Conceptual Data Modeling (ER and EER diagrams)

Midterm

What are the main components of the ER model?

Entities

Attributes

Relationships

What does an attribute type represent

Property of an entity

What is a simple (atomic) attribute

Cannot be divided into parts

Has only one value for an entity

single-valued attribute

Can have multiple values for an entity

Multi-valued attribute

Can be nested composite and multivalued attributes

Complex attribute

Has values from user input

Stored attribute

Value derived from other attributes

Derived attribute

May have a null value

Optional attribute

Must have a valid value, no NULL

Required attribute

Uniquely identifies instances of an entity type

inidentifier

What is the difference between a simple and composite identifier

Simple is a single attribute, while composite is multiple

Can an entity type have more than one key

Yes, can have several candidate keys

What is the notation for key attributes in ER diagrams

Underlined

How are attributes displayed in ER diagrams

In ovals connected to their entity types

What does a multivalued attribute type look like in ER diagrams?

Displayed in double ovals

How is a derived attribute represented in ER diagrams?

dotted ovals

What does a required attribute look like in ER diagrams

presented in bold letters

What does a relationship represent in the ER Model?

An association between one or more entities

Can relationships have attributes?

Yes, they can describe features of the association

What is a cardinality constraint?

Limits combinations of entity instances

Exists independently of other entities

Strong entity

Dependent on a strong entity for existence

Weak entity

Represents a many-to-many relationship with attributes

Associative entity

What is the degree of a relationship type

Number of entity types participating in it

What is a unary relationship

relationship type of degree 1

What is a binary relationship

relationship type of degree 2

What is a ternary relationship

Relationship type of degree 3

What is a n-nary relationship

Relationship type of degree n

Subgrouping with distinct attributes

subtype

Generic entity type with subtypes

Supertype

Defines relationship between supertype and subtypes

IS-A relationship

Defining subclasses starting from a superclass

Specialization

Defining a superclass from subclasses.

Generalization

A constraint that specifies whether you must be a member of a subtype

Completeness

What do disjointness constraints specify

Whether an instance can be in multiple subtypes

What does the disjoint rule state/

An instance can be a member of one subtype

What does the completeness constraint specify

An instance must belong to at least one subtype

Subclass with several possible superclasses

Category

Why are entity clusters used in EER diagrams

To simplify diagrams with many entities

What are the shortcomings of the EER model regarding temporal constrains

It cannot model time-related conditions

It cannot guarantee the consistency across multiple relationship types