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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
10.2
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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
10.3
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Entity-Relationship (E-R) ModelingKey Terms
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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Entity-Relationship (E-R) ModelingKey Terms
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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Entity-Relationship (E-R) ModelingKey Terms
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
10.12