Unit 3: Entity-Relationship Modeling IT238: Data Modeling and Database Design Instructor: Qing Yan,...

Unit 3: Entity-Relationship Modeling

IT238: Data Modeling and Database Design

Instructor: Qing Yan, M.D., Ph.D.

Unit 3 Objectives

• Demonstrate how to develop a Presentation Layer ER model

• Demonstrate how to progress from the Presentation layer to the Design-specific ER Model

Chapter 1 – Database Systems: Architecture and Components2

Unit 3 To-Do List

• Complete the reading Textbook and Web

• Participate in the discussion board30 points

• Attend the introductory seminar or complete FLA quiz20 points

• Complete the unit assignment50 points


Assignment rubric

• 1. specified unique identifiers 10• 2. specified relationship types that exist among

various entity types 20• 3. captured business rules within ERD 15• 4. list semantic integrity constraints where business

rule could not be captured in the ERD 5• Total (Sum of all points) 50

Chapter 3 – Entity-Relationship Modeling 4

The storyline

• The factory has several departments. • A department may have many employees, but at

least three. Every employee works for one and only one department. Every department has a manager – only one manager per department. Clearly, a manager is an employee of the company; but, all employees are not managers.


The storyline

• A department may have many machines, and every machine is assigned to a specific department.

• A machine may go for maintenance numerous times. • Maintenance is done on a machine only once on a

given day. • Some machines are so new that they may not have

gone for maintenance yet.


The storyline

• Products are produced on machines. A product can be an assembly of several different components or a single piece.

• Every product/component goes through one or more machines for appropriate production operations.

• Likewise, several products may go through a particular machine for some standard process.


The storyline

• Designers design the products. Some well qualified

designers may design more than one product. All designers are employees of the factory.

• Operators, who are also employees of the factory, operate these machines. Due to multiple shifts, several operators operate the same machine. All operators are routinely assigned to work on only one machine and no operator is kept idle.

• A machine is never kept idle either, except when it is out for maintenance.


The requirements

• The database should capture an employee’s name, which would include first name, last name, and middle initial. It must also capture gender, address, and salary. An employee’s salary cannot exceed his/her manager’s salary. Social Security number (SSN) uniquely identifies an employee.

• Likewise, department number, department name, type, and location, must be captured. The department number and department name are both unique identifiers of a department.


The requirements

• Every machine will have a unique machine number. It will also consist of other details like name of machine, type, and vendor’s name. When a machine goes for maintenance, maintenance date for that machine needs to be captured.

• A product is identified by its component ID. Component name and description must also be recorded. It should be possible to compute the number of components in a product.


The requirements

• When a component goes through machining, the starting time, completion time, and hours of machining for each product on every machine must be captured.

• The information about a designer includes his/her qualification, specialization field, and experience in years.

• Operators, who are responsible for operating the machines, belong to a labor union and have certain skill sets associated with them.



Notation for drawing the ER diagram

Figure 3.2 Summary of Presentation Layer ER diagram notationFrom: Umanath, N.S., & Scamell, R. (2007). Data Modeling and Database Design. Boston: Thomson Course Technology.


Inclusive Arc Exclusive ArcNoninclusive Arc

R

R

m n Cardinality Ratio of m:n Chen's Notation

E1 Optionally Related To E2(Partial Participation of E1 in R)

E2 Mandatorily Related to E1(Total Participation of E2 in R)

[Existence Dependency of E2 in R]

E1 E2

E1 E2

RE1 E2 Cardinality Ratio of m:n Crow's Foot Notation

Figure 3.2 Summary of Presentation Layer ER diagram notation (continued)

Notation (continued)

From: Umanath, N.S., & Scamell, R. (2007). Data Modeling and Database Design. Boston: Thomson Course Technology.

Key Concepts

• Entity-relationship (ER) diagram (ERD) – The part of an ER model that portrays entity types,

attributes for each entity types, and relationships among the entity types.

• Presentation Layer ER model/schema – An entity-relationship modeling tool that serves as

a communications device between the analyst and the end-user community; consists of a Presentation Layer ER diagram and semantic integrity constraints


Key Concepts

• Synthesis approach – A bottom-up approach for developing an ER

diagram where attributes are grouped into entity types based on apparent commonalities among the attributes.

• Analysis approach – A top-down approach for developing an ER

diagram that begins by identifying entity types and then focuses on gathering attributes that appear to belong to individual entity types.


Vocabulary

• Exclusive arc-Indicates that two or more relationship types are mandatorily exclusive

• Inclusive arc-Indicates that two or more relationship types are mandatorily inclusive

• Noninclusive arc-Indicates that two or more relationship types are optionally inclusive


Vocabulary

• Restrict rule-A deletion rule where the deletion of a parent entity in a relationship is restricted if all child entities related to the parent in the relationship should not be deleted.

• Cascade rule-A deletion rule where the deletion of a parent entity in a relationship also causes all child entities related to the parent in the relationship to be deleted.


