Chapt 2 Entity Relationship Modeling

8/10/2019 Chapt 2 Entity Relationship Modeling

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Lecture Notes on Database Chapter Two

ENTITY RELATIONSHIP MODELING

Overview

Introduction

Entity & Entity Cl!!

Attri"ute!

#ni$ue Identi%ier

Reltion!i'!

E()'le * Cri""en Air Trvel

E()'le * Credit Crd

N)in+ Reltion!i'

Introduction

The Systems Analyst uses education, experience, his own judgement and proven

methodologies to make sense of and to model organization behavior. e may then go onto represent such behavior!activity!event!structure in data. e may use "#"s to create a

model of the activities $processes% taking place in an organization. This includes

identifying the information that flows in the organization, where it flows to and from, andwhere it is stored. "espite the nomenclature, the "#" does not focus on the details of

data, so these &data stores' are not well defined in the "#". To create models which

would provide a picture of data stores we have to turn to (ntity )elationship *odeling.

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n the /01-s there were several competing data modeling tools2 the file system model, thehierarchical model $+*'S *S database system%, and the 3etwork model $45"AS67,

oneywell's "S database system% developed by 4harles +achman. n /01-, the

relational model was introduced by (.#. 4odd. n /018, 9eter 4hen introduced (ntity:)elationship *odeling, which was a unifying methodology for file and database design

$A4* Transactions on "atabase Systems, *arch, /018%.

(ntity:)elationship *odeling allows us to define the re;uirements for data storage. t

involves identifying the significant things or (3TT(S in an organization, the relevant

properties or ATT)+ones, *ary Smith?s payment, etc.

Attri"ute

A data attribute is a characteristic common to all or most instances of a particularentity. Synonyms include property, data element, field. (.g. 3ame, address,

(mployee 3umber, pay rate are all attributes of the entity employee. An attributeor combination of attributes that uni;uely identifies one and only one instance of

an entity is called an identi%ier. (.g. (mployee 3umber is a primary key for

(mployee.

Reltion!i'

A data relationship is a natural association that exists between one or more

entities. (.g. (mployees process payments. Crdinlitydefines the number of

occurrences of one entity for a single occurrence of the related entity. (.g. anemployee may process many payments but might not process any payments

depending on the nature of her job.

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Entity & Entity Cl!!

An (3TT6 is an object or event that the analyst wants to document. )emember thatthis object or event is being and happening in and!or out of the organization. The analyst

has to discover and represent these things, first, diagrammatically in an ()", and then indata. The entity is represented diagrammatically by a &box'with a name. Thename is in

all capitals and is singular.

The &box' may be of any size or shape. The analyst $you% gets to choose the size and

shape. +y stretching or shrinking the box, you find an appropriate size, large enough tohold any text you wish to enclose, and small enough to fit enough boxes on one page to

represent the system you wish to depict. 6ou must also allow enough size to connect

relationship lines to the box.

The analyst, through a study of the organization, discovers and identifies the things which

populate the organization space. These things are then classified by putting them inboxes and giving the boxes names. So that the entity name $the name in the box%, is the

name for a class of thing, not a single occurrence of the thing. So that the entity

A)95)T, represents a class of thing we call &airport', and consists of one or more thingssuch as 9iarco, @igie, and @ere +ird, each of which is an occurrence of the entity

A)95)T.

Attri"ute!

6ou would have considered attributes earlier when searching for entities. Thecharacteristics of entities, the things that draw them to your attention, are their attributes.

#or instance if we are talking about air travel, the first thing that you will probably notice

on the ticket is the name of the passenger, then the destination, the date and the flight

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A)95)T

e.g. 9iarco @igie

@ere +ird


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information. These are the attributes of ticket, or more specifically, as we shall soon see

in the example, coupon. #ormally stated, an attribute is any detail that serves to ;ualify,

identify, classify, ;uantify or express the state of an entity, or provide a description of athing of significance.

An attribute could be text, or numbers. t may also be a picture, as provided for in*icrosoft Access database management software. 7ess obviously, it may be a feel, a

smell, or some other intangible. #or data processing purposes, we tend to concentrate on

text and numbers, but other attribute types could be represented, particularly in theemerging area of multimedia.

