Chapter 5 Questions

Supertype/subtype entities are said to have a(n) ________ relationship.

IS-A

What is meant by the degree of a relationship?

In the E-R model, the degree of a relationship indicates how many entity classes are participating in the relationship. If two entity classes are in the relationship, then the relationship is said to be of degree two and is called a binary relationship. If three entity classes are in the relationship, then the relationship is said to be of degree three and is called a ternary relationship. So, for example, if we have the entity classes CUSTOMER and PRODUCT in a relationship, then they will have a relationship of degree two—a binary relationship.

An entity instance of an entity class is the representation of a particular entity and is described by the values of the attributes of the entity.

True

An entity is something that users want to track

True

An entity that represents something that can exist on its own is called a strong entity.

True

An entity's maximum cardinality in a relationship indicates the number of entity instances that must participate in the relationship.

True

An entity's minimum cardinality in a relationship indicates whether or not an entity must participate in the relationship

True

An identifier of an entity instance is one or more attributes that name or identify entity instances.

True

E-R modeling recognizes both relations classes and relationship instances.

True

E-R models use an identifying relationship to connect entities that are ID-dependent.

True

Entities can be associated with one another in a relationships.

True

Entities of a given type are grouped into entity classes.

True

Entities with an IS-A relationship should have the same identifier.

True

ID-dependent entities are a common type of weak entity.

True

In E-R modeling, an attribute describes the characteristics of an entity.

True

In a 1:N relationship, the parent entity is on the one side of the relationship, and the child entity is on the many side of the relationship.

True

In an E-R model, the three types of maximum cardinality are 1:1, 1:N, and N:M

True

One example of a database design using an ID-dependent relationship is the archetype/instance pattern.

True

One example of a database design using an ID-dependent relationship is the association pattern.

True

Relationships are classified by their cardinality.

True

Relationships between supertypes and subtypes are caleld IS-A relationships.

True

Relationships of degree two are referred to as binary relationships.

True

Subtype entities contain, apart from the identifier, only attributes that do not appear in the supertype.

True

Subtypes can be exclusive or inclusive.

True

The degree of a relationship is the number of entity classes in the relationship

True

The method of constructing data models in the text is the extended entity-relationship (E-R) model.

True

The notation 1:N shows a relationship's maximum cardinalities.

True

The principle difference between an entity and a table is that you can express a relationship between entities without using foreign keys.

True

There are three types of recursive relationships: 1:1, 1:N, and N:M

True

When transforming a data model into a relational design, relationships of all degrees are treated as combinations of binary relationships.

True

Discriminators can be ________.

attributes

The characteristics of a thing are described by its ________.

attributes

Attributes may be ________.

multivalued

When an entity has a relationship to itself, we have a(n) ________.

recursive relationship

An entity that holds specialized attributes that distinguish it from one or more other similar entities is a ________.

subtype

An entity whose existence depends on the presence of another entity but whose identifier does not include the identifier of the other entity is a(n) ________.

weak entity

An entity whose identifier includes the identifier of another entity is a(n) ________.

weak entity

Minimum cardinality refers to ________.

whether or not an instance of one entity class is required to be related to an instance of another entity class

Which of the following is not true about subtype entities?

All subtypes of a supertype must have the same attributes.

In today's E-R models, attributes of relationships are still used.

False

One example of a database design using a strong relationship is the multivalued attribute pattern.

False

Recursive relationships only exist for one-to-one relationship.

False

Relationships among instances of a single entity class are called redundant relationships

False

The degree of a relationship is expressed as relationship's maximum cardinality.

False

The supremetype entity contains the attributes that are common to all subtypes.

False

When designing a database, first identify the entities, then determine the attributes, and finally create the relationships.

False

All weak entities must have a minimum cardinality of 1 on the entity on which it depends.

True

An ID-dependent entity is an entity whose identifier includes the identifier of another entity.

True

An attribute that determines which subtype is appropriate is called a discriminator.

True

An entity class is described by the structure of the entities in that class.

True

An entity instance is an occurrence of a particular entity.

True

Which of the following is considered to be a weak entity?

The association entity in an association pattern

In a minimum cardinality, minimums are generally stated as ________.

0 or 1

You are given an E-R diagram with two entities, ORDER and CUSTOMER, as shown above, and are asked to draw the relationship between them. If a given customer can place only one order and a given order can be placed by at most one customer, which of the following should be indicated in the relationship symbol between the two entities?

1:1

You are given an E-R diagram with two entities, ORDER and CUSTOMER, as shown above. What does the symbol next to the ORDER entity indicate?

A minimum cardinality of "optional"

You are given an E-R diagram with two entities, ORDER and CUSTOMER, as shown above. What does the symbol next to the CUSTOMER entity indicate?

A minimum cardinality of "required"

Distinguish between an entity class and an entity instance.

An entity class is the collection of all entities of a given type. The entity class is described by the structure of the entities in that class. An entity instance is a representation of a particular entity within the entity class. The entity instance is described by the values of the attributes of the entity. An entity class, then, is a collection containing many entity instances.

In an E-R model, the three types of minimum carinality are mandatory, optional, and indeterminate.

False

A binary relationship is a relationship based on numerical entity instance identifiers.

False

A binary relationship is a relationship between two or more entities

False

A composite identifier is defined as a composite attribute that is an identifier.

False

A compound identifier is an identifier consisting of two or more attributes.

