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Logical Data Structures. (Entity Relationship Diagrams). Entities “something about which an e.g. organisation keeps data” customer . Attributes “the properties of an entity” e.g. address. Relationships “the connections e.g. between entities” customer

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Logical data structures

Logical Data Structures

(Entity Relationship Diagrams)


Entities “something about which an e.g.

organisation keeps data” customer

Attributes “the properties of an entity” e.g.

address

Relationships “the connections e.g. between entities” customer

places

order


Entities

An entity may be physical:

an emplyoyee, a part, a machine

Or conceptual:

a project, an order, a course

A particular employee is an “entity occurrence” of the employee entity


Attributes

An EMPLOYEE has: A PROJECT has:

an age a project leader

a salary a start date

an address a cost


Identifying Entities

One or more attributes must form a unique identifier (or key) for a given occurrence of an entity.

EMPLOYEES have:

an employee number (or first name + second name)

PROJECTS have:

a project code


Relationships

Relationships link entities:

An employee works on a project

A part is made on a machine and appears on an order

An employee attends a course


Terms used in LDS

LDSData Storagee.g.

Entity File customer

Entity Record fred smith

Occurrence

Atribute Field address


Describing Entities and Relationships

Factory

Entity (Master)

Relationship (1:many)

“makes”

Component

Entity (Detail)


Relationships and Entities

Given a factory

we can tell which

components it makes

Factory

Given a component

we can tell which

factory makes it

Component


One to Many Relationships

For any occurrence of A

there may be 0,1 or many

occurrences of B

A

B


One to One Relationships

Usually one entity should be an attribute of the other

Factory Manager

Sometimes the diagram requires one-to-one

Factory


Masters can also be details

Division

Factory

Component


Many to Many Relationships

component

order

appears on

One component

Many orders

Is for

One order

Many components


M:N Relationships hide missing entities

order

component

A “link” entity

order item

What is the identifier for an “order item”? – order no + component no

What are its attributes? – qty, price, description


Optional Masters

Some orders come via sales staff, others directly from customers. So the link may be to a sales person or to another entity (the customer)

Sales Person

Order


The complete LDS

division

sales person

factory

order item

order

component


Developing an LDS

Select initial entities

Identify direct relationships

Construct initial diagram

Identify additional characteristics


Select initial entities

Use existing data documents and files to identify:

Entities

Attributes


Identify direct relationships

Look for links from one entity to another, e.g. references to other entities within an entity’s attributes

b

c

a

d

A entity-entity cross reference grid may be useful

a

b

c

d


  • Draw initial diagram

  • 2. Determine degree of relationship

  • 3. Resolve M:N relationships

  • 4. Check 1:1 relationships and resolve


A wines and spirits wholesaler stocks a range of approximately 400 different lines and delivers by van on regular weekly runs to customers who are mainly off-licenses and clubs. The majority of orders are received by telephone and any one customer may place several orders during the week which must be accumulated and delivered in one consignment. When a customer telephones he expects to be advised on the availability of an item requested and, if not in stock, the alternatives that are available. At present, sales order forms are filled in by the telephone salesmen and sent to the warehouse where they are used for the despatch of the items. Hand-written despatch notes are used as the basis of the invoices which are typed and sent by post sometime later.

Prepare a logical data structure (LDS) which reflects the above system.


  • Identify entities approximately 400 different lines and delivers by van on regular weekly runs to customers who are mainly off-licenses and clubs. The majority of orders are received by telephone and any one customer may place several orders during the week which must be accumulated and delivered in one consignment. When a customer telephones he expects to be advised on the availability of an item requested and, if not in stock, the alternatives that are available. At present, sales order forms are filled in by the telephone salesmen and sent to the warehouse where they are used for the despatch of the items. Hand-written despatch notes are used as the basis of the invoices which are typed and sent by post sometime later.

  • Customer

  • Customer order

  • Item

  • Despatch note

  • Invoice


2. Cross reference table approximately 400 different lines and delivers by van on regular weekly runs to customers who are mainly off-licenses and clubs. The majority of orders are received by telephone and any one customer may place several orders during the week which must be accumulated and delivered in one consignment. When a customer telephones he expects to be advised on the availability of an item requested and, if not in stock, the alternatives that are available. At present, sales order forms are filled in by the telephone salesmen and sent to the warehouse where they are used for the despatch of the items. Hand-written despatch notes are used as the basis of the invoices which are typed and sent by post sometime later.

order

cust

item

d/n

inv

cust

order

item

d/n

inv


Initial Diagram approximately 400 different lines and delivers by van on regular weekly runs to customers who are mainly off-licenses and clubs. The majority of orders are received by telephone and any one customer may place several orders during the week which must be accumulated and delivered in one consignment. When a customer telephones he expects to be advised on the availability of an item requested and, if not in stock, the alternatives that are available. At present, sales order forms are filled in by the telephone salesmen and sent to the warehouse where they are used for the despatch of the items. Hand-written despatch notes are used as the basis of the invoices which are typed and sent by post sometime later.

customer

Despatch note

order

invoice

item


Final LDS approximately 400 different lines and delivers by van on regular weekly runs to customers who are mainly off-licenses and clubs. The majority of orders are received by telephone and any one customer may place several orders during the week which must be accumulated and delivered in one consignment. When a customer telephones he expects to be advised on the availability of an item requested and, if not in stock, the alternatives that are available. At present, sales order forms are filled in by the telephone salesmen and sent to the warehouse where they are used for the despatch of the items. Hand-written despatch notes are used as the basis of the invoices which are typed and sent by post sometime later.

customer

Despatch note

order

order

line

invoice

item


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