Chapter 2Conceptual ModelingConceptual Modeling

Entity Relationship DiagramsEntity Relationship Diagrams

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ObjectivesObjectives• Definition of terms• Importance of data modeling• Write good names and definitions for entities,

l ti hi d tt ib trelationships, and attributes• Distinguish unary, binary, and ternary relationships• Model different types of attributes entities• Model different types of attributes, entities,

relationships, and cardinalities• Draw E-R diagrams for common business situationsDraw E R diagrams for common business situations• Convert many-to-many relationships to associative

entities

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• Model time-dependent data using time stamps

Business RulesBusiness Rules

• Statements that define or constrain someStatements that define or constrain some aspect of the business

• Assert business structure• Assert business structure

• Control/influence business behavior

• Are expressed in terms familiar to end users

• Govern how data are stored and handled.

• Our DB App will (hopefully) automate business rules

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business rules

E/R ModelingE/R Modeling

• The E/R model is used to construct a conceptual pdata model –

a representation of thea representation of the structure and constraintsof a database and is theof a database and is the technology independent.

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E-R ModelingE R Modeling

• An E-R model is based on:

– Entities

– Relationships between entities

– Attributes of entities and relationships

• E-R diagram

– a graphical representation of an E-R modelg p p

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Sample E-R Diagram (Figure 2-1)

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Basic E-R notation (Figure 2-2)

Entity symbols

Attribute symbols

A special entity that is

symbols

yalso a relationship Relationship

symbols

Relationship degreesspecify number of number of entity types involved

Relationship cardinalitiesspecify how many of

h

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each entity type is allowed

Modeling Business Rules• Business rules

– Statements that define the business.– Expressed in natural language for end users– Expressed in data models for system developers

A G d B i R l i• A Good Business Rule is: (Table 2-1)

– Declarative – what, not how– Precise – clear, agreed-upon meaning, g p g– Atomic – one statement– Consistent – internally and externally

Expressible structured natural language– Expressible – structured, natural language– Distinct – non-redundant– Business-oriented – understood by business people

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A Good Data Name is:

R l t d t b i t t h i l• Related to business, not technical

• Meaningful and self-documenting

• Unique

• ReadableReadable

• Composed of words from an approved list

R t bl• Repeatable

• Written in standard syntax

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Modeling Entities and Attributes

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EntitiesEntities• Things in the real world, physical or not: person,

place object event conceptplace, object, event, concept

• Entity Type– collection of entities that share properties or

characteristicsEntity type is always SINGULAR– Entity type is always SINGULAR

– Represented by a rectangleStudent

• Entity Instance– each of the instances of an entity type

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What Should an Entity Be?What Should an Entity Be?• SHOULD BE:

– An object that we are trying to model

– An object that will have many instances in the database

• SHOULD NOT BE:– A user of the database system

– An output of the database system (e.g. a report)

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Figure 2-4 Example of inappropriate entities

Inappropriate System user

System outputpp p

entities

Appropriate entities

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Attributes

• Attribute - property or characteristic of an entity p p y ytype that is of interest to the organization.

• Classifications of attributes:Classifications of attributes:– Required versus Optional Attributes

– Simple versus Composite AttributeSimple versus Composite Attribute

– Single-Valued versus Multi-valued Attribute

Stored versus Derived Attributes– Stored versus Derived Attributes

– Identifier Attributes

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Figure 2-7 A composite attribute

An attribute broken into component parts

Figure 2-8 Entity with multivalued attribute (Skill) and derived attribute (Years Employed)and derived attribute (Years Employed)

MultivaluedMultivaluedan employee can have more than one skill

Derivedfrom date employed

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more than one skilland current date

Identifiers (Keys)( y )

• Identifier (Key) - An attribute (or combination of ib ) h i l id ifi i di id l iattributes) that uniquely identifies individual instances

of an entity type• Simple Key versus Composite Key• Simple Key versus Composite Key• Candidate Key – an attribute that could be a

key…satisfies the requirements for being a keyy q g y

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Criteria for Selecting Identifiersg

• Will not change in value over the life of geach instance of the entity type.

• Will not be NULL.Will not be NULL.