An ellipse is used in the ()" to represent attributes. The ellipse is connected to the

entity using a line. The attribute's name, a noun, is written within the ellipse. $n *Sord, the ellipse shape with a text box inside%. This style of notation is used here since it

is the simplest type that most clearly identifies the attributes. 7ater we will discard this

style in favour of a more compact style.

hile it is not essential to show attributes on an entity relationship diagram, it is helpful

to clarify the nature of the entity and identify attributes early, for later use when definingdatabase tables.

An attribute describes only one entity, and it must describe only the entity against whichit is shown. This may seem obvious at first glance, but we may find in a real organization

that different entities may have attributes that appear to be similar. #or example, both the

entity student and the entity lecturer may have an attribute that may be called address.

As a rule of thumb, do not use more than seven attributes to describe an entity.

Sometimes relationships may be disguised as attributes, or attributes of one entity may be

mas;uerading as attributes of another. #or example, consider the case of employees andtheir departments in an organization. The entity (*9756(( may be shown as having

an attribute &department number', this is wrong. &"epartment number' is really an

attribute of the entity "(9A)T*(3T.

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date issued

time issued+5A)"3C 9ASS


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A 4omposite Attribute consists of other attributes and connects to its constituents using a

solid line=

A *ulti:@alued Attribute is one that may have multiple values. t connects to the entity

using a double line=

35T(= After 3ormalization, an entity may have only one value for an attribute at anytime. )epeated attributes must be removed from the entity. This is the first rule of

normalization, the removal of repeating attributes. e will look at 3ormalization more

closely later on.

A "erived Attribute is one whose value may be calculated $derived% from other attributes.

Since it can be derived, there is no need to store it. t connects to the entity using adashed line.

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address

street

city

statephoneE

ST


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An attribute name must be in the singular. f, when naming, it appears that the attribute

should be plural, this is an indication that missing entities with their own attributes may

be hiding here. An attribute can become an entity when it is a thing of significance,which has its own attributes and relationships.

#ni$ue Identi%ier

(ach uni;ue instance of an entity must be uni;uely identifiable by a combination ofattribute$s% and!or relationships $key%. 6ou must seek out candidate attributes that may

uni;uely identify an entity. Student ", invoice number, etc. are good candidate

attributes that would uni;uely identify the entities ST


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Layout Rules These are designed to make the diagrams easy to read and to

maximize the ;uality and accuracy.

H Arrange your diagram so the entity boxes line up and relationship lines are

mainly straight and horizontal or vertical. *inimize crossing lines. hen relationship

lines must cross, try to reduce clutter and use diagonal lines. Avoid using many closelyparallel lines. These are often difficult to follow.


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.-N /One0to0Mny1

N-. /Mny0to0One1

N-M /Mny0to0Mny1

E()'le o% .-.-

A 9rofessor teaches *A36 4lasses.

A 4lass is taught by 53( and only one 9rofessor.

E()'le o% M-N-

A Student enrolls in *A36 classes.

A 4lass contains *A36 Students.

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. Mteaches9)5#(SS5) 47ASS

enrolls

Crade

MN

7ast 3ame

Student "

ST


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*any to *any $3=*% )elationships must be transformed into two separate 5ne to *any

$/=3% )elationships. To do this a 4omposite (ntity is created. t is represented by a new

entity &box'.

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Crade

4lass )oom EStudent "

N.

N. (3)577*(3T

7ast 3ame

Student "

ST


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Entity Relationship Model of the ticket:

Figure 2.2 %R/ o! the ticket

The nucleus of this simple system is the coupon. t is significant as the lowest commondenominator and has vital information such as its class and status. t can only exist

within the context of a ticket, from which it inherits the date of issue and fare.

(ach of the boxes in the ()" in #igure . above is an entity and the line between is a

relationship. The line has a forked $many% ending on the left and a single $one% ending on

the right, indicating that there can be many coupons on one ticket= a many to onerelationship. The line is solid to show that the relationship is mandatory. The

relationship can be read from the left to right to tell us that=

(ach 45


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7et us look at the ()" of the relationship between the T4M(T and the #7CT.

