Each supertype entity instance must be a member of some subtype.

7. The ____ specifies that an entity instance can simultaneously be a member of two or more subtypes.
Correct answer: overlap rule
For example, a PART (supertype) can be manufactured (subtype) or purchased (subtype) or both.

8. The entity type at the top of a hierarchy is sometimes called the____.
Correct answer: root
In a hierarchical arrangement, a subtype has only one supertype.

9. ___ solves the problem of presenting huge information system data models to users and developers.
Correct answer: Entity clustering
Entity clustering groups a set of entity types and relationships into a single abstract entity type, condensing unwieldy data details.

10. Entity clustering usually results in
Correct answer: a simplified view of the E-R model.
Entity clustering condenses entity types and relationships into a single abstract entity type, simplifying the E-R model.

11. A business rule on which actions are limited is a(n)
Correct answer: anchor object.
An anchor object limits actions. For example, a course (action object) must have a course number.

12. A(n) ____ states something that must always be true.
Correct answer: integrity constraint
An example would be “A student’s enrollment data must be earlier than their graduation date.”

13. A(n) ____ causes the execution of one or more actions.
Correct answer: executive
An example would be “When a student has a GPA below 2.0, the student goes on academic probation.”

14. A(n) ________ is a story or script that describes how a business reacts to a given situation.
Correct answer: scenario
Scenarios are used as an approach to identifying and testing business rules.

15. A subtype _____ is an attribute whose value determines to which subtype a supertype instance belongs.
Correct answer: discriminator
A subtype discriminator for Employee might be a code for Hourly, Salaried, or Consultant.

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True/False Questions

1. An important construct in the original E-R data model is supertype/subtype relationships.
Correct answer: FALSE
Supertype/subtype relations were incorporated into the EER data model.

2. In an EER diagram, a subtype is connected with a line to a circle, which in turn is connected by a line to each subtype.
Correct answer: FALSE
A SUPERTYPE is connected with a line to a circle, which in turn is connected by a line to each subtype.

3. A member of a subtype must be a member of the supertype.
Correct answer: TRUE
Is the reverse also true?

4. A member of a supertype must also be a member of one or more of the subtypes.
Correct answer: FALSE
A member of a supertype may or may not be a member of the subtypes, it depends on the business situation.

5. Generalization is a bottom-up process.
Correct answer: TRUE
Generalization defines a more general entity type from a set of more specialized entity types.

6. The disjoint rule specifies that an entity instance can simultaneously be a member of two or more subtypes.
Correct answer: FALSE
The overlap rule specifies that an entity instance can simultaneously be a member of two or more subtypes.

7. The disjointness constraint is an attribute of the supertype whose values determine the target subtype or subtypes.
Your answer: FALSE
The subtype discriminator is an attribute of the supertype whose values determine the target subtype or subtypes.

8. A supertype/subtype hierarchy has a hierarchical arrangement where each subtype has only one supertype.
Correct answer: TRUE
In a supertype/subtype hierarchy, subtypes that are lower in the hierarchy inherit attributes from all supertypes higher in the hierarchy.

9. In a supertype/subtype hierarchy, attributes are assigned at the lowest logical level possible.
Correct answer: FALSE
Attributes are assigned at the highest logical level possible.

10. An entity cluster should focus on an area of interest to a group of users.
Correct answer: TRUE
Which entity types and relationships are grouped together depends on your purpose.

11. A structural assertion is a statement derived from other knowledge in the business.
Correct answer: FALSE
A derivation is a statement derived from other knowledge in the business.

12. A structural assertion says that something of importance to the organization either exists or exists in relationship with other things of interest.
Correct answer: TRUE
A structural assertion can be as simple as the definition of a term or relationship.

13. A corresponding object is a business rule (fact) on which actions are limited.
Correct answer: FALSE
An anchor object is a business rule (fact) on which actions are limited.

14. A timer action assertion, if true, leads to the existence of the corresponding object.
Correct answer: FALSE
An enabler may lead to the existence of a corresponding object.

15. The total specialization rules states that each entity instance of a supertype is allowed not to belong to any subtype.
Correct answer: FALSE
Total specialization states that each entity instance of a supertype must belong to a subtype.

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Essay Questions

1. When should you consider using supertype/subtype relationships?
You should consider using supertype/subtype relationships when there are attributes that apply to some, but not all, of the instances of an entity type and/or when the instances of a subtype participate in a relationship unique to that subtype.
In a hospital example, OUTPATIENT and RESIDENT_PATIENT fit these criteria. Both patient types have common attributes, yet have attributes unique to their subtype.

2. How are attributes assigned and inherited in supertype/subtype hierarchies?
Attributes should be assigned at the highest logical level possible in the hierarchy.
Subtypes that are lower in the hierarchy inherit attributes not only from their immediate supertype, but from all supertypes higher in the hierarchy.

3. Why would you want to use entity clustering?
Entity clustering groups entities and associated relationships into a single abstract type. This allows huge systems to be modeled without unwieldy data and can show developers and users different views with varying amounts of detail.

4. What is the difference between an E-R data model and an EER data model?
An EER model is an extended E-R data model. An EER model more accurately represents the complex data encountered in the business environment. Most importantly, EER models support supertype/subtype relationships.

5. What categories can business rules be divided into?
Business rules can be divided into derivations, structural assertions, and actions assertions. A derivation is a statement derived from other knowledge in the business. A structural assertion is a statement that expresses some aspect of the static structure of the organization. An action assertion is a constraint or control on the actions of the organization.


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