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Entity-Relationship Design. Information Level Design. TOP DOWN DATA ANALYSIS.

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Entity relationship design

Entity-Relationship Design

Information Level Design


Top down data analysis
TOP DOWN DATA ANALYSIS

Computer systems are extremely complicated and cannot be developed without careful planning. The most common MIS is the Systems Development Life Cycle. This approach is to build a model of the information system based on the objectives and goals it must meet. This is called top down modeling.


Babysitter service
Babysitter Service

The AITP Service Club wants to run a babysitting service. Customers call to request a sitter and the Club Coordinator assigns an employee to sit for the customer from a list of employees available for the particular day requested.


Entity relationship model
Entity-Relationship Model

  • A logical representation of the data of an organization or business area in graphical form


Enterprise E-R Diagram

Employee

Customer

Job


Data Flow Diagram

Context Diagram

Request

Customer

Confirm-

ation

Babysitter Information

System

Availability

Assignment

Employee


Data flow diagram level 1
Data Flow DiagramLevel 1

Request

Assignment

Availability

Confirm-

ation

1.

Assign

Employee

Avail

Times

2.

Record

Avail-

ability

New

Current

Avail

Times

Job

Assign

D1 | Customer

D2 | Employee

D3 | Jobs


Communications model
Communications Model

  • A representation of the location at which data is stored and processed and the communications links that connect them.


Entity relationship models
Entity Relationship Models

A good E-R model has

  • One table for every entity in the business system

  • Correctly drawn relationships indicating 1-1 or 1-m cardinalities

  • Optionality indicators to support needed referential integrity


Entity
ENTITY:

A person, place, object, event, or concept about which the organization wishes to maintain data.

  • Must need to store data

  • Must have at least two attributes

  • Must have at least two records


Entity types
ENTITY TYPES

classes of people, objects or concepts about which we wish to store data.

  • become tables in a new computer system.

  • Instances are rows

  • Attributes are columns


Attribute
ATTRIBUTE:

A description or property of a given entity type.

  • Must depend on the entity key alone

  • Must contain information that we explicitly need

  • Must have the same data type for all entity occurrences


Relationship
RELATIONSHIP: .

A connection between entity instances in different entity classes

  • Must specify what row connects with what row in associated tables

  • Must not describe processing



Narrative description
Narrative Description

  • The conceptual modeling process starts with a narrative description of the process. This is a direct, active depiction of what the system should do. This is the basis of the initial data and process models.


Discovering entities
Discovering Entities

  • Entities are normally described by NOUNS and ADJECTIVES

  • Entities do not change anything.

  • Entity occurrences are records, entity types are files.

  • Reports are derived output and not entities.


Discovering entities1
Discovering Entities

  • Entities with only one attribute are usually modeled as attributes of another entity.

  • Entities that have only one record are usually modeled as a set of parameters and not as files.

  • Include only files (entity types) that are needed by a system. Extra entities require maintenance and space that can add considerably to the cost of a system.


Converting a text description into an e r model
Converting a text description into an E-R model:

1. Review the conceptual description of the business area for nouns that describe the system.

2. Each entity type should have more than one potential instance.

3. Each entity type should have more than one attribute.

4. Each entity type should be relevant..


Relationships
Relationships

A relationship is a connection between records in one table and those in another.

  • Instructor assigned to class (section)

  • Student enrolled in class (section)


Relationship1
RELATIONSHIP.

Does not describe processing or change any data. Relationship names should be passive (ordered by).

  • CARDINALITY Refers to the number of records that a relationship connects to a given child record in a relationship.

  • PARTICIPATION (Optionality) Refers to whether a record must exist in one table before a related one is inserted into another.


Diagrams 1 m relationships
Diagrams: 1:m Relationships

InstructorID

Section

CourseSection

Instructor

InstructorID


Diagrams m n relationships
Diagrams:m:n Relationships

CourseSection

StudentID

Student-Section

Section

CourseSection

Student

StudentID


Optionality referential integrity
Optionality(Referential Integrity)

Records in a table that have a relationship with another table may be restricted by optionality requirements.

  • Relationship Optional

  • Relationship Mandatory (referential integrity enforced)


Optionality
Optionality

Optional (0 allowed)

0

Mandatory (1 or more required)

1


Optionality1
Optionality

A constraint should be mandatory only if the relationship must be known whenever a record is first entered. Most relationships are optional.


Maintaining integrity
Maintaining Integrity

If a parent record is deleted then an optionality relationships can be maintained in several ways

  • Cascade delete

  • Cascade update

  • Cascade null



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