CT075!3!2 DTM Topic2 EntityRelationshipModelling

Data ManagementCT075-3-2

Entity Relationship Modelling


Topic & Structure of the lesson

• Entity Relationship Modelling – Terminologies– ER Modelling Notations– How to build an ER Model

Slide 2 (of 22)


Learning outcomes

At the end of this lecture you should be able to:•explain terminologies used.•explain the use of ER Modelling notations.•demonstrate how to build an ER Model.

Slide 3 (of 22)


Key terms you must be able to use

– Entity• Attributes• Value of an attribute

– Primary Key– Foreign Key– Relationship

• degree of relationship– one to one– one to many– many to many

• multiplicity • cardinality

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Terminologies

Entity Relationship Diagrams (ERDs) illustrate

the logical structure of databases.

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Terminologies

• Entity– an object about which the system need to store data,

such as Customer or Product– Attributes of an entity

• data items or elements that make up that entity• E.g. – customerName, customerAddress – those significant

to the system

– Value of an attribute• E.g. – John Barrett is the value of the attribute

customerName

Slide 6 (of 22)


Terminologies

• Primary Key– of an entity is an attribute or a set of attributes whose

values uniquely identify one occurrence of that entity– Entity primary keys are underlined as shown here:

• E.g. – Customer (customer#, initial, surname, customerAddress, customerPhone)

• Foreign key– an attribute in one entity which is the primary key of

another entity– acts as a link or navigation route between related

entities

Slide 7 (of 22)


Terminologies

• Relationship– a link between two entities which is significant for the

system– E.g. – customer places an order

• “places” describes a relationship between Customer and an order

• Degree of relationship– can be

• one to one

• one to many

• many to many

– multiplicity and cardinality are used to refer to the degree of relationship

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Entity Relationship modelling

• Notations– wide variety

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Figure 1: Variation 1 – Chen Notation from Elmasri book



• Notations

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Figure 2: Variation 2 – “Information Engineering” notation from Oracle Designer/ 2000 CASE



• Notations

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Figure 3: Variation 3 – Visible Analyst Notation



• Notations

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Figure 4: Variation 4 – Sybase PowerDesigner Notation



• Notations

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Figure 5: Variation 5 – Popkin System Architect Crow’s Feet Notation


Chen Notation

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Entity Weak Entity Attribute Key attribute

Multivalued attribute

Derived attribute

RelationshipsWeak

Relationships

Cardinality Recursive Relationship


Quick Review Question

• List the various notations for– entities– relationships– cardinalities– multiplicities

Slide 15 (of 22)


How to build an ER Model – (1)[i]

• Identify data objects (entities) about which the system needs to

store data.

• For each entity, list its attributes and check the attributes for

completeness.

• Using the practical guidelines listed below, make any necessary

improvements.

– Data items should be put into logical groups – groups that go

together.

– For each data group, or entity, there should be a key that uniquely

identifies individual members of that entity.

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How to build an ER Model – (2) [i]

– There should be no redundant data in the model – Data is

deemed redundant in the following situations:

• the data is never used by the system

• the same data items (CustomerName, CustomerAddress) are

stored in more than one place in the system.

• Data in one place can be derifed from data held in another place in

the system.

Slide 17 (of 22)


How to build an ER Model – (3) [i]

• Investigate and record relationships. – link entities so that all significant real-life relationships are

captured.

• Check the entity descriptions against the data

dictionary descriptions – Make sure that each process on the data flow diagram has

available to it all the data it needs;

[i] Britton, C., Doake, J., Chapter 5: Data Modelling, Software System Development

– A gentle introduction, McGraw-Hill

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Summary

• Entity Relationship Modelling – Terminologies– ER Modelling Notations– How to build an ER Model

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Q & A Session

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Next Session

• Logical Data Modelling

Slide 22 (of 22)

Documents

CT075!3!2 DTM Topic2 EntityRelationshipModelling