CGS2545 Chapter 3

Terms for Chapter 3

Business rule: A statement that defines or constrains some aspect of the business. It is intended to assert business structure or to control or influence the behavior of the business.

Term: A word or phase that has a specific meaning for the business.

Fact: An association between two or more terms.

Entity-relationship model (E-R model): A logical representation of the data for an organization or for a business area.

Entity-relationship diagram (E-R diagram): A graphical representation of an entity-relationship model.

Entity: A person, place, object, event or concept in the user environment about which the organization wishes to maintain data.

Entity type: A collection of entities that share common properties or characteristics.

Entity instance: A single occurrence of an entity type.

Strong entity type: An entity that exists independently of other entity types.

Weak entity type: An entity whose existence depends on some other entity type.

Identifying owner: The entity type on which the weak entity type depends.

Identifying relationship: The relationship between a weak entity type and its owner.

Attribute: A property or characteristic of an entity type that is of interest to the organization.

Composite attribute: An attribute that can be broken down into component parts.

Simple attribute: An attribute that cannot be broken down into smaller components.

Multivalued Attribute: An attribute that may take on more than one value for a given entity instance.

Derived attribute: An attribute whose values can be calculated from related attribute values.

Identifiers: An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type.

Composite Identifiers: An identifier that consists of a composite attribute .

Relationship types : A meaningful association between (or among) entity types.

Relationship instance : An association between (or among) entity instances where each relationship instance includes exactly one entity from each participation entity type.

Associative entity: An entity type that associates the instance of one or more entity types and contains attributes that are particular to the relationship between those entity instances.

Degree: The number of entity types that participate in a relationship.

Degree -> Unary relationship : A relationship between the instance of a single entity type.

Degree -> Binary relationship : A relationship between the instances of two entity types.

Degree -> Ternary relationship : A relationship between the instances of three entity types.

Cardinality constraint : Specifies the number of instances of one entity that can (or must) be associated with each instance of another entity.

Minimum Cardinality: The minimum number of instances of one entity that may be associated with each instance of another entity.

Maximum Cardinality: The maximum number of instances of one entity that may be associated with each instance of another entity.

Time stamp : A time value that is associated with a data value.

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Questions from Companion Website

Multiple Choice

1. The E-R model is most often used as a tool during the ___ phase of database development.
Correct answer: analysis
The E-R model is used to construct a conceptual data model.

2. Good business rules are
Correct answer: expressible, atomic, and business-oriented.
These are just a few of the characteristics of a good business rule.

3. A good data name should be
Correct answer: readable, unique, and repeatable.

4. Which is NOT included in the definition of an entity?
Correct answer: Action
An entity is a person, place, object, event, or concept.

5. Which is NOT an example of a strong entity type?
Correct answer: STUDENT_ID
STUDENT_ID depends upon the existence of the STUDENT entity, so it is not a strong entity type.

6. A property or characteristic of an entity type is a(n)
Correct answer: attribute.
A property or characteristic of an entity is an attribute.

7. If EMPLOYEE is the entity type, then SMITH, JOHN is the entity ____.
Correct answer: instance
SMITH, JOHN is a single occurrence of an entity type.

8. An attribute whose value can be calculated from related attribute values is a(n) ___ attribute.
Correct answer: derived
For example, a customerís total bill can be calculated by finding the sum of all line items.

9. An attribute that can be broken down into component parts is a ____ attribute.
Correct answer: composite
An example is Address, which can be broken down into street, city, state, etc.

10. A meaningful association between entity types is a(n)
Correct answer: relationship type.
In an E-R model this is usually denoted by a diamond symbol.

11. The most common type of relationship encountered in data modeling is the ____ relationship.
Correct answer: binary
A binary relationship is a relationship between the instances of two entity types.

12. The _______ specifies the number of instances of one entity that can (or must) be associated with each instance of another entity.
Correct answer: cardinality constraint
A minimum or maximum cardinality may be defined for relationships.

13. An example of a good relationship name is
Correct answer: Teaches.
This is a verb phrase and is descriptive.

14. Which is NOT a basic construct of an E-R model?
Correct answer: Identifiers
The basic constructs are relationships, entity types, and attributes.

15. An example of a multivalued attribute might be
Correct answer: College_Degree
College_Degree might have multiple values for an individual.

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True/False Questions

1. The E-R model was introduced in an article by Chen in the 1980s.
Correct answer: FALSE
The E-R model was introduced in the 1960s.

2. Business rules should be determined primarily by interviewing executives in the business.
Correct answer: FALSE
Business rules should be determined from many sources, including policies, manuals, contracts, and brochures.

3. Data names should relate to business but not be overly technical.
Correct answer: TRUE
Customer would be a good data name; Field4DB2 would not be a good data name.

4. Diagrams are not usually used for data definitions.
Correct answer: FALSE
E-R diagrams will often accompany data definitions.

5. An E-R diagram consists of entities and resources.
Correct answer: FALSE
An E-R diagram contains entities and relationships.

6. There is no standard notation for an E-R diagram.
Correct answer: TRUE
Some notations are better than others, but there is no standard notation.

7. A strong entity type does not need an identifier.
Correct answer: FALSE
A strong entity type always has a unique characteristic or an identifier.

8. STUDENT_REGISTRATION_FOR_CLASS is a good entity type name.
Correct answer: FALSE
Entity type names should be more concise.

9. A cardinality constraint specifies the maximum number of attributes an entity may have.
Correct answer: FALSE
A cardinality constraint specifies the maximum number of instances one entity may be associated with each instance of another entity.

10. An entity can have only one defined attribute.
Correct answer: FALSE
Entities can have many attributes.

11. A multivalued attribute is an attribute that is common to many entities.
Correct answer: FALSE
A multivalued attribute is an attribute that may take on more than one value for a given entity instance.

12. A relationship is an association between attributes.
Correct answer: FALSE
A relationship is an association between entity types.

13. The degree of a relationship is the number of entity types that participate in a relationship.
Correct answer: TRUE
The three most common relationship degrees are unary, binary, and ternary.

14. A ternary relationship is a simultaneous relationship between more than three entity types.
Correct answer: FALSE
A ternary relationship is a simultaneous relationship among only three entity types.

15. A time stamp is a time that is saved whenever the database is opened.
Correct answer: FALSE
A time stamp is a date and time that is associated with a data value.

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Essay Questions

1. Why is the E-R model so popular?
The E-R modelís popularity is based on its relative ease of use, widespread CASE tool support, and the belief that entities and relationships are natural modeling concepts in the real world.

2. Why is data modeling often considered the most important part of the systems development process? The characteristics of the data, and the facts and rules are essential for ensuring data integrity. Data, not processes, are the most complex aspects of many information systems and require a central role in structuring system requirements. Data tend to be more stable than the business processes, so an information system based on data will be useful for a longer period of time.

3. What are some guidelines for naming data objects?
Data names should be related to the business, not technical characteristics.
They should be meaningful or self-documenting.
Data names should be unique, readable, and composed of words taken from an approved list.
Data names should also be repeatable, meaning that different people will develop almost the same name for an object.

4. What is the difference between an entity type and an entity instance?
An entity type is a collection of entities that share common properties or characteristics, for example: EMPLOYEE.
An entity instance is a single occurrence of an entity type, for example: Sally Jones.

5. Describe several types of attributes.
A simple attribute has no component parts.
A composite attribute is one that can be broken down into component parts.
A multivalued attribute can have multiple values for a single instance of an entity.
A derived entity is one whose values can be calculated from other attribute values.


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