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IST 210 Entity-Relationship Model & Diagram

Todd S. BacastowIST 210: Organization of data

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IST 210 Design Principles

The client has some vague idea of what he/she wants. YOUR task is to design a DB that represents these thoughts and onlythese thoughts.

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IST 210 Data ModelingData modeling is a formalismEntity-Relationship Model (ER) Entity Relationship model:

a visual model attempts to capture the structure of data

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IST 210 What is Visual Modeling?

Business Process

Order

Item

Ship via

Computer System

Visual Modeling is modeling using standard graphical notations

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IST 210 Visual Modeling is a Communication Tool

Use visual modeling to capture business objects and logic

Use visual modeling to analyze and design your database

Copyright © 1997 by Rational Software Corporation

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IST 210 Entity-Relationship (E-R) Modeling

Premises The real world consists of entitiesEntities in one enterprise are relatedDatabase model = relevant entities + relationships among them

Entities and relationships have attributes

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IST 210 E-RFeatures

Relatively easy to model real-world Translates easily to relational data model

Basic ConceptsEntity sets Relationship sets Attributes

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IST 210 EntitiesDefinition

Entityan object that is distinguishable from other objects.

a particular student, a particular book

Entity seta set of entities of the same type that share the same set of properties

students, books

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IST 210 EntitiesExamples of these entity types are:

Person: employeestudentcustomersupplier

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IST 210 Entity TypesPlace:

citystatecounty

Object: vehiclebuildingairplaneanimal

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IST 210 Entity TypesEvent:

purchasesaleregistrationgame

Concept: accountcoursework groupdepartment

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IST 210 Weak EntitiesWeak entities are entities, but with a difference---Weak entities only exist because some other entity exists. For example:

two entities employee and salary-historysalary-history is a weak entity the record of an employee’s salary history could only exist if a record of an employee also existsE.g., Joe Smith’s salary history wouldn’t make much sense if Joe Smith doesn’t exist in the data base.

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IST 210 Finding EntitiesEntities are normally located through the various user views identified during analysisSearch for entities by:

looking for nounsfinding objects/things that have many instances (i.e., things stored in tables) have descriptive/defining properties (attributes)

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IST 210 AttributesDefinition:

Attribute: a descriptive property possessed by all members of an entity set. Attribute set: the set of all attributes possessed by members of an entity set.

e.g., book = (author,title,publisher,barcode, etc.) e.g., student = (ID,surname,firstname,course, etc.) e.g., subject = (code,name,lecturer,timetable, etc.)

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IST 210 AttributesProperties of Attributes

Domain - the seat of permitted values for each attribute Type:

data type simple vs. composite attributes single-valued vs. multivaluedderived vs. stored

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IST 210 Simple & Composite AttributesSimple attributes cannot be decomposed without loss of meaning, e.g.,

Title: Mr., Ms., Mrs., Dr.

Composite attributes can be further decomposed without loss of meaning, e.g.,

Name: first name + last nameDate: day + month + yearAddress: street + city + state + zip

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IST 210 Single-Valued AttributesSingle-valued attributes occur once only for each entity instance, e.g.

IDNameDepartment

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IST 210 Multi-Valued AttributesMulti-valued attributes can occur more than once for the same entity instance; e.g.

Sports person playsHobbies person likesSkills person has achievedCourses person has takenDegrees person completed

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IST 210 Stored & Derived AttributesA derived attribute is one that can be calculated from other stored data

not usually stored as an attribute occasionally stored to speed information/query requests when these occur frequently

Maybe shown on ERD as attribute name within a dashed-line ellipse

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IST 210 RelationshipsRelationship: an association among 2 or more entities

E1: student - R: takes - E2: subject e.g. Sara Smith takes IST210

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IST 210 Relationship setsRelationship set: a relation among entity sets:

e.g. (Smith,IST210) takesRelationship sets

have degree may have attributes e.g. final mark attribute for takesrelationship

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IST 210 E-R DiagramEntity

Relationship

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IST 210 Entity Relationship Diagram Symbols

The symbols used in an ERD are not standardised so -

diagrams may appear somewhat different depending upon who created themsymbols used for our purposes will follow the format

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IST 210 Graphical Elements of the ER Diagram

Its components are: rectangles representing entity sets. ellipses representing attributes. diamonds representing relationship sets.

Relationship

• Customer Account

Entity

• Customers• Accounts

Attribute

• id• name

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IST 210 Basic Symbols

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IST 210 Relationship

STUDENT COURSETake

Connect two or more entity setsRepresented by diamondsThink of the “value” of a relationship set as a tableOne row for each list of entities, one from each set, that are connected by the relationship

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IST 210 Finding RelationshipsOften represented by a Verballows answers to questions not available through individual entity types

DEPENDENTHasEMPLOYEE

Weak Entity

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IST 210 Attributes & RelationshipsRelationship can have 1 or more attributes

depend on relationshipdo not belong to the entitiesfor example the date on which a course was completed belongs to the relationship rather than either of the participating entitiesshown in same way as entity attributes

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IST 210 Multiplicity or Cardinality

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IST 210 Cardinality Symbology

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IST 210 One-to-One

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IST 210 One-to-Many

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IST 210 Many-to-Many

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IST 210 Recursive

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IST 210 Common ConstraintsKeys we will discuss nextSingle-value constraints must have a certain value

Null valueReferential integrity constraints “broken link to a web page”Domain constraints a negative account balanceGeneral constraints limits you set

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IST 210 Keys

Primary key Foreign key Primary key

1 Wood Bob C Ist 357

2 Kent Chuck B Ist 115

3 Smith Jane A Ist 357

4 Boone Dan B Ist 357

ID Last First Grade Class

Student

Ist 357 48 Jones

Ist 115 120 Brower

Ist 20 120 Mennis

Name #Stud Instructor

Class

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IST 210 KeysA key is a set of attributes whose values can belong to at most one entityIn E-R model, every E.S. must have a keyMore than one key: a set of attributes is the “designated” keyIn E-R diagrams: underline all attributed of the designated key

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IST 210 A Multi-attribute Key

dept+number (e.g. IST 210) form a key for courses.Possibly, hours+room also forms a key. But we have not designated it as such.

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IST 210 Weak Entities & KeysWeak entity sets:

Does not have a primary keyare meaningful only in the context of a relationship with an owning strong entity set

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IST 210 We Made it!