Entity Relationship Modeling

Entity

Relationship

Modeling

Entity Relationship Modeling

• Origin and use

- Article (Chen, 1976): ‘The Entity Relationship Model, towards a unified view of data’

- Entity Relationship Modeling (ERM) still is the international de facto standard for data modeling in data intensive

domains (databases, data warehouses)

• Resulting data model

- Entity Relationship Diagram (ERD) - Not much semantics can be modeled - Many different design styles

Diamond notation (Chen)

role

Name Gender

max cardinality

‘many to one’

attributes

Employ

Employee

ment

Department entity types

works for

N 1

relationship

Depno

Empno Name

identifying

attribute

Crow’s foot notation (Martin)

role

Name Gender

max cardinality

‘many to one’

attributes

Employment

Employee Department

entity types

works for

relationship

Depno

Empno Name

identifying

attribute

OMT-style notation (Rumbaugh)

role

Name Gender

max cardinality

‘many to one’

attributes

Employment

Employee Department

entity types

works for

relationship

Depno

Empno Name

identifying

attribute

UML-style notation (Booch)

role

Name Gender

max cardinality

‘many to one’

attributes

Employment

Employee Department

entity types

works for

relationship

Depno

Empno Name

identifying attribute

* 1

Diamond, Crow’s foot, OMT and UML notations for relationships

Person Instrument

Person Instrument

Person Instrument

plays

N N Employee Department

Employee Department

Employee Department

works N for 1

plays

plays

works for

works for

Person plays Instrument

Employee works for Department

* * * ¹

Oracle ^notation

(Barker)

role

max cardinality

‘many to one’ relationship name

works for

Employment EMPLOYEE

# Empno Name Gender

DEPARTMENT

# Depno Name

min cardinality

‘zero’, ‘one’

Note: the stroke for max cardinality ‘one’ is sometimes omitted

identifyer

entity type name

attributes

IE ^notation

(Martin)

role

max cardinality

many to one relationship name

works for

Employment

Employee Empno

Name Gender

Department Depno

Name

min cardinality zero, one

Note: the stroke for max cardinality ‘one’ is sometimes omitted

entity type name

identifyer

attributes

Supporting tools

• Most used tools nowadays - PowerDesigner (Sybase):

uses Information Engineering (IE) notation (Martin), also UML - Oracle Designer (Oracle):

uses Oracle notation (Barker) tools are nowadays incorporated in Oracle’s JDesigner

- ERwin (Platinum):

uses several notations: OMT, IE, Oracle, UML

• Conceptual or logical

- Some tools (like PowerDesigner) call an ERD a conceptual data model, others (like ERwin) call an ERD a logical data model

- Our opinion: an ERD is a logical level model, not a model at the conceptual level

Notational choice

• Information Engineering (IE) notation

- Box notation for entity types, including attributes - Only binary relationships

- James Martin’s notation for maximum and minimum cardinalities

• Resulting data model

- Entity Relationship Diagram (ERD) - Not much semantics can be modeled - Many different design styles

Starting document

Rooms are on floors and identified by giving the floor and the room number (like 1, 2, but 2a, 2b is possible as well) on that floor. Floors are identified by floor numbers like 1, 2, 3. We call the first and second room on floor 1 room 1.1 and room 1.2 and the first and second room on floor 2 we call room 2.1 and room 2.2, etc.

For each floor we want to register the number of emergency exits on that floor. When a floor has no emergency exits, we register that as well. For rooms we want to be able to register the number of seats in that room and when the room is

equipped with some facility (like ohp, pc, beamer, video, …), we

want to be able to register that as well.