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Btec National - IT SYSTEMS Btec National - IT SYSTEMS ANALYSIS AND DESIGN ANALYSIS AND DESIGN 1 IT Systems Analysis IT Systems Analysis and Design and Design Entity Relationship Entity Relationship Diagrams Diagrams

Btec National - IT SYSTEMS ANALYSIS AND DESIGN 1 IT Systems Analysis and Design Entity Relationship Diagrams

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Btec National - IT SYSTEMS ANALYSIS AND DESIGN 3 Entity Relationship Diagrams ERD –An ERD represents the entities and the relationships that exists between them. –An ERD diagram is created to establish what tables within a database are required and what foreign keys are needed

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Page 1: Btec National - IT SYSTEMS ANALYSIS AND DESIGN 1 IT Systems Analysis and Design Entity Relationship Diagrams

Btec National - IT SYSTEMS ANALYSBtec National - IT SYSTEMS ANALYSIS AND DESIGNIS AND DESIGN

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IT Systems Analysis and IT Systems Analysis and DesignDesign

Entity Relationship DiagramsEntity Relationship Diagrams

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AimsAims

Learn about Entity Relationship Diagrams Learn about Entity Relationship Diagrams (ERD) and what they are used for.(ERD) and what they are used for.

Understand the decomposed processUnderstand the decomposed process

Create an ERD diagramCreate an ERD diagram

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Entity Relationship DiagramsEntity Relationship Diagrams

ERDERD

– An ERD represents the entities and the An ERD represents the entities and the relationships that exists between them.relationships that exists between them.

– An ERD diagram is created to establish what An ERD diagram is created to establish what tables within a database are required and tables within a database are required and what foreign keys are neededwhat foreign keys are needed

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Entity Relationship DiagramsEntity Relationship Diagrams

ERD – EntitiesERD – Entities

– Real world things that are part of the system Real world things that are part of the system under investigation.under investigation.

– Examples include Products, Customers, Examples include Products, Customers, Orders.Orders.

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Entity Relationship DiagramsEntity Relationship Diagrams

ERD – RelationshipERD – Relationship– Relationships illustrate how two entities share Relationships illustrate how two entities share

information in the database structure. information in the database structure. – Relationships exists between each of the Relationships exists between each of the

entities examples are shown below:entities examples are shown below:1 M

M M

1 1

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Entity Relationship DiagramsEntity Relationship Diagrams

Products EmployeeOrderM M M 1

Makesinvolves

A many to many relationship as shown above is not allowed because it will create duplicate data, and repeating groups. For more information use the link below:

http://www.cms.livjm.ac.uk/johnwillitts/RelDBDes/13Norm/SuppRepeatingGrp/repeating_groups.htm

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Entity Relationship DiagramsEntity Relationship DiagramsOrderIDOrderID EmployeeIDEmployeeID ProductIDProductID OrderDateOrderDate DeliverbyDeliverby CommentsComments

11 100100 A1A1 29/03/200929/03/2009 29/04/200929/04/2009 Ring the bellRing the bell

11 100100 A2A2 29/03/200929/03/2009 29/04/200929/04/2009 Ring the bellRing the bell

11 100100 A3A3 29/03/200929/03/2009 29/04/200929/04/2009 Ring the bellRing the bell

Order table

This is the result of a many to many relationship, as you can see there is repeating groups of data (A1, A2,A3), the OrderID primary key has lost it’s uniqueness and there is loads of redundancy i.e. the order date 29/03/2009 is stored more than once.

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Entity Relationship DiagramsEntity Relationship Diagrams

Products EmployeeOrderM 1

Makes

To resolve this many to many relationship we have to add another table which is a decomposed table of order and products, called Order Products.

Order Products

M 1

Consists

M

Included

1

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Entity Relationship DiagramsEntity Relationship DiagramsOrderIDOrderID ProductIDProductID

11 A1A1

11 A2A2

11 A3A3

Order Products

By using the decomposed table it reduced redundancy and removed repeating groups from the Order table, this results in a better functioning database.

OrderIDOrderID EmployeeIDEmployeeID OrderDateOrderDate DeliverbyDeliverby CommentsComments

11 100100 29/03/200929/03/2009 29/04/200929/04/2009 Ring the bellRing the bell

Order table

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Entity Relationship DiagramsEntity Relationship Diagrams

ERD – AttributeERD – Attribute– An attribute is a specification that defines a property An attribute is a specification that defines a property

of an entity. An attribute of an entity usually consists of an entity. An attribute of an entity usually consists of a name and a value for example:of a name and a value for example:

OrderIDOrderID EmployeeIDEmployeeID OrderDateOrderDate DeliverbyDeliverby CommentsComments

11 100100 29/03/200929/03/2009 29/04/200929/04/2009 Ring the bellRing the bell

These are all attributes, they have a name and a value.

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ConclusionConclusion

Learnt about Entity Relationship Diagrams Learnt about Entity Relationship Diagrams (ERD) and what they are used for.(ERD) and what they are used for.

Discussed the decomposed processDiscussed the decomposed process

Created an ERD diagramCreated an ERD diagram