Chapter 5 Questions
To represent an archetype/instance pattern in an E-R model, ________. A) create a new ID-dependent entity with a 1:N relationship B) create a new weak, but not ID-dependent entity with a 1:N relationship C) create a new strong entity with a 1:1 relationship D) create a new ID-dependent entity with a 1:1 relationship
A
Explain the ambiguity in the broad definition of a weak entity.
The broad definition of a weak entity includes any entity that relies on another entity for its existence in the database. This is ambiguous since this definition can apply to any entity that is involved in a relationship with a minimum cardinality of 1. If the minimum cardinality is 1, then an entity instance in that entity class cannot exist in the database unless there is a related entity instance in the associated entity class. The more restrictive definition of a weak entity indicates that an entity is weak if it logically depends on the other entity for its existence.
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
A composite attribute is an attribute that ________. A) is multivalued B) describes a characteristic of the relationship C) consists of a group of attributes D) is calculated at run-time
C
A hash mark across the relationship line near an entity indicates ________. A) a maximum cardinality of "zero" B) a maximum cardinality of "one" C) a minimum cardinality of "optional" D) a minimum cardinality of "required"
D
Recursive relationships can have all of the following maximum cardinalities except ________. A) 1:1 B) 1:N C) N:M D) M:M
D
A binary relationship is a relationship based on numerical entity instance identifiers.
False
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 identifier may be ________. A) composite B) optional C) a relationship D) minimal
A
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.
Distinguish between entity identifiers and keys.
An entity identifier is an attribute or group of attributes that is used to name or identify instances of an entity class. Unlike a key in the relational model, an entity identifier is more of a logical concept. It is simply an attribute that the users think of as distinguishing between entity instances in their environment.
A for-use-by pattern must involve what kind of entity? A) Strong entity B) Supertype entity C) Weak entity with one identifying relationship D) Weak entity with two identifying relationships
B
A binary relationship is a relationship between two or more entities
False
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.
What is an ID-Dependent entity? Include an example.
There are some entities that cannot be completely identified without being associated with another entity that provides additional identifying information. A common example is an apartment, where just the apartment number by itself (for example, the number 12) does not tell you which apartment is being described—you have to know which building the apartment is in. Thus the entity APARTMENT needs to be associated with the entity BUILDING. In fact, the entity APARTMENT needs to include the identifier of BUILDING in the identifier of APARTMENT for the identifier of APARTMENT to be complete. An entity that requires the inclusion of another entity's identifier as part of its own composite identifier is called an ID-Dependent entity.
The principle difference between an entity and a table is that you can express a relationship between entities without using foreign keys.
True
When transforming a data model into a relational design, relationships of all degrees are treated as combinations of binary relationships.
True
What are supertype and subtype entities? Include an example.
When instances of an entity can be categorized into different types with each type sharing some common characteristics while having certain characteristics that are unique to each type, the common attributes can be separated from the unique ones. This is represented in E-R diagrams through the use of supertype and subtype entities. The supertype entity class contains the attributes that are common to all of the subtypes. The subtype entity classes represent the different types, or categories, of the supertype. Only the attributes that are unique to a given subtype tend to be listed as the attributes of that subtype. Each subtype has a relationship with the supertype. All subtypes should have the same identifier as the supertype since they represent different perspectives of the same thing. As an example, consider VEHICLE (VIN, year, make) as a supertype, with subtypes CAR (VIN, model, seatingCapacity) and MOTORCYCLE (VIN, horsepower). All vehicles have VINs, a year, and a make. CARs additionally have a model and a seating capacity and MOTORCYCLES additionally have a horsepower rating.