Figure 2.0 %R/ o! TIC%T to FLI3T Relationship

This shows that a coupon is in some way related to a flight. The diagram now reads from

left to right=

(ach 45


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(ach T4M(T must be made up of one or more 45


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N)in+ Reltion!i'

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45*9A36

A445


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The name for each end of the relationship is placed near the appropriate end in lower case

as shown above.

To read any relationship simply but definitively, the following syntax is used=

must be(ach $and every% (3TT6 F A end:name:/

may be

53( A3" 5376 53( (3TT6 F + $ever%

&is that true5)53( 5) *5)( (3TT6 F + plural

And conversely=

must be

(ach $and every% (3TT6 F + end:name:/

may be

53( A3" 5376 53( (3TT6 F A $ever%

&is that true5)

53( 5) *5)( (3TT6 F A plural

Entity Reltion!i' Di+r)!

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end0n)e0.

end0n)e03

(3TT6 A (3TT6 +


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Overview

This section discusses the following topics with respect to (ntity )elationship "iagrams$()"s%=

De%inition nd #!e AnEntity Relationship Dia%ra! &ERD' is a graphical representation of the

relationships among tables in a database

This representation can also be referred to as the ?data model,? ?database schema,?

or ?database diagram? among other names

An ()" represents the lo%ical relationshipsbetween tables2 it has nothing to do

with the way the data isphysicallystored in the database

There is no standard way to represent an ()", however some common

conventions discussed below

SP7 Server 1.-, includes a database diagramming tool that can easily create

()"s

9rior to version 1.-, SP7 Server did not have this capability so ()"s were

typically created in *icrosoft Access or with a third:party product like @isio

3ote that unlike Access, the SP7 Server 1.- diagram tool does not draw

relationships lines directly connecting to keys $i.e., the columns used to establish the

relationship%2 however, you can manipulate their position on the screen to point to theappropriate location

There is no standard way to draw an ()"2 several different conventions are used

T"le! Tables$a.k.a. entities or relations% are drawn as boxes

Colu!ns$ a.k.a. fields or attributes% are listed as rows by name inside the table

box #eysare usually annotated as 9M : primary key2 #M : foreign key2 and AM :

alternative key $an AM is a column that meets the re;uirements of a 9M but is not

designated as such2 it is a good candidate for sorts and indexes%

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"efinition and


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The SP7 Server diagram tool does not follow this convention2 instead, the

primary key column$s% is designated with a ?key? icon to the left as shown below=

ithin the SP7 Server diagram tool, you can right:click on the table name bar

and choose ?4olumn 9roperties? to display detail for each column $data type,

nullability, etc.%2 there are also other display options available

Reltion!i'! Relationshipare represented by lines between the table boxes

A one(to(one relationshipmay be represented by a line without an endpoint2

sometimes it will be annotated with a ?/? or, in SP7 Server, with a ?key? icon on eitherend

A one(to(!any relationshipmay be represented by a line with a dot on the ?many?

end2 sometimes the line will be annotated with an ?3? or, in SP7 Server, with an

infinity symbol on the ?many? end

As discussed in )elationships, a !any(to(!any relationshipis simply two one:

to:many relationships as show below=

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http://www.frick-cpa.com/ss7/Theory_Relationships.asp#NNhttp://www.frick-cpa.com/ss7/Theory_Relationships.asp#NN


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Sometimes the ()" graphically distinguishes between dependent and

independent relationships

A dependent relationshipis one where the 9M of a dependent table is also a #M2

typically the dependent table box is drawn with rounded corners "ndependent tablesare drawn with s;uare corners

The SP7 Server diagram tool does not distinguish between independent and

dependent relationships

5ther designations sometimes used include=

o ): means ?one to zero or one? relationship

o :means ?one to one or more? relationship

o n: means ?one to exactly n? relationship

ERD %or te 4Pu"!4 Dt"!e

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ERD %or te 4Nortwind4 Dt"!e

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Reltion!i'!

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Overview

This section discusses the following topics with respect to relationships=

5!ic Crcteri!tic! o% Reltion!i'! n a relational database all data is organized into tables often referred to as

entities or relations. An advantage of the relational database model is its ability to establish

associations or ?relationships? between the tables.

A relationshipis a logical linking between two entities $tables% that describe how

they are associated with each other.

