Conceptual Data Modeling 4. 1 CSE2132 Database Systems Week 4 Lecture Conceptual Data Modeling.

Conceptual Data Modeling 4. 1

CSE2132 Database Systems

Week 4 Lecture

Conceptual Data Modeling


Aims of Database Design

· Reduce data redundancy.

· Provide stable data structures that can be readily changed with changing user requirements.

· Allow users to make ad hoc requests for data.

· Maintain complex relationships between data elements.

· Support a large variety of decision needs.

Hawryszkiewycz I J (1984) Database Analysis and Design p 2


Design Criteria

- data availability

- data reliability

- data currency

- data consistency

- data flexibility

- data efficiency

Verify these criteria are satisfied via technical review


Database Design

- the process of developing database structures from user requirements for data

- a structured methodology

Structured Methodology - a number of ordered formalprocesses with known inputs and expected outputs

Objectives

1. derive relationships2. evolve to meet user requirements3. user requests are met within reasonable time limits


Conceptual Design in the System LifecycleProject Identification

and Selection

Project Initiationand Planning

Logical design

Analysis

Physical design

Implementation

Maintenance

Conceptual Data Modeling

Enterprise Modeling

Logical Database Design

Physical Database Design

Database Implementation

}

DB Maintenance


The Entity-Relationship Model

· Represents major data objects and the relationships between them.

· Widely used commercially in Australia.

· Originated by Chen in 1976 and adapted in the 'Information Engineering' approach.

· An Entity model is composed of textual descriptions of objects and a graphical representation called an Entity Relationship Diagram


ER Diagram Notation

Entity

Weak Entity

Relationship

Attribute

Multivalued Attribute

Identifying Relationship

Derived Attribute

Associative Entity


Entity Relationship Modelling

Entities form relationships with one another and each entity may be described by one or more attributes.

Customer Sale OrderPlaces

(Placed By)

ENTITY RELATIONSHIP ATTRIBUTE

CUSTOMER

Any person or organization who purchases goods

Customer Number, Customer Name, Customer Phone


Entity Type

· Sometimes referred to as an entity set.

· A set of objects about which we wish to store information.

Smith, Jones, Brown

· Entity instances i.e members of the set LECTURER

· Entity Type or Entity Set is a classification of similar instances.

LECTURER


Entity Type

In practice Entity Instance is often shortened to 'Instance’ and Entity type is shortened to Entity.

• Entity names should be brief but unambiguous i.e clearly reflect the meaning .

• Entities should be named in the singular and are often recognized as nouns or noun phrases.

• Entities can model any object set of interest i.e physical, conceptual, events etc.


Entity Type versus Weak Entity Type• A strong entity type (i.e. an entity type) exists independently of

other entity types.

• A weak entity type is an entity type whose existence depends on some other entity type. The entity type on which the weak entity type depends is called the identifying owner

EMPLOYEE DEPENDENThas

Emp_ID E_Name D_Name D_O_B


Attributes• An attribute is a property or characteristic of an entity type that is of interest to

the organization. Years_Employed is a derived attribute. Skill is a multi-valued attribute. Address is a composite attribute

EMPLOYEE

Years_Employed

Employee_ID

Employee_Name

Address

Skill

Date_Employed

Street

Postcode


Attribute Types

A data item or element that describes one entity type.

STUDENT RESULT

Student#, Subject Code, Year, Semester, Grade

S1 COT423 1990 1 N

S1 COT423 1990 2 HD

Primary Key or Identifier

Some or all of the attributes describing an entity type which serve to uniquely identify each entity instance.


Value Sets or Domains

Each attribute type describes one entity type and is based on an underlying domain or value set.

EMPLOYEE

Emp# Emp-Gender Emp-Name Emp-Hire-Date

E1 Female Jones 20/6/89

DEPENDENT

Emp# Depend-Name Depend-Sex Birthdate

E1 Smith Male 23/6/46


Relationship

A named association between two entity types.

CUSTOMER

SALES ORDER

Places

(Placed By)

Entity type

Relationship Type

Entity Type


Relationship

CUSTOMER

SALES ORDER

Places

(Placed By)

Relationship names should indicate the role or reason for the association in both directions. A relationship may have attributes.


Relationship Instances

· A relationship instance is an association between two entity instances.

CUSTOMER SALES ORDER

Smith Places Order# 651

Jones Places Order# 375

· A relationship type is a classification of relationship instances.


Places

(Placed By)


Degree of Relationship

Binary

Binary Employee Salary

Order Part

Employee Employee

Computer Op. System

Software

1 1

1 m

1

m

m

n

m

n

p

is paid

contain

is aleader/member

is aleader/member

runs on

Unary

Ternary


Relationship Cardinality (Maximal Cardinalities)

Employee

Project

Leads

(Lead by)

Customer

Sales Order

Places

(Placed by)

Supplier

Item

Supplies

(Supplied by)

One to One One to Many Many to Many

Relationships represent the 'Business Rules' of the organisation.


· The entity participation in the relationship may be mandatory or optional.


