IE 224 Chapter 5 Questions
Entities of a given type are grouped into an ________. A) entity class B) entity relationship C) entity instance D) entity attribute E) None of the above.
A
Common variants of the E-R model include: A) IE Crow's Foot B) IDEF1X C) UML D) A and B E) A, B, and C
E
An entity instance is the occurrence of a particular entity.
T
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.
T
An entity is something that users want to track.
T
An entity that represents something that can exist on its own is called a strong entity.
T
An identifier of an entity instance is one or more attributes that name or identify entity instances.
T
E-R modeling recognizes both relationship classes and relationship instances.
T
E-R models use an identifying relationship to connect entities that are ID-dependent.
T
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.
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) A and B E) A and C
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 E) None of the above.
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 E) create a new weak, but not ID-dependent entity with a 1:1 relationship
A
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 E) create a new weak, but not ID-dependent entity with a 1:1 relationship
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.
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) A and C E) B and C
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) A and C E) B and C
B
Supertype/subtype entities are said to have a(n) ________ relationship. A) HAS-A B) IS-A C) recursive D) redundant E) multivalue
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) 0:1 B) 1:1 C) 1:N D) N:1 E) N:M
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" E) None of the above.
C
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 E) is an identifier
C
An attribute that determines which subtype should be used is a ________. A) supertype B) subtype C) discriminator D) A and C E) B and C
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 E) None of the above.
C
The occurrence of a particular entity is called an ________. A) entity class B) entity relationship C) entity instance D) entity attribute E) None of the above.
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) A or C E) B or C
C
Which of the following is not a key element of an E-R model? A) Identifiers B) Entities C) Objects D) Attributes E) Relationships
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" E) None of the above.
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" E) None of the above.
D
An identifier may be ________. A) composite B) a single attribute C) a relationship D) A and B E) A, B, and C
D
Attributes may be ________. A) composite B) element C) multivalued D) A and C E) B and C
D
Discriminators can be ________. A) exclusive only B) inclusive only C) decisive only D) A or B E) B or C
D
In a minimum cardinality, minimums are generally stated as ________. A) 0 B) 1 C) N D) A or B E) A, B, or C
D
The characteristics of a thing are described by its ________. A) identifiers B) entities C) objects D) attributes E) relationships
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 1:N relationship to one of two parent entities E) create a new weak, but not ID-dependent entity with a 1:N relationship to one of two parent entities
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) 0:1 B) 1:1 C) 1:N D) N:1 E) N:M
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" E) None of the above.
D
An entity whose identifier includes the identifier of another entity is a(n) ________. A) strong entity B) weak entity C) ID-dependent entity D) A and C E) B and C
E
Recursive relationships can have which of the following maximum cardinalities? A) 1:1 B) 1:N C) N:M D) A or B E) A, B, or C
E
Which of the following is considered to be a weak entity? A) ID-dependent entity B) subtype entity C) association entity D) B and C E) A, B, and C
E
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) Subtypes have attributes that are required by the supertype. E) Subtypes can produce a closer-fitting data model.
E
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) 0:1 B) 1:1 C) 1:N D) N:1 E) N:M
E
A binary relationship is a relationship based on numerical entity instance identifiers.
F
A binary relationship is a relationship between two or more entities.
F
A composite identifier is defined as a composite attribute that is an identifier.
F
A compound identifier is an identifier consisting of two or more attributes.
F
A relationship's maximum cardinality indicates the maximum number of entities that can participate in the relationship.
F
A single relationship class involves only one entity class.
F
An exclusive subtype pattern has one supertype entity that relates to one or more subtype entities.
F
An identifier of an entity instance must consist of one and only one attribute.
F
An identifier serves the same role for a table that a key does for an entity.
F
Data modelers agree that weak, non-ID-dependent entities exist and are important.
F
Entities containing optional sets of attributes are often represented using subtypes.
F
ID-dependent entities are associated by a nonidentifying relationship.
F
In E-R modeling, entities within an entity class may have different attributes.
F
In a 1:N relationship, the term parent refers to the N side of the relationship.
F
In an E-R model, the three types of minimum cardinality are mandatory, optional and indeterminate.
F
In today's E-R models, attributes of relationships are still used.
F
One example of a database design using a strong relationship is the multivalued attribute pattern.
F
Recursive relationships only exist for one-to-one relationships.
F
Relationships among instances of a single entity class are called redundant relationships.
F
The degree of a relationship is expressed as the relationship's maximum cardinality.
F
The supremetype entity contains the attributes that are common to all subtypes.
F
When designing a database, first identify the entities, then determine the attributes, and finally create the relationships.
F
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).
A data model is a plan for a database design.
T
A relationship's minimum cardinality indicates the number of entity instances that must participate in the relationship.
T
A relationship's minimum cardinality indicates whether or not an entity must participate in the relationship.
T
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.
T
All weak entities must have a minimum cardinality of 1 on the entity on which it depends.
T
An ID-dependent entity is an entity whose identifier includes the identifier of another entity.
T
An attribute that determines which subtype is appropriate is called a discriminator.
T
An entity class is described by the structure of the entities in that class.
T
Entities can be associated with one another in relationships.
T
Entities of a given type are grouped into entity classes.
T
Entities with an IS-A relationship should have the same identifier.
T
ID-dependent entities are a common type of weak entity.
T
In E-R modeling, an attribute describes the characteristics of an entity.
T
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.
T
In an E-R model, the three types of maximum cardinality are 1:1, 1:N and N:M.
T
One example of a database design using an ID-dependent relationship is the archetype/instance pattern.
T
One example of a database design using an ID-dependent relationship is the association pattern.
T
Relationships are classified by their cardinality.
T
Relationships between supertypes and subtypes are called IS-A relationships.
T
Relationships of degree two are referred to as binary relationships.
T
Subtype entities contain only attributes unique to the subtypes.
T
Subtypes can be exclusive or inclusive.
T
The degree of a relationship is the number of entity classes in the relationship.
T
The method of constructing data models used in the text is the extended entity-relationship (E-R) model.
T
The notation 1:N shows the relationship's maximum cardinality.
T
The principal difference between an entity and a table is that you can express a relationship between entities without using foreign keys.
T
There are three types of recursive relationships: 1:1, 1:N and N:M.
T
When transforming a data model into a relational design, relationships of all degrees are treated as combinations of binary relationships.
T
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.
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.
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.
What is an ID-Dependent entity?
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.
What are supertype and subtype entities?
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.