• No intelligent identifiers (containing e.g. locations or people that might change)locations or people that might change)

• Substitute new, simple (e.g., surrogatett ib t ) k f l it kattribute) keys for long, composite keys

(e.g., entity type of Game: Game# instead f H T d Vi it T )

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of Home_Team and Visitor_Team)

Figure 2-9 Simple and composite identifier attributes

The id ifi i identifier is boldfaced and underlined

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Practice: Person

• Produce an E-R diagram for the following situation:

We have one entity, called Person, with the following tt ib t ID (th id tifi ) N hi h i dattributes: ID (the identifier); Name, which is composed

of one or more given names and one or more family names; one or more aliases; an address (composed of ; ; ( pstreet, city, state, zip); date of birth; and age, which can be calculated from the date of birth.

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Relationshipsp

• Association between two or more entities• Represented by connecting lines

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Figure 2-10 Relationship types and instances

a) Relationship type (Completes)

b) Relationship instances

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Relationshipsp

• Degree• Cardinality constraints

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Degree of relationships – from Figure 2-2

One entity related to

th f

Entities of two Entities of

h d ffanother of the same entity type

two different types related to

three different types related to each other

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y yprelated to each other

Fig. 2-12: (b) Binary relationships

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Interpreting Cardinalities

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Interpreting Cardinalities

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Cardinality ConstraintsCardinality Constraints

• Cardinality Constraints - the ynumber of instances of one entity that can or must be associated with each instance of

th titanother entity.

• Minimum CardinalityIf th ti l– If zero, then optional

– If one or more, then mandatory

• Maximum CardinalityMaximum Cardinality– The maximum number

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Quick CheckQuick Check

• So now you try it Add cardinality constraints to the• So now you try it. Add cardinality constraints to the following diagram– A person is national of zero or more countriesA person is national of zero or more countries

– A country has one or more people.

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Fig. 2-16: Introducing cardinality constraint(a) Basic relationship(a) Basic relationship

(a) Relationship with cardinality constraints

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Figure 2-17 Examples of cardinality constraints

a) Mandatory cardinalities

A patient must have recorded at least one history and can

A patient history is recorded for one and

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at least one history, and can have many

recorded for one and only one patient

Figure 2-17 Examples of cardinality constraints (cont.)

b) One optional, one mandatory

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Maximum Cardinalities

• Looking at the MAXIMUM cardinality on BOTH sides, we classify relationships as: , y p

1:1

1:M

M:N

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Practice: Products

• We have two kinds of entities: Products and Categories. For each product we keep its id g p p(identifier), name, price, wholesale price, and profit margin, which is calculated from the price and the

h l l i F h t k it idwholesale price. For each category we keep its id (identifier) and its name. Each product belongs to zero or more categories and each category can have zero or g g ymore products.

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Practice : CD

• Let us model a CD with three entities: CD, PERSON and SONGand SONG.– A CD has a number, which is its identifier, and a title– A person has an ID and a name, divided into first, last– A song has an ID, a title and a length– We keep track of which person is a song’s author. A person

can author many songs and a song has exactly one authorcan author many songs and a song has exactly one author.– We keep track of which people perform on a CD. Zero or

more people can perform on a CD, and people can perform on CDzero or more CDs.

– We keep track of which songs are included on a CD. One or more songs are included on a CD, and a song is included in

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zero or more CDs.

Strong vs.Weak Entitiesg

• Strong entity– Exists independently of other types of entities– Has its own unique identifier– Identifier underlined with single-line– Identifier underlined with single-line

• Weak entity– Dependent on a strong entity …cannot exist on its ownp g y– Does not have a unique identifier (only a partial identifier)– Partial identifier underlined with double-line

E i b h d bl li– Entity box has double line

• Identifying relationshiplinks strong entities to weak entities

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– links strong entities to weak entities

Figure 2-5 Example of a weak identity and its identifying relationshiprelationship

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Strong entity Weak entity

Weak Entity Strong Entity

N Strong entity

Now a strong entity(unique ID)

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Practice: Course Sections

• Model the courses using two entities: COURSE and SECTION Each course has a number and a title and maySECTION. Each course has a number and a title, and may have zero or more sections. Each section has a number and the term offered, which is composed of semester and year.

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Practice: Weak Entities

• Give another example of a weak entity type.

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Relationships with AttributesRelationships with Attributes

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Figure 2-11a A binary relationship with an attributeAttribute on a relationship (Link Attribute/Associative)

Here, the date completed attribute pertains specifically to the employee’s completion of a course it is an

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to the employee s completion of a course…it is an attribute of the relationship

Practice: Relationships with Attributes• #8, page 103. The figure below shows a grade report that is

mailed to students at the end of each semester. Prepare an ERD reflecting the data contained in the gradePrepare an ERD reflecting the data contained in the grade report. Assume that each course is taught by one instructor.

