VCE IT Theory Slideshows - ITA

VCE IT Theory Slideshows - ITA

By Mark Kellymark@vceit.com

Vceit.com

Entity Relationship Diagrams(ERD)

This is the VCAA sample ERD,published at

http://www.vcaa.vic.edu.au/vcaa/vce/studies/infotech/it-applications/publications/IT-Apps-samp.pdf

ENTITY

ENTITY

RELATIONSHIP

ENTITY

ATTRIBUTE

RELATIONSHIP

ENTITY

ATTRIBUTE

RELATIONSHIPKEY FIELD

In the real world, primary field

names are often underlined.

In the real world, primary field

names are often underlined.

Reading the ERD

A Teacher

Reading the ERD

supervises

Reading the ERD

subjects

Reading the ERD

Each subject has a name attribute

Reading the ERD

The primary key for each

subject is the SubjectID field

Reading the ERD

A teacher also supervises

classes

Reading the ERD

Students belong to classes

But …

• The VCAA sample ERD has NO CARDINALITY• i.e.no indications of

– 1:1– 1:many– many:1– many:many relationships

We have to assume…

No cardinality is shown. Can one teacher supervise more than one subject or class?

We have to assume…

Can one subject or class be supervised by more than teacher?

This ERD does not say. Use the case study and common sense.

Unofficial additions…

The ‘many’ end of a relationship can be shown as “N” or “” …

N

1

Unofficial additions…

“Crow’s feet” notation of “many”

“Crow’s feet” notation of “1”

Optional Advanced Crowsfeet

If we wished to indicate that a school may enrol many students, or may not enrol any students at all. We also wish to indicate that each student attends exactly one school. This diagram indicates this optionality and cardinality: 

SCHOOL

STUDENTEach school enrolls

at least zero

and at most many

students

Each student attends

at least one

and at most one

school

The VCAA sample exam question

List the three basic elements of entity relationship diagrams and provide an example of each from the diagram. (3 marks)

ENTITY

ATTRIBUTE

RELATIONSHIPKEY FIELD

The answer

By Mark Kellymark@vceit.comvceit.com

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VCE IT THEORY SLIDESHOWS