H10 ERD Basics

8/21/2019 H10 ERD Basics

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Entity Relationship Diagrams

Basic Elements and Rules


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Introduction to Entity-Relationship (E-R) Modeling

Notation uses three main constructs

Data entities

Relationships

Attributes

Entity-Relationship (E-R) Diagram

A detailed, logical representation of the

entities, associations and data elementsfor an organization or business

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Entity-Relationship (E-R) ModelingKey Terms

Entity A person, place, object, event or concept in the

user environment about which the organizationwishes to maintain data

Represented by a rectangle in E-R diagrams

Entity Type A collection of entities that share common

properties or characteristics

Attribute A named property or characteristic of an entity

that is of interest to an organization

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Candidate keys and identifiers

Each entity type must have an attributeor set of attributes that distinguishes one

instance from other instances of the sametype

Candidate key

Attribute (or combination of attributes) thatuniquely identifies each instance of an entitytype


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Examples

Identify a few entity types, instances,attributes and candidate keys for:

DePaul Campus Connect RegistrationSystem

Illinois Bureau of Motor Vehicles System

Amazon.com Product Information

System


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Depicting Entities and Attributes

Draw a portion of the ERD for each of these systems:

DePaul Campus Connect Registration System

Illinois Bureau of Motor Vehicles System

Amazon.com Product Information System


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Conceptual Data Modeling andthe E-R Diagram

Goal Capture as much of the meaning of the data as

possible

If you know the rules of normalization,referential integrity, foreign keys, etc., this isgood but not as important now. It is much moreimportant to get the organizational data modelcorrect, i.e. to understand the actual datarequirements for the organization.

Result A better design that is scalable and easier to

maintain


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Identifier

A candidate key that has been selected as theunique identifying characteristic for an entity

type Selection rules for an identifier

1. Choose a candidate key that will not change itsvalue

2. Choose a candidate key that will never be null

3. Avoid using intelligent keys

4. Consider substituting single value surrogatekeys for large composite keys


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Relationship

An association between the instances ofone or more entity types that is of

interest to the organization

Association indicates that an event hasoccurred or that there is a natural linkbetween entity types

Relationships are always labeled withverb phrases


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Cardinality

The number of instances of entity B thatcan be associated with each instance ofentity A

Minimum Cardinality The minimum number of instances of entity B

that may be associated with each instance ofentity A

This is also called modality.

Maximum Cardinality The maximum number of instances of entity B

that may be associated with each instance ofentity A


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Naming and DefiningRelationships

Relationship name is a verb phrase

Avoid vague names

Guidelines for defining relationships

Definition explains what action is being takenand why it is important

Give examples to clarify the action

Optional participation should be explained

Explain reasons for any explicit maximumcardinality


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Naming and DefiningRelationships

Guidelines for defining relationshipsExplain any restrictions on participation in

the relationship

Explain extent of the history that is keptin the relationship

Explain whether an entity instanceinvolved in a relationship instance can

transfer participation to anotherrelationship instance

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Documents

H10 ERD Basics