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Practice: Relationships with Attributes

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Homework Assignmentg• Exercise #15(a,g), page #104• Exercise #17 page #106• Exercise #17, page #106• Exercise #25, page #107

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Associative EntitiesAssociative Entities

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Associative Entities• One of the hardest concepts in E-R modeling• An associative entity is a relationship transformed• An associative entity is a relationship transformed

into an entity• Each instance of an associative entity represents an y p

instance of the relationship• Needed to represent ternary relationships, and for

h d t t l ti hi i tcases when we need to convert a relationship into an entity, to relate it to other entities.

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Associative Entities - Example• Previous model: a database of courses taken by employees.

For each employee we keep its SSN (identifier), name and birth date and for each course we keep its id and title Webirth date, and for each course we keep its id and title. We also keep the date the employee completed that course.

oo

S h l d i f i b h

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• Suppose that we also want to record information about the institutions issuing the certificates. HOW???

Figure 2-11b An associative entity (CERTIFICATE)

oo

Associative entity is like a relationship with an attribute, but it is also considered to be an entity in its own righty g

Note that the many-to-many cardinality between entities in Figure 2 11a has been replaced by two one to many

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Figure 2-11a has been replaced by two one-to-many relationships with the associative entity

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IssuesFill in the missing di li i

INSTITUTION

cardinalities.

Institution IDInstitution Address (City,

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( yState, Zip)

Fig. 2-11: (b) An associative entity (CERTIFICATE)

oo

Employee ID

oo

p y _Course_ID

What is an alternative to assign the PK?

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Associative Entities• An entity - has attributes• A relationship - links entities together• A relationship - links entities together• When should a relationship with attributes instead be

an associative entity? y– All relationships for the associative entity should be many– The associative entity could have meaning independent of

the other entitiesthe other entities– The associative entity preferably has a unique identifier,

and should also have other attributes– The associative entity may participate in other relationships

other than the entities of the associated relationship

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Homework Assignmentg• Exercise #15(f), page #104

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Ternary RelationshipsTernary Relationships

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Ternary RelationshipsTernary Relationships

• Relationships of degree 3• Relationships of degree 3

• Associates three entities at the same time

• Can't we just live with binary relationships?

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Ternary Relationship - ExampleTernary Relationship Example

• Say we wanted to keep track of which Person got which Degrees from which University.

tPERSON UNIVERSITY

got

DEGREEDEGREE

5555

• Can this be modeled with binary relationships?

Ternary Relationship - ExampleTernary Relationship Example

• Yes, with the help of an associative entity, p y

PERSON UNIVERSITYDIPLOMAPERSON UNIVERSITYDIPLOMA

DEGREE

• Ternary relationship “got” is now an associative

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entity, DIPLOMA. Cardinality constraints?

Figure 2-12 c) Example of a ternary relationship

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Note: a relationship can have attributes of its own

Fig. 2-14: Ternary relationships as an associative entity

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As a general rule, ternary relationships should be converted to associative entities.

Fig. 2-18: Cardinality constraints in a ternary relationship

Wh t th b i l ?

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What are the business rules?

Practice: Ternary RelationshipsPractice: Ternary Relationships

• Give another example of a ternary relationship• Give another example of a ternary relationship.

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Homework AssignmentHomework Assignment

• Exercise #15(b) page #104• Exercise #15(b), page #104

• Exercise #18, page 106

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Unary RelationshipsUnary Relationships

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Unary RelationshipsUnary Relationships

• Relationships of degree 1• Relationships of degree 1

• Also known as recursive relationships

• Two or more entities in the relationship are of the same type

• Example: we want to represent when an employee supervises another employee.p y p p y

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Example: SupervisesExample: Supervises

• Example: we want to represent when an• Example: we want to represent when an employee supervises another employee.

