ITE305: Chapter 4 - Entity Relationship Modeling

12 Questions

What is the primary purpose of an Entity-Relationship Diagram (ERD)?

To represent the conceptual database as viewed by the end user

What is the difference between an entity and an entity instance?

An entity is a table in the relational environment, while an entity instance is a single entity occurrence

What is the purpose of a domain in the ER model?

To determine the possible values for a given attribute

What is the notation used to represent an entity in an ERD?

A rectangle with the entity name

What is an optional attribute in the ER model?

An attribute that does not require a value

What is the purpose of the attribute box in the Crow's Foot notation?

To specify the attributes of an entity

What is the purpose of an entity identifier in an Entity-Relationship Model?

To uniquely identify each entity instance

What is a composite attribute?

An attribute that can be further subdivided to yield additional attributes

What is the primary key in a relational table?

A column that contains a unique identifier for each row

What is a multivalued attribute?

An attribute that can have many values

What is the purpose of underlining in an Entity-Relationship Diagram (ERD)?

To identify key attributes

What is a single-valued attribute?

An attribute that can have only a single value

Study Notes

Entity Relationship Modeling

ERD represents the conceptual database as viewed by the end user.
ERDs depict the database's main components: entities, attributes, and relationships.

Types of ERD Notations

Chen notation
Crow's Foot notation
UML notation

Entities

An entity is an "object" of interest to the end user.
An entity refers to the entity set, not to a single entity occurrence.
An entity in the ERM corresponds to a table, not to a row, in the relational environment.
The ERM refers to a table row as an entity instance or entity occurrence.
An entity is represented by a rectangle that contains the entity's name, usually written in all capital letters.

Attributes

Attributes are characteristics of entities.
In Crow's Foot notation, attributes are written in the attribute box below the entity rectangle.

Required and Optional Attributes

A required attribute is an attribute that must have a value; it cannot be left empty.
An optional attribute is an attribute that does not require a value; therefore, it can be left empty.
In Crow's Foot, a required attribute is indicated with a vertical line.

Domains

A domain is the set of possible values for a given attribute.
The domain for a grade point average (GPA) attribute is written (0,4).
The domain for a company's date of hire attribute consists of all dates that fit in a range (for example, company startup date to current date).

Identifiers (Primary Keys)

Identifiers are one or more attributes that uniquely identify each entity instance.
The entity identifier is mapped as the table's primary key (PK).
Identifiers are underlined in the ERD.

Composite Identifiers

Ideally, an entity identifier is composed of only a single attribute.
A composite identifier is a primary key composed of more than one attribute.

Composite and Simple Attributes

A composite attribute is an attribute that can be further subdivided to yield additional attributes.
Examples: ADDRESS can be subdivided into street, city, state, and zip code.
A simple attribute is an attribute that cannot be subdivided.
Examples: age, gender, and marital status.

Single-Valued and Multivalued Attributes

A single-valued attribute is an attribute that can have only a single value.
Examples: a person's gender, a student's GPA, an employee's SSN.
Multivalued attributes are attributes that can have many values.
Examples: a person may have several college degrees, a household may have several different phones, each with its own number.