Discriminators can be ________. A) attributes B) entities C) relationships D) subtypes
A
Entities of a given type are grouped into an ________. A) entity class B) entity relationship C) entity instance D) entity attribute
A
For a relationship to be considered a binary relationship it must satisfy which of the following conditions? A) It must involve exactly two entity classes. B) It must have a maximum cardinality of 1:1. C) It must have a maximum cardinality of 1:N. D) Both A and B are correct
A
In a minimum cardinality, minimums are generally stated as ________. A) 0 or 1 B) 1 or N C) M or N D) one or many
A
Maximum cardinality refers to ________. A) the most instances of one entity class that can be involved in a relationship instance with another entity class B) the minimum number of entity classes involved in a relationship C) whether or not an instance of one entity class is required to be related to an instance of another entity class D) whether or not an entity is a weak entity
A
To represent a multivalued attribute in an E-R model, ________. A) create a new ID-dependent entity with a 1:N relationship B) create a new weak, but not ID-dependent entity with a 1:N relationship C) create a new strong entity with a 1:1 relationship D) create a new ID-dependent entity with a 1:1 relationship
A
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? A) N:M B) 1:1 C) 1:N D) N:1
A
An entity that holds specialized attributes that distinguish it from one or more other similar entities is a ________. A) supertype B) subtype C) discriminator D) parent
B
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) ________. A) strong entity B) weak entity C) ID-dependent entity D) subtype entity
B
An entity whose identifier includes the identifier of another entity is a(n) ________. A) strong entity B) weak entity C) optional entity D) required entity
B
Supertype/subtype entities are said to have a(n) ________ relationship. A) HAS-A B) IS-A C) recursive D) redundant
B
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 ________. A) create a new, separate E-R diagram with just the new information B) add the relationship and the weight attribute to the existing data model C) add the relationship but not the weight attribute D) add the weight attribute but not the relationship
B
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? A) N:M B) 1:1 C) 1:N D) N:1
B
A circle across the relationship line near an entity indicates ________. A) a maximum cardinality of "zero" B) a maximum cardinality of "one" C) a minimum cardinality of "optional" D) a minimum cardinality of "required"
C
A line-item pattern must involve what kind of entity? A) Strong entity B) Supertype entity C) Weak entity with one identifying relationship D) Weak entity with two identifying relationships
C
An attribute that determines which subtype should be used is a(n) ________. A) supertype B) subtype C) discriminator D) identifier
C
An occurrence of a particular entity is called an ________. A) entity class B) entity relationship C) entity instance D) entity attribute
C
Attributes may be ________. A) complete B) optional C) multivalued D) required
C
Minimum cardinality refers to ________. A) the most instances of one entity class that can be involved in a relationship with one instance of another entity class B) the minimum number of entity classes involved in a relationship C) whether or not an instance of one entity class is required to be related to an instance of another entity class D) whether or not an entity is a weak entity
C
When an entity has a relationship to itself, we have a(n) ________. A) supertype/subtype relationship B) archetype/instance relationship C) recursive relationship D) Both A and C are correct
C
Which of the following is considered to be a weak entity? A) A subtype entity B) The archetype in an archetype/instance pattern C) The association entity in an association pattern D) The parent entity in a parent-child relationship
C
Which of the following is not a key element of an E-R model? A) Relationships B) Entities C) Objects D) Attributes
C
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) A maximum cardinality of "zero" B) A maximum cardinality of "one" C) A minimum cardinality of "optional" D) A minimum cardinality of "required"
C
The characteristics of a thing are described by its ________. A) identifiers B) entities C) objects D) attributes
D
To represent an association pattern in an E-R model, ________. A) create a new ID-dependent entity with a 1:1 relationship to one other entity B) create a new weak, but not ID-dependent entity with a 1:1 relationship to one other entity C) create a new strong entity with a 1:1 relationship to one other entity D) create a new ID-dependent entity with a N:1 relationships to two parent entities
D
Which of the following is not a common variant of the E-R model? A) IE Crow's Foot B) IDEF1X C) UML D) Object-oriented
D
Which of the following is not a good reason for designing a data model? A) It's easier to change things in the data model than it is in the actual DBMS. B) The data model provides a simplified view of the database that can be shown to people who don't need to see all the details. C) Creating a data model will help you understand the application area better. D) The data model can be discarded after the database is operational.
D
Which of the following is not true about subtype entities? A) Subtypes may be exclusive. B) The supertype and subtypes will have the same identifier. C) Subtypes are used to avoid a situation in which some attributes are required to be null. D) All subtypes of a supertype must have the same attributes.
D
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? A) N:M B) 1:1 C) 1:N D) N:1
D
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) A maximum cardinality of "zero" B) A maximum cardinality of "one" C) A minimum cardinality of "optional" D) A minimum cardinality of "required"
D
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
In an E-R model, the three types of minimum carinality are mandatory, optional, and indeterminate.
False
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
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.
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).
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.
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
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
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
The notation 1:N shows a relationship's maximum cardinalities.
True
There are three types of recursive relationships: 1:1, 1:N, and N:M
True