W ld i h hi lik hi• We could start with something like this:

SUPERVISORSupervises

SUPERVISEE

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Example: SupervisesExample: Supervises

• But supervisors can have their own supervisors• But supervisors can have their own supervisors

• Both supervisor and supervisee are employees, so we need a recursive relationship with roleswe need a recursive relationship, with roles

Supervises

EMPLOYEE

Supervisee

EMPLOYEESupervisor

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Example: SupervisesExample: Supervises

• And of course add cardinalities• And, of course, add cardinalities

• Should always define roles – even more important when cardinalities are differentwhen cardinalities are different

Supervises

Supervisee V

EMPLOYEESupervisor

EMPLOYEE

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What are the business rules?

Fig. 2-12 (a) Unary relationships

Practice: Add missing cardinalities.

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g

Figure 2-17 Examples of cardinality constraints (cont.)

c) Optional cardinalities

A person is pmarried to at most one other person, or may not be married at all

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Example: Bill Of MaterialsExample: Bill Of Materials

Has ComponentsWhole PartHas Components

• Part-Whole is also a recursive relationship:

Has ComponentsPart V

Q iITEM

Whole>

Quantity

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Fig. 2-13: (b) Two ITEM bill-of-materials instancesFig. 2 13: (b) Two ITEM bill of materials instances

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Fig. 2-13 (c ) : An associative entity - bill of materials structure

This could just be a relationship with

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This could just be a relationship with attributes…it’s a judgment call

Practice: Unary RelationshipsPractice: Unary Relationships

• Give another example of a unary relationship.p y p

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Homework AssignmentHomework Assignment

• Exercise #10, page #103., p g

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Attributes or Entity?Attributes or Entity?

• Sometimes you will wonder whether to represent y pdata as an attribute or an entity. This is a common dilemma.

• Let us look at a few situations.

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Figure 2-15a and 2-15b Multivalued attributes can be represented as relationships/Associative Entity

simple

composite

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Attributes or Entity?Attributes or Entity?

• So when SHOULD an attribute be linked to an entity type via a relationship?– Attribute refers to a concept in the data model

– Multiple entity instances share the same attribute

• Example on next page

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Fig. 2-15: Using relationships and entities to link related attribute(c) Composite attribute of data shared with other entity types( ) p y yp

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M lti l R l ti hiMultiple Relationships

M th l ti hi b t• More than one relationship between the same entity types (Fig. 2-21)

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Fig. 2-21: Examples of multiple relationships

( ) E l d d t t(a) Employees and departments

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Entities can be related to one another in more than one way.

Fig. 2-21: (b) Professors and courses (fixed upon constraint)

A N B i R lA New Business Rule:An instructor who isscheduled to teach a course must be qualifiedcourse must be qualifiedto teach that course??

????

Here, minimum cardinality constraint is 2, what’s for?

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Homework AssignmentHomework Assignment

• Exercise #11, page #103, p g

• Exercise #15(d,i), page #105

• Exercise #18, page #106Exercise #18, page #106

• Exercise #20, page #106

• Exercise #23 page #107Exercise #23, page #107

• Exercise #26, page #108

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Modeling Time-Dependent DataModeling Time Dependent Data

Figure 2-19. Simple example of time-stampingg p p p g

This attribute is both

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This attribute is both multivalued and composite.

Fig. 2-20: (a) E-R diagram not recognizing product reassignment

Fig. 2-20: (b) E-R diagram recognizing product reassignment

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Fig. 2-20: (a) E-R diagram not recognizing product reassignment

H t fl t d t li

In the middle of year, due to a reorganization of the sales function some products are reassigned to different product lines

PRODUCTLINE

How to reflect product line changed over time?

Sales Product Product-Line

$50,000 P B$ ,

$40,000 P A(out of $50,000)

Assigned

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PRODUCT ORDERPlaced

Fig. 2-20: Pine Valley Furniture product databaseSolution: adding a new relationship of “Sales_for_product_line”

(b) E-R diagram recognizing product reassignment

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Fig. 2-20: Pine Valley Furniture product database(c) E-R diagram with associative entity for product assignment to product line ( ) g y p g p

over time

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Th E i R l i hi (E R) M d lThe Entity Relationship (E-R) Model

Congratulation !!

You have just learnedYou have just learned one of the most important modelingimportant modeling concept (E-R) for developing the datadeveloping the data base systems.

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Recognizing Different ERDRecognizing Different ERD Notations

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Fig. 2-22: Microsoft Visio

Notation for Pine Valley

Furniture E-R diagram

Different modelingDifferent modeling software tools may have different notation f thfor the same constructs

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Fig. 2-22: E-R diagram for Pine Valley Furniture Company

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