Entity-Relationship Design

Information Level Design

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

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

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

Enterprise E-R Diagram

Employee

Job

Customer

Data Flow DiagramContext Diagram

Babysitter InformationSystem

Customer

Employee

Request

Confirm-ation

Assignment

Availability

Data Flow DiagramLevel 1Request

Confirm-ation

1.AssignEmployee

Availability2.Record Avail-ability

Assignment

D1 | Customer

D2 | Employee

New

Current AvailTimes

D3 | Jobs

JobAssign

AvailTimes

Communications Model

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

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:

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

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:

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: .

A connection between entity instances in different entity classes

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

• Must not describe processing

LOGICAL AND PHYSICAL COMPONENTS

Logical Physical

Entity Class Table

Instance Row

Attribute Column

Relationship Junction Tableor Foreign Key

Identifier Primary Key

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

• 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 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: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

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

Instructor assigned to class (section)

Student enrolled in class (section)

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

Section

Instructor

CourseSection

InstructorID

InstructorID

Diagrams:m:n Relationships

Section

Student

CourseSection

StudentID

Student-Section

CourseSection

StudentID

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

0

1

Optional (0 allowed)

Mandatory (1 or more required)

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

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

Cascade delete Cascade update Cascade null

Data