)elationships are used to enforce data integrity and facilitate joins to provide

access to multiple tables at the same time.

5ther database models $e.g., hierarchical or network% use explicit pointers to

associate pieces of data.

)elationships are represented by common data values stored in two tables.

(xample= a relationship is established between the customer and orders tables by

placing a customer " column in both tables2 each ?order? can then be ?related? to aparticular customer by the value in this common column.

The common columns that relate two tables are called &keys' $primary or foreign

key%.

n some cases a key can consist of one or more column in which case it is referred

to as a co!posite key$

A pri!ary key $9M% is a column $s% that uni;uely identifies each row in the

primary or ?parent? table.

The 9M not only ensures there are no duplicate rows but it also provides a

mechanism that allows you to reference all of the attributes $columns% of a specificrow simply by referring to one numeric value.

(xample= 6ou can reference any information about a particular customer simplyby referring to that customer?s ".

The 9M should be a small numeric key so that it is easy to sort, store and search.

A forei%n key $#M% in related or ?child? table inherits the primary key of the

?parent? or primary table.

Thus, a 9M:#M combination establishes a relationship between the parent and

child tables.

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+asic 4haracteristics

mplementing )elationships in a "atabase

5ne:to:*any )elationships

5ne:to:5ne )elationships

)ecursive )elationships


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The cardinalityof a relationship defines how many instances of each entity relate

to each other.

There are three primary types of cardinality= one:to:one2 one:to:many2 and many:

to:many.

A recursi*e or refle+i*e relationshipis a relationship within a single table2 i.e.,

the same table acts as both the child and parent. The relationships among tables $entities% are shown graphically in an (ntity

)elationship "iagram $()"%.

I)'le)entin+ Reltion!i'! hen designing a database, an entity$table% is anything that contributes to the

business operation and can be described in terms of accessible data.

n other words, if you were describing the activity being modeled by the database,

the entities would typically be the nouns in your description. Relationships between entities &tables' are lo%ical, not physical$i.e., they don't

exist as separate objects in a database%.

)elationships are defined as a logical link between the primary key $9M% of the

parent table with the foreign key $#M% of the child table.

hen a relationship is created, the parent table contributes its primary key to the

child table where it is referred to as a foreign key.

n other words, the relationship is created by storin% the *alue of the # for the

parent in the -# colu!n of the child$

The general SP7 syntax for adding a primary key to a table is=

CONSTRAINT constraint6naePRIMARY 6EY$colun6nae% The general SP7 syntax for adding a foreign key to a table is=

CONSTRAINTconstraintQname 7OREIGN 6EY$columnQname%

RE7ERENCESparentQtableQname $columnQnameQinQparentQtable%

3ote that the #M must references a column in another table.

A #M can only reference a column that has either a 9)*A)6 M(6 constraint or

a


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The classic example of a one:to:many relationship is that of ?customers? and

?orders2? each customer $parent% can have many orders, but each order $child% can be

related to only one customer.

To implement this relationship, a ?customer "? column is added to the ?orders?

table $#M% and referenced to ?customer "? column in the ?customers? table $9M%.

Mny0to0Mny /N0N1 n a many:to:many relationship, !any instances of one entity are associated

with !any instances of another entity$

The classic example of a many:to:many relationship is that of ?orders? and ?parts?.

(ach order can consist of many parts andeach part can be used in many orders.

This type of relationship cannot be established directly.

A third table, called a /unction tableor associatin% table, is used to establish the

relationship.

A junction table typically has a composite 9M that consists of two or more

columns that also serve as #Ms.

n effect you have two one:to:many relationships linked via the junction table that

together produce a single many:to:many relationship.

n the classic ?5rder (ntry? database example, the junction table is referred to as

the ?order detail? table2 it implements the many:to:many relationship between ?orders?and ?parts?.

The ?order detail? table contains an ?orderQ "? column that functions as a #M and

relates it to the ?orders? table2 the result is a one:to:many relationship between ?orders?

and ?order details?.

The ?order detail? table also contains a ?partQ "? column that functions as a #M

and relates it to the ?parts? table2 the result is a one:to:many relationship between

?parts? and ?order details?.

