CH 5- Data Modeling with the Entity Relationship Modell

Question 1

data model

Answer 1

• plan or blueprint for a database design.
• > is more generalized and abstract than a database design.

• It is easier to change a data model then it is to change
a database design, so it is the appropriate place to work through conceptual database problems

Data Model = Conceptual Design

Question 2

Design stages

Answer 2

1) Conceptual design (conceptual schema)
2) Logical design (logical schema)
3) Physical design (physical schema)

Question 3

Entity-Relationship (E-R) model

Answer 3

-set of concepts and graphical symbols that can be used to create conceptual schemas.

Question 4

Entities

Answer 4

-Something that can be readily identified and that users want to track

a) Entity class—a collection of entities of a given type
b) Entity instance—the occurrence of a particular entity

• There are usually many instances of an entity in an entity class.

Question 5

Attributes

Answer 5

• describe an entity’s characteristics
• All entity instances of a given entity class have the same attributes, but vary in the values of those attributes.
• They were originally shown in data models as ellipses.
• Data modeling products today commonly show attributes in rectangular form.

Question 6

Identifiers

Answer 6

• attributes that name, or identify entity instances.

* The identifier of an entity instance consists of one or more of the entity’s attributes.

Question 7

Composite identifiers

Answer 7

-identifiers that consist of two or more attributes.

• Identifiers in data models become keys in database designs.
– Entities have identifiers.
– Tables (or relations) have keys.

Question 8

relationships

Answer 8

-relationship class can involve two or more entity classes.

Question 9

degree of relationship

Answer 9

-Number of entity classes in the relationship

a) binary relationship= two entities (degree 2)
b) Ternary relationship= three entities (degree 3)

Question 10

Cardinality

Answer 10

• means “count,” and is expressed as a number
  a) Maximum cardinality= is the maximum number of relationship instances in which an entity can participate.
  b) Minimum cardinality= is the minimum number of relationship instances in which an entity must participate.

Question 11

Types of maximum cardinality

Answer 11

– One-to-One [1:1]
– One-to-Many [1:N]
– Many-to-Many [N:M]

Question 12

one-to-many

Parent and Child Entities

Answer 12

– The entity on the one side of the relationship is called
the parent entity or just the parent.

– The entity on the many side of the relationship is
called the child entity or just the child.

Question 13

Minimum cardinality

Answer 13

• Minimums are generally stated as either zero or one:
– IF zero [0] THEN participation in the relationship by the entity is optional, and no entity instance must
participate in the relationship.

– IF one [1] THEN participation in the relationship by the
entity is mandatory, and at least one entity instance must participate in the relationship.

Question 14

Strong Entity

Answer 14

-an entity that represents something that can exist on its own.
Examples (PERSON, AUTOMOBILE, BUILDING)

Question 15

Weak Entity

Answer 15

-an entity whose existence depends on
the presence of another entity.

Example (APARTMENT – depends on BUILDING)

Question 16

ID-dependent entity (a weak entity)

Answer 16

-an entity (child) whose identifier includes the identifier of another entity (parent).

• The ID-dependent entity is a logical extension or subunit of the parent:
– BUILDING : APARTMENT
– PAINTING : PRINT

• The minimum cardinality from the ID-dependent entity to the parent is always one.

Question 17

identifying relationship

Answer 17

-The solid line connecting an ID-dependent entity and its parent

Question 18

non-ID-dependent weak entities

Answer 18

-The identifier of the parent does not appear in the

identifier of the weak child entity

Question 19

nonidentifying relationship

Answer 19

The dashed line connecting strong

entities or a non-ID dependent weak entity to its parent

Question 20

subtype entity

Answer 20

-a special case of a supertype entity

• The supertype contains all common attributes, while the subtypes contain specific attributes.
• The supertype may have a discriminator attribute which indicates the subtype.

Question 21

Exclusive subtype

Answer 21

-one supertype relates to at

most one subtype

Question 22

Inclusive subtype

Answer 22

one supertype can relate to one or more subtypes

Question 23

Subtypes: IS-A Relationships

Answer 23

• Relationships connecting supertypes and subtypes are called because a subtype is a supertype.
• The identifier of the supertype and all of its subtypes must be identical
• Subtypes are used to avoid value-inappropriate nulls.

Question 24

Strong Entity Relationship Types

Answer 24

– 1:1 Strong Entity Relationship
– 1:N Strong Entity Relationship
– N:M Strong Entity Relationship

Question 25

recursive relationship

Answer 25

when an entity type has a relationship to itself:
– 1:1 Recursive Relationships
– 1:N Recursive Relationships
– N:M Recursive Relationships