Places

(Placed By)

· A Customer may place many Sales Orders.

· A Sales Order must be placed by one Customer.

Relationship Cardinality (Minimal Cardinalities)


Associative Entity• The presence of a relationship with one or more attributes suggests

that the relationship should perhaps be represented as an entity.

• An associative entity is an entity type that associates the instances of one or more entity types and contains attributes that are peculiar to the relationship between those entity instances.

EMPLOYEE completes COURSE

EMPLOYEE COURSECERTIFICATE

C_No Date_Completed


Abstractions in Conceptual Database DesignClassification abstraction: Class BICYCLE (black bicycle, my bicycle .... )

Aggregation abstraction: Aggregation BICYCLE (wheel, seat, handle bar .. )

Generalization abstraction: Generalization VEHICLE(bicycle, car, truck .. )

VEHICLE

BICYCLE CAR

WHEELPEDAL

SEATHANDLEBAR

STANDARD BARSTRAIGHT BAR

RACER BAR

GENERALISATION

AGGREGATION

CLASSIFICATION


Synonyms or Aliases

· Be careful to differentiate between things and their names.

ITEM = PART = PRODUCT ???

· SYNONYM or ALIAS

The same object called different names by different people.

· This problem concerns entity types, relationship types and attribute types.


Homonyms

· Different things called the same name by different people ??

EMPLOYEE START DATE

· In the Personnel area this may mean the date an employee started with the company.

· In a given department this may mean the date an employee started with the department.

· This problem also concerns entity types, relationships types and attribute types.


Identifying Entity Types

· Entity type names are generally nouns or noun phrases.

· Can the entity type be defined in one or two sentences ?

· What are some typical attributes describing the entity type ??

· Which attributes uniquely identify each entity instance ?


Identify the Entity Types

ABC COMPANY SALES ORDER

Order # ____________ Order Date __ / __ / __

Customer Name __________________________________

Customer Address________________________________

_________________________________________________

Item# Description Qty-ord Unit-Price Ext-Price

____ _______________ _______ $_______ $_______

____ _______________ _______ $_______ $_______


Building an ER Model

· List the major entity types in the system and note the relationship types between them.

· Represent the entity types by named rectangles.

· Identify relationship types between entity types including relationship degree and nature.


Entity Type Proforma

Entity Name ________________ Aliases___________________Definition ______________________________________________Purpose ______________________________________________

Name of Entity Super-type _______________________________________Sub-type Predicate _______________________________________

Sub-type Class Disjoint / Overlapping Total / Partial

Number of Instances ___________________________________________Inserts per period ______________ Deletes per period ________

Insert, Update & Delete Procedures


Relationship Definition

Entity 1 Name ____________ ____ Entity 2 Name __________________Relationship Name (E1 to E2) _______________ Aliases _______________Relationship Name (E2 to E1) _______________ Aliases _______________

Degree (E1 to E2) 1:1, 1:m, m:1, m:nNature (E1 to E2) M:O, O:M, O:O, M:M

Referential Integrity Rules Cascade (C), Nullify (N), Restrict (R)E1 to E2 Delete ________ Update ________E2 to E1 Delete ________ Update ________

Definition, Description & Purpose


Attribute Definition

Entity Name ___________________________

Attribute Name Domain Name Primary Foreign Key Nulls Default Validation Rules Key References OK Value


Domain Definition

Domain Name ___________________ Aliases _______________________Definition ____________________________________________________

Valid Values and Meanings ________________________________________________________________________________________________________

Data Type ________ Size ________

Validation Rules

Comments


ER Modelling Exercises

For each of the following problems prepare an Entity Relationship diagram. For relationship types you should name the relationship in both directions;indicate the degree i.e. 1:1, 1:m, m:n; and the nature i.e optional or mandatory. Entity types should be defined in one or two sentences and some possible attribute types identified together with the likely primary key of the entity type. You should also attempt to identify some of the domains or value sets on which the attribute types are based.

1. A Company is made up of several Divisions with each Division employing many Salespersons. Salespersons write many Orders with each Order relating to only one Salesperson and one Customer. Many Items may be ordered on an Order and a given Item may appear on many Orders.

2. The Pearcey Centre for Computing runs a number of evening Courses with each course being identified by a four character course code. Each course runs at a certain time on a certain evening. Students may enrol in any number of courses concurrently provided the courses are not running at the same time on the same evening.


A Possible Answer to 1.

1. A Company is made up of several Divisions with each Division employing many Salespersons. Salespersons write many Orders with each Order relating to only one Salesperson and one Customer. Many Items may be ordered on an Order and a given Item may appear on many Orders.

Company

Division Salesperson

Item

Order

Customer

has

employs

belongs to(contains many)

places

Conceptual Data Modeling 4. 1 CSE2132 Database Systems Week 4 Lecture Conceptual Data Modeling.

Documents

Transcript of Conceptual Data Modeling 4. 1 CSE2132 Database Systems Week 4 Lecture Conceptual Data Modeling.