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ithin the ?order detail? table, the ?orderQ "? and the ?partQ "? together are

defined as a co!posite pri!ary key$

Taken all together, the two one:to:many relationships $orders:RorderQdetail and

parts:RorderQ detail% constitute a many:to:many relationship between ?orders? and

?parts?.

One0to0One /.0.1 n a one:to:one relationship, one instance of one entity is associated with a

sin%le instance of another entity.

This type of relationship is not very common.

n order to evaluate the need for a one:to:one relationship, ask yourself why you

can?t just use one large table.

+y implementing a one:to:one relationship you are in effect vertically partitioninga table $splitting it into two pieces%.

There are typically two reasons for vertically partitioning a table by implementing

a one:to:one relationship= /% security and % performance.

(xample= 6ou split the employee table into two pieces so that salary and other

sensitive information can be stored in separate table $security%.

(xample= Some columns in a large table are not accessed fre;uently or contain

large data types so they are moved to a separate table $performance%.

n the illustration above, the ?publishers? table is split into two pieces2 one to hold

the smaller, more commonly accessed fields and another to hold the less fre;uently

re;uested data fields that include large data types $the ?logo? field is an image datatype and the ?prQinfo? field is a text data type%.

Recur!ive Reltion!i' A recursi*erelationship refers to a relationship established within a table.

n other words, a table is ?related? to itself.

)ecursive relationships are usually established in situations where it is useful to

perform a self:join.

The classic example of a self(/oin is the ?employee:manager? report. n this

situation, the ?employees? table contains name, " and ?reports to? columns. The

?reports to? column contains the " $not the name% of the employee?s manager. n order

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to prepare a report that lists employees and their managers by their naes, a self:join

is re;uired..

n this situation, the ?reports to? column is typically made a #M that references the

9M ?employee "? field.

(stablishing this internal relationship allows referential integrity to be enforced

$e.g., you cannot insert a manager " into the ?reports to? column that does not exist

in the employee " column%.

A recursive relationship is sometimes referred to as a refle+i*e relationship$

The above was adapted from a website by "avid ). #rick 4o., 49A $http=!!www.frick:

cpa.com%

E8AMPLE T9O

TOYOTA TRINIDAD AND TO5AGO LIMITED

5AC6GRO#ND

Toyota Trinidad and Tobago 7imited $TTT7% business type is classified as an automotivedealership. TTT7. specializes in providing sales and services of their vehicles that are of

high ;uality, safe, reliable and durable. Thus, they offer a range of vehicles to meet theneeds of their customers, from the adventurous to the business type.

To support its sales and services, TTT7 puts its customers at the center of everything itdoes by delivering custom made vehicles to the buyer's specifications and providing

;uality service in the timeliest manner. ence, the company has developed a vast

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database on its customers that includes their preferences for styling, model types, colours,

prices and other features.

The building compound is partitioned into a reception area, sales area $which includes a

showroom%, a service bay $including a garage%, warehousing facilities, offices and ample

parking for its customers.

Toyota's computers and peripherals are connected through a local area network. This

network enables the departments and the people within those departments to shareinformation and resources. The primary purpose for maintaining and operating the

network is to share identical data that is always available to, and modifiable by, different

people simultaneously.

n dealing with the service aspect of TTT7, the database structure will support the heavy

flow of transactions that take place on a daily basis. Although this project considers only

the service aspect of TTT7, the description of the business operations helps to establish

several processes namely, inventory, job orders, invoicing.

(ach transaction will generate a set of procedures supported by a database design moduleand within each module a set of entities will be defined. The remaining description of

operations will add the re;uired detail and will help define other processes, entities,

attributes, relationships, etc.

t is true that in order for any business to survive and thrive, its business objectives must

be synchronized with its business type. ith this in mind, the business objectives must

be defined precisely and in detail. Since the business objectives are so closely tied to thebusiness type, they help audit the initial database components. #or example, while the

business objectives are likely to add attributes and entities to the database design, they

should not yield database components that are in conflict with those re;uired by thebusiness type.

Co)'ny O":ective!