Vocabulary

• Superclass (SC)-A generic class of entity that includes one or more entity type occurrences.

• Subclass (sc)-An entity type that is a member of a superclass.


Vocabulary

• Generalization-A process that crystallizes the common properties (attributes) shared by a set of entity types into a generic entity type (SC).

• Defining attribute-The attribute in the superclass used to define the predicate in an attribute defined specialization.


Vocabulary

• Aggregation-A construct that allows modeling a “whole/part” association as an “Is-a-part-of” relationship between a superclass and a subclass.



Application of ER Modeling Grammar to the Conceptual Modeling Process

The ER model for conceptual modeling serves two major purposes

– As a communication/presentation device used by an analyst to interact with the end-user community (i.e., the Presentation Layer ER Model/Schema)

– As a design tool at the highest level of abstraction to convey a deeper level of understanding to the database designer (i.e., the Design-Specific ER Model/Schema)



The ER Model

The ER model includes:• An ER diagram (ERD) portraying entity types,

attributes for each entity type, and relationships among entity types

• Semantic integrity constraints that reflect the business rules about data not captured in the ER diagram.



The ER Model (continued)



Notation

Figure 3.2 Summary of Presentation Layer ER diagram notationFrom: Umanath, N.S., & Scamell, R. (2007). Data Modeling and Database Design. Boston: Thomson Course Technology.


Inclusive Arc Exclusive ArcNoninclusive Arc

R

R

m n Cardinality Ratio of m:n Chen's Notation

E1 Optionally Related To E2(Partial Participation of E1 in R)

E2 Mandatorily Related to E1(Total Participation of E2 in R)

[Existence Dependency of E2 in R]

E1 E2

E1 E2

RE1 E2 Cardinality Ratio of m:n Crow's Foot Notation

Figure 3.2 Summary of Presentation Layer ER diagram notation (continued)

Notation (continued)



Presentation Layer ER Model(An End-User Communication/Presentation

Device)

• Not a Science !• Art? Engineering? - Artistic engineering or

Engineered art !• Heuristic (Intuitive), iterative process• Recursive incremental refinement• Target Audience: End-user community• Technology-independent



The Synthesis Approach to Generate an Initial Set of Entity Types and Attributes

• List all discernable data elements and treat them as attributes

• Group these attributes based on apparent commonalities

• Designate each cluster of attributes as an entity type• Review leftover data elements and investigate the

possibility that some of them serve as links among the entity types previously identified

• Designate these links as relationship types



The Analysis Approach to Generate an Initial Set of Entity Types and Attributes

• Begin by searching for things that can be labeled by singular nouns and call these things entity types

• Gather properties that appear to belong to individual entity types and label them as attributes of a particular entity type

• Be sensitive throughout the process to the identification of relationships among the various entity types



Bearcat, IncorporatedInitial Set of Entity Types

• Read through the vignette on pages 75-77• Identify entity types

– Plant - Company ?– Project - Manager ?– Employee - Supervisor ?– Dependent– Credit Union Account– Hobby



Bearcat, IncorporatedRevised Set of Entity Types

• Read through the vignette on pages 75-77• Identify entity types

– Plant - Company x- Company x– Project - Manager x- Manager x– Employee - Supervisor x- Supervisor x– Dependent– Credit Union Account– Hobby



Development of Presentation Layer ER Model

• Divide story into meaningful “chunks” and identify relationship(s) among entity types

• Version 0 of ER diagram results: a punctuated equilibrium

• Go back and read the story (strike out story lines already captured if need be) and refine entity types and relationship types of the ER diagram based on additional facts discerned



Development of Presentation Layer ER Model

• Gather business rules that can’t be captured in the ERD into a list of Semantic Integrity Constraints, a supplement to the ERD

• Repeat last two steps (as many times as necessary) until all aspects of the story (i.e., the business rules) have been accounted for in the ER model



Box 1

Bearcat Incorporated is a manufacturing company that has several plants in the northeastern part of the United States. These plants are responsible for leading different projects that the company might undertake, depending on a plants’ function. A certain plant might even be associated with several projects but a project is always under the control of just one plant. Some plants do not undertake any projects at all. If a plant is closed down, the projects undertaken by that plant cannot be canceled. The project assignments from a closed plant must be temporarily removed in order to allow the project to be transferred to another plant.

PLANT

PROJECT

Undertaken_by

n

1

Note: The relationship shown models the italicized text

Development of Presentation Layer ER Model (continued)



Box 2

Employees work in these plants and each employee works in only one plant. A plant could employ many employees but must have at least 100 employees. A plant with employees cannot be closed down. Every plant is managed by an employee who works in the same plant; but every employee is not a plant manager nor can an employee manage more than one plant. Company policy dictates that every plant must have a manager. Therefore, an employee currently managing a plant cannot be deleted from the database. If a plant is closed down, the employee no longer manages the plant but becomes an employee of another plant. Some employees are assigned to work on projects and in some cases might even be assigned to work on several projects simultaneously. For a project to exist, it must have at least one employee assigned to it. Projects might need several employees depending on their size and scope. As long as an employee is assigned to a project, his or her record cannot be removed from the database. However, once a project ends it is removed from the system and all assignments of employees to that project must be removed.