False

A relationship's maximum cardinality indicates the maximum number of entities that can participate in the relationship.

False

A single relationship class involves only one entity class.

False

An exclusive subtype pattern has one supertype entity that relates to one or more subtype entities.

False

An identifier of an entity instance must consist of one and only one attribute.

False

An identifier serves the same role for a table that a key does for an entity.

False

Data modelers agree that weak, non-ID-dependent entities exist and are important.

False

Entities containing multivalued attributes are often represented using subtypes.

False

ID-dependent entities are associated by a nonidentifying relationship.

False

In E-R modeling, entities within an entity class may have different attributes.

False

In a 1:N relationship, the term parent refers to the N side of the relationship.

False

What is meant by the cardinality of a relationship?

In the E-R model, there are two types of cardinality—the maximum cardinality and the minimum cardinality. The maximum cardinality is the maximum number of entity instances that can participate in a relationship instance. There are three types of maximum cardinality: one-to-one (1:1), one-to-many (1:N) and many-to-many (N:M). The minimum cardinality is the minimum number of entity instances that must participate in a relationship instance. This number is generally zero (0) or one (1). If the number is zero, then participation by that entity instance is optional (O)—it does not have to be in a relationship with an instance of the other entity. If the number is one, then participation by that entity instance is mandatory (M)—it must participate in a relationship with an instance of the other entity. In a binary relationship there are four (4) possible sets of minimum cardinalities—(O-O), (O-M), (M-O) and (M-M).

For a relationship to be considered a binary relationship it must satisfy which of the following conditions?

It must involve exactly two entity classes.

Recursive relationships can have all of the following maximum cardinalities except ________.

M:M

You are given an E-R diagram with two entities, ORDER and CUSTOMER, as shown above, and are asked to draw the relationship between them. If a given customer can place many orders and a given order can be placed by at most one customer, which of the following should be indicated in the relationship symbol between the two entities?

N:1

You are given an E-R diagram with two entities, ORDER and CUSTOMER, as shown above, and are asked to draw the relationship between them. If a given customer can place many orders and a given order can be placed by one or more customers, which of the following should be indicated in the relationship symbol between the two entities?

N:M

Which of the following is not a common variant of the E-R model?

Object-oriented

Which of the following is not a key element of an E-R model?

Objects

A for-use-by pattern must involve what kind of entity?

Supertype entity

How is minimum cardinality expressed in IE Crow's Foot E-R diagrams?

The Crow's Foot E-R diagram notation uses a circle to indicate a minimum cardinality of zero (or "optional") and a hash mark to indicate a minimum cardinality of one (or "mandatory"). Each symbol is placed on the relationship line just beyond the symbol indicating maximum cardinality.

How is maximum cardinality expressed in IE Crow's Foot E-R diagrams?

The Crow's Foot E-R diagram notation uses a hash mark to indicate a maximum cardinality of one and a crow's foot to indicate a maximum cardinality of many. Each symbol is placed immediately next to the rectangle symbolizing an entity. These symbols are combined in various forms to signify 1:1, 1:N and N:M relationships.

Describe and discuss the main elements of the Entity-Relationship (E-R) model. Include an example.

The E-R model is basically made up of entities, attributes and relationships. Entities represent the things people want to keep track of, such as customers and products. Each "thing" is presented by an entity or entity class—in this case CUSTOMER and PRODUCT. Individual members of each entity class are called entity instances—for example CUSTOMER John Smith and PRODUCT Canned Tomato Soup. Each entity is described by a set of attributes. For example, a CUSTOMER will have a CustomerNumber, a LastName and a FirstName, while a PRODUCT will have a ProductNumber and a Description. These, of course, have specific values for each entity instance. Finally, the entities are connected by relationships. There are relationship classes between entity classes and relationship instances between entity instances. For example, in general a CUSTOMER buys a PRODUCT, and a PRODUCT is sold to a CUSTOMER. Specifically, CUSTOMER John Smith buys a PRODUCT Canned Tomato Soup.

Which of the following is not a good reason for designing a data model?

The data model can be discarded after the database is operational.

Maximum cardinality refers to ________.

The most instances of one entity class that can be involved in a relationship instance with another entity class

A data model is a plan for a database design

True

A weak entity is an entity that cannot exist in the database without ( and is logically dependent upon) another type of entity also existing in the database.

True

A line-item pattern must involve what kind of entity?

Weak entity with one identifying relationship

A circle across the relationship line near an entity indicates ________.

a minimum cardinality of "optional"

A hash mark across the relationship line near an entity indicates ________.

a minimum cardinality of "required"

Suppose you have created a data model for an airline based on a variety of forms, reports, user interviews, data sources, etc. Your client comes forward with a new desired report that includes passenger weights and a relationship between passengers and employees, neither of which is in your E-R diagram. You should ________.

add the relationship and the weight attribute to the existing data model

An identifier may be ________.

composite

A composite attribute is an attribute that ________.

consists of a group of attributes

To represent a multivalued attribute in an E-R model, ________.

create a new ID-dependent entity with a 1:N relationship

To represent an archetype/instance pattern in an E-R model, ________.

create a new ID-dependent entity with a 1:N relationship

To represent an association pattern in an E-R model, ________.

create a new ID-dependent entity with a N:1 relationships to two parent entities

An attribute that determines which subtype should be used is a(n) ________.

discriminator

Entities of a given type are grouped into an ________.

entity class

An occurrence of a particular entity is called an ________.

entity instance