3 entity relationship diagrams

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ACE2141-3 Tony Cornah 2006/07 1

Entity-Relationship Diagrams

Steps in producing an E-R Diagram (ERD)

Identify the Entities.

Identify the relationships.

Consider 1-to-1 relationships.

Deal with M-to-N relationships.

Check for dependent existence.

Draw the diagram.

Check for Connection Traps.

Redo the Entities table if necessary.


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Entities

Three rules.

Each entity must have a key uniquely identifying it.

The number of values for an attribute must be the same

for all instances of an entity.

Any relationship between entities must be 1-to-N.

Identify the Entities.

Summarise the information from system descriptions

and the Data Flow Diagram.

Identify the things described by this data.

Check that the entities satisfy the first two of the Three

Rules.


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Member of Staff

Staff IDName

Address

Course

CourseCodeTitle

Summary

Timetabled Class

Day

Time

Location

CourseCode

StaffID

(Day, Time, Location, StaffID)

(Weds, 17:15, G29, A123)

(Mon, 17:15, G29, B456)

(Thurs, 19:15, G38, B456)


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Identify the Entities in the following description:

When a customer telephones the hotel, Chris, the receptionist,

checks the booking log for an available room, which he offers to

the customer, along with the price. If the customers is happy

with that room, Chris fills in a duplicate booking form with the

customers name, address and telephone number, and the dates,room number, room occupancy and price of the booking. He

then updates the booking log by writing the customers name

against the appropriate dates for the booked room. At various

convenient points during the day, the receptionist sends all the

original booking forms filled in so far to the customers as

confirmation. He then files the duplicates in the booking form

file.


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Identify the relationships.

Identify pairs of entities which have a connection. Check that the relationships satisfy the third of the

Three Rules.

Can one A have (more than) one B?

Can one B have only one A? Consider 1-to-1 relationships.

Either: Incorporate one entity into the other.

Or: Keep them separate.

Deal with M-to-N relationships.

Create a new entity.

Create two new 1-to-N relationships.

Relationships


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Relationships are

a Staff Member teaches a Class (taught by

)

a Classbelongs to a Course (has)

Can a Staff Member teach more than one Class?

Can a Classbe taught by only one Staff Member?

Can a Classbelong to (more than) one Course?

Can a Course have only one Class?

Yes

Yes

No

Yes

Yes

No

Can a Course have more than one Class?

Can a Classbelong to only one Course?


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Member of Staff Course

Timetabled Class


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Now consider some modifications

some Classes have (has) a Practical Session

some Classes are taught by more than one Staff Member

Can a Class have (more than) one Practical Session?

Can a Practical Sessionbe for by only one Class?



Yes

Yes

Yes

No

Yes

No




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Timetabled Class

Practical session

(only 1 per class)

Teacher For


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Check for dependent existence.

Can A exist without a corresponding B?

Can B exist without a corresponding A?

Can a Staff Member exist without teaching?

Can a Course exist without timetabled

classes? Can a timetable slot exist without a Staff

Member to teach and/or belonging to a

Course?

Yes

No

No


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Draw the diagram.

Entities as named boxes.

Relationships as arrows from the 1 to the N.

Absolute (i.e. not dependent) existence as dotted

half line.

Describe each end of the relationship.


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Timetabled Class

Teaches

Taught By

Has

Belongs To


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Hall of Residence Student

University SiteHolds Details Of

Registered At

Contains

Is In

If these were the relationships identified then there would be no way

of discovering where a student lived.

If students could live in any hall (or in private accommodation):

Lives In

Resident of

If students must live in a hall on their site:

Check for Connection Traps.


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Produce an Entity-Relationship Diagram

for the Hotel Booking System

Other Entity-Relationship Diagram

notations: http://members.iinet.net.au/~lonsdale/docs/erd.pdf http://ace2141.group.shef.ac.uk/SupplementaryMaterial/erd.pdf

http://www.smartdraw.com/examples/view/index.aspx?catID=.Examples.

SmartDraw.Software_Design.Entity_Relationship_Diagrm_-_ERD

The future UML Class Diagrams: (Unified Modelling Language) http://www.agilemodeling.com/artifacts/classDiagram.htm


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ROOM

CUSTOMER

BOOKING LOG ENTRY

there are redundant relationships here

Appears In

Refers To

Is Reserved By

Reserves

Makes

Is For

Appears In

Contains

Consists Of

Made Up Of


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Creating a database in Access

Open Access

New Blank Database

Specify values for

Save in: choose directory

File name: type file name

Create


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Creating a table using a Wizard Tables tab

Double clickCreate table by using wizard

Select required Sample Table Customers

Choose required Sample Field and select with "

Can Rename Field if required

Next to choose Primary Key and a new name for the tableif required

Can Modify the Table Design if required then Finish

If design is modified, Close the design view and clickYes

when asked if you want to Save the changes to the table


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Creating a table without a Wizard

Tables tab

Double click on Create table in Design view Fill in Field Name: Booking Date

Select Data Type:Date/Time

Etc.

References to other tables must match in type but

Use Number to refer to AutoNumber fields

Select the table key

select the row(s) containing the key item(s)

KEY icon orEdit - Primary Key

Close the design view and clickYes when asked if youwant to Save the changes to the table and name it


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Consider these tables1) for composite foreign keys

2) with the previous entity-

relationship diagram

Redo the Entities table if necessary.


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Creating relationships

Open the existing database and select Relationships

(Right click and Show Table), Add all the tables and Close

For each relationship

Drag the attribute from the One side to the N side

Select Enforce Referential Integrity ClickCreate

Close, saving the changes to, Relationships


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3 entity relationship diagrams

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