1

EMPLOYEE PLANT

PROJECT

Works_in

Managed_by

Undertaken_by

Assigned

n

n

m n

1

1 1

Development of Presentation Layer ER Model (continued)



n

1

EMPLOYEE PLANT

PROJECT

DEPENDENT

BANK ACCOUNT

Works_in

Managed_by

Undertaken_by

Assigned

Dependent_of

Held_by_D

manager

Supervised_by

Held_by_E

Fname

Gender

Emp#

SalaryAddress

Name

Lname

Minit

Date_hired

Pl_name

Pnumber

Budget

Building

No_of_employees

Mgr_start_dt

Hours

Plocation

Pnumber

Pr_name

Related_how

GenderBirthdate

Dname

Balance

Account_id

Acct_type

Account#

n

n

m

n

m n

HOBBY

Participates

Participant

n

Annual_cost

Hrs_per_wkHb_name

Gi_activity

Io_activity

Name_tag

No_of_dependents

- - - - - - - -

1

1 1

1

1

1

1

m

worker

managed by

employer

Responsible

Controlled

Assignee

Assignment

Usage

Account Holder

Holder of

Supervisee

Supervisor

Depends_on

Having

Dependent

20

1

EMPLOYEE PLANT

PROJECT

DEPENDENT

BCU_ACCOUNT

Works_in

Managed_by

Undertaken_by

Assigned

Dependent_of

Held_by_D

manager

Account of

Supervised_by

Figure 3.3 Presentation Layer ER diagram f or Bearcat, I ncorporated

Held_by_E

Fname

Gender

Emp#

SalaryAddress

Name

Lname

Minit

Date_hired

Pl_name

Pnumber

Budget

Building

No_of_employees

Mgr_start_dt

Hours

Plocation

Pnumber

Pr_name

Related_how

GenderBirthdate

Dname

Account_id

n

n

n

m

n

m n

HOBBY

Participates

Participant

n

Annual_cost

Hrs_per_wkHb_name

Gi_activity

Io_activity

Name_tag

No_of_dependents

- - - - - - - -

1

1 1

1

1

1

1

m

worker

managed by

employer

Responsible

Controlled

Assignee

Assignment

Usage

Account Holder

Belongs to

Holder of

Supervisee

Supervisor

Depends_on

Having

Dependent

Presentation Layer ER Diagram



Presentation Layer Semantic Integrity Constraints

Semantic integrity constraints are grouped into the following categories:– Attribute level business rules

– Entity level business rules

– Business rules governing entity deletion

– Miscellaneous business rules



Design-Specific ER Model(Technology-Independent Design Tool)

• Not a Science !• Art? Engineering?

Not an Art anymore; more of an Engineering

• Systematic process• Target Audience: Database Designer/Administrator

– Coarse granularity

– Fine granularity



Coarse-granular Design-Specific ER Model

• Tasks– Collect additional attribute characteristics relevant to

design/implementation (e.g., data type, size) and prepare an updated list of Semantic Integrity Constraints

– Introduce a technically more precise notation, viz., (min, max) notation, for expressing the structural constraints of a relationship type

– Map deletion rules to the ER diagram



Deletion Constraints

Restrict Rule (R): If a parent entity in a relationship is deleted and if all child entities related to this parent in this relationship should not be deleted, then the deletion of the parent should be disallowedCascade Rule (C): If a parent entity in a relationship is deleted and if all child entities related to this parent in this relationship should also be deleted, the cascade rule applies

Note: Conventionally, when a deletion constraint is not specified, the restrict rule is implied by default



Deletion Constraints (continued)

Set Null Rule (N): If a parent entity in a relationship is deleted and if all child entities related to this parent in this relationship should be retained but no longer referenced to this parent, the ‘set null’ rule applies

Set Default Rule (D): If a parent entity in a relationship is deleted and if all child entities related to this parent in this relationship should be retained but no longer referenced to this parent but should be referenced to a predefined default parent, the ‘set default’ rule applies



Fine-granular Design-Specific ER Model

• Tasks– Map the additional attribute characteristics to the ER

diagram– Decompose ER constructs preparatory to logical

model mapping• Decomposition of multi-valued attribute

• Decomposition of m:n cardinality constraint (ratio)

– Generate an updated list of Semantic Integrity Constraints



Presentation Layer ER Model• Diagram + Integrity Constraints

Coarse-Granular ER Model

Fine-Granular ER Model

Summary


Q & A

• Questions?

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Unit 3: Entity-Relationship Modeling IT238: Data Modeling and Database Design Instructor: Qing Yan,...

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