/. To maintain and manage sustainable sales growth

This objective re;uires designing Toyota's operations to foster customer loyalty

and to attract new customers by=

a. *aintaining an inventory of products that meet customer needs.

b. *aintaining and improving order response time.

c. "elivering ordered products ;uickly and efficiently.d. *aintaining customer contact through follow:up actions.

e. *aintaining price competitiveness through operational cost control.

. To maximize net sales returns by maintaining and improving cost controls through=

a. (fficient data access and data:to:information transformation to improveinventory management

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b. The customer order process.

c. The vendor order process.

d. The shipping process.e. The back:order process.

f. The product return process.

5u!ine!! Rule! & ERD!

Te ;o" Order Module

A customer is someone who has purchased a Toyota vehicle in the past.

/. 4ustomers of Toyota own cars. This business rule establishes two entities,

45+Q5)"(). This establishes a /=* relationship

between service advisor and job order as a service advisor can generate more than onejob order.

D. hen a customer brings in a car for servicing, the car is entered on a job order. Arelationship is created between 4A) and >5+Q5)"(). 5nly one car can be enteredon one job order but over time, one car can generate many job orders.

Te ;o" Order Module ERD

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enters

owns4A)45+ 5)"()


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Te Invoicin+ Module

The invoicing module will be based on the following business rules.

/. An invoice is generated only when a customer makes a purchase. This business rule

establishes that 4


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Te Invoicin+ Module ERD

Te Servicin+ Module

/. There are different types of employees. This establishes the entity of(*9756((QT69(. (ach employee is of one employee type. This establishes a /=/

relationship between (*9756(( and (*9756((QT69(.

. The employee of type garage foreman accepts the vehicle and the job order from theservice advisor. This business rule establishes three entities, 4A), >5+ 5)"() and

(*9756((. There is a mandatory relationship between job order and car and an

optional relationship between >5+ 5)"() and (*9756((, in that vehicle ismandatory to >5+ 5)"() and >5+ 5)"() is optional to (*9756((. There is

only one vehicle per job order. There is a /=* relationship between >5+ 5)"() and

(*9756(( as an employee could have performed many service jobs on onevehicle.

B. The garage foreman analyses the job order information and assigns the job or job

segments to the appropriate technical team. This establishes the entity of T(A*, and

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is returned to

has

>5+ S((TS()@4(A"@S5)

(*9756((

contains

is written in9A)T 3@54( 73(

generates

writes

4


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establishes that T(A* is mandatory to >5+ 5)"(). Also, this establishes a

relationship between T(A* and (*9756(( such that the T(A* is made up of as

many as five (*9756((S and each (*9756(( belongs to only one T(A*.There is a /=* relationship between T(A* and >5+Q5)"() as one T(A* can

work on more than one >5+Q5)"().

D. 9arts re;uired for the service job are sourced from Toyota's on:site warehouse. These

parts are used in the car. This rule shows the relationship between 9A)TS and 4A)

and establishes that more than one part can be used on a car.

G. 9A)TS are assigned to the job they are to be used for. This establishes a relationship

between 9A)TS and >5+Q5)"(), since 9A)TS used on the car will be placed on

the >5+Q5)"().

8. Specialized tools used in a job must be signed for by the technician using the

e;uipment. This creates the entity T557S, and established that >5+Q5)"() is

mandatory to T557S as such tool used must be tied to a service job performed. Thereis a /=* relationship between >5+Q5)"() and T557S as one >5+Q5)"() may

re;uire more than one T557.

Te Servicin+ Module ERD

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assignedto

receives >5+ 5)"()

used on

workson

9A)T 4A)

uses

make up

is of(*9756(( (*9756((T69(

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Reltionl Sce)

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nvoice

nvoice "4ustomer "

7icense 9latenvoice "ate

(mployee "

>ob 5rder "

(mployee

(mployee "Title

#irst 3ame7ast 3ame

AddressTown+irthdate)ecruitment "ate

Termination "ate9hone 3umber

*obile 3umber(mployee Type "

Team

Team "(mployee /(mployee (mployee B

(mployee D(mployee G

(mployee Type

(mployee Type "(mployee Type

"escription

9art

9art "9art "escription9art 4ost9art 9rice

9art Puantity9art *in. Puantity

nvoice 7ine

nvoice 7ine "7ine 3umber

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Documents

Chapt 2 Entity Relationship Modeling