Modeling Relationships Modeling Relationships

Farrokh Alemi, Ph.D.

Updated by Janusz Wojtusiak, Fall 2009

Definition of RelationshipDefinition of Relationship

Relationship describes the link between two entities

Different words for the same idea – At the conceptual level (e.g. entities and their

relationships) – At the physical level (tables, data classes and

linkages)

RelationshipsRelationships

A relationship is ALWAYS between two entities.– If you have three entities, you need at least two

relationships to link them.

Relationships Are Identified Relationships Are Identified Through SentencesThrough Sentences

A sentence containing the names of the two entities and a verb phrase in between.  – Verb phrases:  "has," "contains," "visits,"

"prescribes," "travels with," etc. – The two entities "Patient" and "Clinician" can

be made into a sentence such as "A patient visits a clinician." 

Relationships RevealedRelationships Revealed

Is one of the entities a look up table used to provide menu items for an attribute of the other entity. 

Is one of the entities a subcategory to another. 

Do two entities share an attribute. 

Relationships DocumentationRelationships Documentation

The name of first entityThe name of second entityThe verb phrase describing the relationshipThe cardinality of the relationship

– one to one, one to many or many to many

Business justification of the relationship, particularly its cardinality. 

One-to-One RelationshipsOne-to-One RelationshipsThis type of relationship often suggest that

entities have been incorrectly designedDatabase designer must identify a good

reason for using this type of relationshipOne-to-one relationships are used in

specific situations, such as:– To avoid missing data– To model hierarchies– For privacy/security reason– To break very large tables if some fields are

rarely accesses at the same time.

One-to-One RelationshipsOne-to-One Relationships

This type of relationship in realized by using a common identifiers in both entities

Employee

EmployeeIDFirstNameLastName…

EmployeeSecurity

EmployeeIDDateObtainedSecurityLevel…

One-to-One RelationshipsOne-to-One Relationships

More examples?Justification?

One-to-Many RelationshipsOne-to-Many RelationshipsThis is the most important type of relationship in

relational databasesThey are used to represent all types of relationships

in relational databases

Relational databases are about one-to-many relationships

Majority of relationships in databases are of this type.

One-to-Many RelationshipsOne-to-Many Relationships

One-to-Many relationships are represented by listing an identifier of “one” in “many”

Employee

EmployeeIDFirstNameLastName…

EmployeeAddress

AddressIDEmployeeIDDateObtainedLevel…

One-to-Many RelationshipsOne-to-Many Relationships

More examples?

Some Relationships Suggest Some Relationships Suggest New EntitiesNew Entities

Hierarchical Many to many relationships

Definition of HierarchyDefinition of Hierarchy

A collection of super and sub entities.– A super entity is the broadest definition of

several sub entities. – A sub entity is an entity that inherits its

relationship from another entity.

Examples of HierarchiesExamples of Hierarchies

VehiclesOrganismsDrugsParts of a complex deviceAny other examples?

Generalization & SpecializationGeneralization & Specialization

The process of abstracting from narrowly scoped terms into terms of broader scope is a generalization process

Starting with a general term and narrowing its scope is a specialization process.

An Example of Generalization & An Example of Generalization & SpecializationSpecialization

Hiearchy May Simplify Data Hiearchy May Simplify Data StructuresStructures

Specializations of the more general data class Person

Oftentimes, introducing a subtype hierarchy can simplify the information model

Rules for Identifying HierarchiesRules for Identifying Hierarchies

Super-type is the broadest entity,  should have all the attributes that are shared across the sub-entities.  

Each sub-entity should have an attribute that makes it different from other  entities.  – The attributes should be

mutually  exclusive. 

Rules for Identifying HierarchiesRules for Identifying Hierarchies

ConfusionConfusion

It may not be possible for sub types to unambiguously reside in one and only one of the subtypes specified in the hierarchy. – Refine further– If confusing, do not do it

Steps for Identifying a HierarchySteps for Identifying a Hierarchy

1. Shared attributes

2. Discriminator attribute

3. Entity linkages at the super type level

Step two: DiscriminatorStep two: Discriminator

ExampleExample

Step Three: Relations at Super Step Three: Relations at Super Entity LevelEntity Level

Reduces unnecessary relations– Simpler to read– Easier to implement

Advantages of HierarchiesAdvantages of Hierarchies

Stabilize  the overall model with respect to new requirements.

An exampleAn example

New Requirement: “Patient of record” becomes inactive but the

system does not purge it

Insert a new entity in the  Insert a new entity in the  hierarchyhierarchy

Include the new entity in the discriminator attribute

Check that all sub entities are appropriately named

New Data StructureNew Data Structure

Check Sub EntitiesCheck Sub Entities

The first issue is whether the sub entity is distinct form other entities 

Check Super EntityCheck Super Entity

Do the attributes in the super entity apply equally well to the new sub entity

Check DiscriminatorCheck Discriminator

The third issue is whether the discriminator attribute in  the super-

entity applies to all sub types

The importance of  good entity and attribute definitions

HierarchiesHierarchies

Let’s work on some examples.

Many to Many RelationshipsMany to Many Relationships

ExampleExample

Patient Clinician AssociationPatient Clinician Association

Association Class SimplifiesAssociation Class Simplifies

Resolve  all many-to-many linkages A simpler way of capturing and tracking

relationships

In relational databases many-to-many relationships must be implemented through association classes.

Many-to-many relationships Many-to-many relationships

Examples:– Patient – clinician– Insurance company – patient– Nurse – bed

– Can you give more examples?

Back to HierarchiesBack to HierarchiesWhat happens if we have more than one

role in a hierarchy?

Self ReferralSelf Referral

A Person to Person Association A self referring relationship is found in the

same way as all relationships are found by making a sentence containing the name of the entity and a verb phrase.

Take Home LessonsTake Home Lessons

1. We saw that scenarios focus on decisions

2. Scenarios contain use cases which reveal the information exchange

3. Exchanges are used to specify fields

4. Business rules define relations

5. One-to-many is the most important type of relation.