Data Modeling Man is a knot, a web, a mesh into which relationships are tied. Only those...

Data Modeling

Man is a knot, a web, a mesh into which relationships are

tied. Only those relationships matter

Saint-Exupéry

Data modeling

A technique for modeling dataA graphical representation of a databaseThe goal is to identify the facts to be stored in the databaseData modeling is a partnership between the client and analyst

Modeling Scope Model Technology

Motivation

why Goals Business plan Groupware

People who Business units

Organization chart Systems interface

Time when Key events PERT chart Scheduling

Data what Key entities Data model Relational database

Function how Key processes

Process model Application software

Network where Locations Logistics network System architecture

The building blocks

EntityAttributeRelationshipIdentifier

Data model quality

A well-formed data modelA high fidelity image

A well-formed data model

Construction rules obeyedNo ambiguity

All entities, attributes, relationships, and identifiers are definedNames are meaningful to the client

A high fidelity image

Faithfully describes the world it is supposed to representRelationships are of the correct degreeData model is complete, understandable, and accurateThe data model makes sense to the client

Quality improvement

Is the level of detail correct?Are all exceptions handled?Is the model accurate?

Pure geography

NATION

*natname

natpop

natarea

REGION

*regname

regtype

regpop

regarea

CITY

*cityname

citypop

cityarea

Geography revised

NATION

*natnamenatpopnatarea

ADMIN UNIT

*unitnameunittypeunitpopunitarea

CITY

*citynamecitypopcityarea

natcaptype

ADMIN UNIT-CITY

unitcaptype

National capital

Family matters - take 1

husband/wife

MAN

*manidmandob

manfnamemanonamemanlname

WOMAN

*womanidwomandob

womanfnamewomanonamewomanlname

Family matters - take 2

spousePERSON

*iddob

fnameonamelnamegender

Family matters - take 3

PERSON

*iddob

fnameonamelnamegender

MARRIAGE

*begindateenddate

Family matters - take 4

PERSON

*iddob

fnameonamelnamegender

MARRIAGE

*marriagenomarriagestatus

begindateenddate

Family matters - take 5

MARRIAGE

*marriagenomarriagestatus

begindateenddate

PERSON

*iddob

fnameonamelnamegender

child

Bookish matters

BORROWER

*borrowerid…

LIBRARY

*libname…

BOOK

*callnoISBN

booktitleduedate

…

History - take 1

DIVISION

*divname…

DEPARTMENT

*deptname…

EMPLOYEE

*empid…

History - take 2

DIVISION

*divname…

DEPARTMENT

*deptname…

EMPLOYEE

*empid…

POSITION

*begdateenddateposttitle

History - take 3

DIVISION

*divname…

DEPARTMENT

*deptname…

EMPLOYEE

*empid…

POSITION

*begdateenddateposttitle

PAYSLIP

*paydate

PAYSLIPLINE

*pslnopslamount

psltype

History - take 4

DIVISION

*divname…

DEPARTMENT

*deptname…

EMPLOYEE

*empid…

POSITION

*begdateenddateposttitle

PAYSLIP

*paydate

PAYSLIPLINE

*pslnopslamount

PSLTEXT

*psltypepslmessage

A ménage à trois for entities - take 1

AIRCRAFT-AGENT

AGENT

*agentid…

AIRLINE-AIRCRAFT

AIRLINE

*airlinename…

AGENT-AIRLINE

AIRCRAFT

*aircraftcode…

A ménage à trois for entities - take 2

AIRCRAFT

*aircraftcode…

AIRLINE

*airlinename…

AGENT

*agentid…

LEASE

*startdate…

Planning and doing - take 1

EMPLOYEE

*empno…

ACTIVITY

planned hours

DAILY WORK

*workdateactual hours

PROJECT

*projectid…

Planning and doing - take 2

EMPLOYEE

*empno…

ACTIVITY

DAILY WORK

*workdateplanned hoursactual hours

PROJECT

*projectid…

Entity types

IndependentDependentAssociativeAggregateSubordinate

Independent

Often a starting pointProminent in the client's mindOften related to other independent entities

STOCK

*stock codefirm namestock price

stock quantitystock dividend

stock PE

NATION

*nation codenation name

exchange rate

Dependent

Relies on another entity for its existence and identificationCan become independent if given an arbitrary identifier

REGION

*regnameregtyperegpopregarea

…

CITY

*citynamecitypopcityarea

…

Associative

A by-product of an m:m relationshipTypically between independent entitiesCan store current or historical dataCan become independent if given an arbitrary identifierDIVISION

*divname…

DEPARTMENT

*deptname…

EMPLOYEE

*empid…

POSITION

*begdateenddateposttitle

Aggregate

Created from several different entities that have a common prefix or suffixCommonly used with addresses or names

Subordinate

An entity with data that can vary among instances

ANIMAL

*animalid…

SHEEP

fleeceweight…

HORSE

furlong speed…

Generalization

A relationship between a more general element and a more specific element

ANIMAL

*animalidanimaldob

…

SHEEP

fleeceweight…

HORSE

furlongspeed…

GeneralizationMap with one table for each entityFor each of the subtype entities the primary key is that of the supertype entity You must also make this column a foreign key so that a subtype cannot be inserted without the presence of the matching supertype

Aggregation

Aggregation is a part-whole relationship between two entities

Herd Cow

consists of

*Herd Cow

Shared aggregation

One entity owns another entity, but other entities can own that entity as well

Class Student

members

** Class Enrollment Student

Composite aggregation

One entity exclusively owns the other entity

Student Aptitude test

taken

*Student Aptitude test

Data model contraction

AUDIOLIBRARY

…

LP

…

TAPE

…

CD

…

AUDIOLIBRARY

…

AUDIORECORDING

…audio type

Hints on data modeling

The model will expand and contractInvent identifiers where necessaryIdentifiers should have only one purpose – identificationA data model does not imply orderingCreate an attribute if ordering of instances is requiredAn attribute’s meaning must be consistent

Names and addresses

The query testIf an attribute has parts, are any of the parts ever likely to appear in a query?

Have an understanding on representing names and addresses in a data model

STUDENT

*studentid...

ADDRESS

*address typecitystate

post codenation

ADDRESS LINE

*address rankaddress line text

Hints on data modeling

Single instance entities are OKSelect names carefullySynonyms—different words have the same meaning

Get clients to settle on a common word or use views

Homonyms—same word has different meaningsClarify to avoid confusion

Naming associative entitiesConcatenate entity names if there is no obvious real world name

Hints on data modeling

Uncover all exceptionsLabel relationships to avoid ambiguityKeep the data model well-formed and accurate

Meaningful identifiers

An identifier is meaningful when some attributes of the entity can be inferred from the identifier’s value

Advantages Disadvantages

Recognizable and rememberable

Identifier exhaustion

Administrative simplicity Reality changes

Loss of meaningfulness

RecommendationNothing, however, is lost and much is gained by using non-meaningful identifiersNon-meaningful identifiers serve their sole purpose well

To uniquely identify an entity

Attributes are used to describe the characteristics of the entityA clear distinction between the role of identifiers and attributes creates fewer data management problems

The seven habits of highly effective data modelers

ImmerseChallengeGeneralizeTestLimitIntegrateComplete