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ENMA 6010: Entity Relationship Diagrams 1
ENMA 6010:
Entity Relationship Diagrams
Reference: Wikipedia – Entity Relationship Diagrams
© 2010 – Mark PolczynskiAll rights reserved
ENMA 6010: Entity Relationship Diagrams 2
Goals of System Modeling:
1.To focus on important system features while downplaying less important features,
2.To verify that we understand the user’s environment,
3.To discuss changes and corrections to the user’s requirements at low cost and with minimal risk,
4.To verify that we have documented our understanding in a way that would allow others to construct/maintain the system.
ENMA 6010: Entity Relationship Diagrams 3
Characteristics of Good Models:
1.Graphical, with support for detailed text descriptions: Picture is worth a thousand words picture links to a thousand words.
2.Top-down partitionable: Globe Continents Countries etc…
3.Minimally redundant: Make changes in just one place in the model.
4.Transparent: Requires no expertise in model building to understand the model.
ENMA 6010: Entity Relationship Diagrams 4
Object CActivity 1
Start
Decision?
Object A
Activity 2
End
Activity 3
Activity 4Object B
PFD vs. DFD vs. STD vs. ERD
Function XObject A Object B
Objects C
Object C Object C
Control Flow 1
ERD
ERD
STD turns on DFD,Causes PFD to execute
ENMA 6010: Entity Relationship Diagrams 5
Modeling tools we will be using in this course:
•Context Diagram: Scope and boundaries of model.
• Data Flow Diagram: Movement of data and materials – What?
•State Transition Diagrams: Transitions between system wait states - When?
•Process Flow Diagrams: Actions and decisions – How?
•Entity Relationship Diagram: Information stores – Which?
•Causal Loop Diagram: System cause and effect relationships.
•IBIS Analysis: Description of “fuzzy” decisions.
•Monte Carlo Simulation: Entity flow and resource utilization.
Why do we need all
these models?!
ENMA 6010: Entity Relationship Diagrams 6
Different models reveal different aspects of the system we are
modeling.
Some are not as useful as others, but you may not know which is
most useful when you start.
ENMA 6010: Entity Relationship Diagrams 7
Where do we start?!
An experienced system analyst approaching a new system is confronted with the same dilemma that a new system analyst faces:
Where to start the analysis? This course introduces a number modeling andanalysis tools and processes suitable for application to a wide range of systems.
In terms of an optimal sequence of application of these approaches, the best answer is, of course:
Apply them all at once!
ENMA 6010: Entity Relationship Diagrams 8
Start With Genchi Genbutsu:
•Genchi genbutsu is a principal of lean thinking which roughly translates to: “Go and see for yourself”.
•A good place to start your system analysis is to go out and talk to current or perspective system users to see “how things get done”.
•As you interview people, you will be taking noteson what you hear and see.
•In this course, we capture this info in a basic processflow diagram, to which we add detail as we applyother modeling techniques to the system.
•The entity relationship diagram is another excellent tool for capturingthis information.
ENMA 6010: Entity Relationship Diagrams 9
Where should you start?!
•As you gain experience using different system modeling tools, you will, in fact, apply them all at the same time to capture a description of the system you are analyzing.
•However, with experience, you may find it beneficial to entity relationship diagrams first.
•This tool helps to systematically capture the information obtained through interviews in a standardized format.
•The tool also reveals gaps and inconsistencies in your description of the system that you will want to correct early in your analysis.
•But the ERD can be difficult to grasp, so we have delayed discussing it until after presenting some more (hopefully!) intuitive tools.
ENMA 6010: Entity Relationship Diagrams 10
Simple Example – Hospital System Model
Here is a portion of your notes taken whileinterviewing staff in a hospital:
• Patients are assigned to a ward, and aretreated by doctors.
• Wards contain beds occupied by patients.
• Nurses work in a ward, where they attend to patients.
• Patients consume medications dispensed by nurses.
This simplified narrative description of a hospital system is derivedfrom http://www.umsl.edu/~sauter/analysis/er/er_intro.html
We will use the description to illustrate the primary elements ofentity relationship (ER) models.
ENMA 6010: Entity Relationship Diagrams 11
Simple Example – Hospital System Model
Here is a portion of your notes taken while interviewing staff in a hospital:
• Patients are assigned to a ward, and are treated by doctors.
• Wards contain beds occupied by patients.
• Nurses work in a ward, where they attend to patients.
• Patients consume medications dispensed by nurses.
When creating an ER model, your primary focus is on on:
• “Things” (entities) in the system about which the system needs tostore information, and…
• What information needs to be stored for each entity.
ENMA 6010: Entity Relationship Diagrams 12
Common types of entities about which systems need to store information:
• Tangible objects - MRI machine, lab report
• Roles played - Patient, doctor
• Organizational units - Ward, admissions
• Sites/locations - Factory, warehouse
• Events - Purchase, payment
Relationship to DFD: - DFD models flow of data and materials, - ERD models stored information about data and materials.
ENMA 6010: Entity Relationship Diagrams 13
Object CActivity 1
Start
Decision?
Object A
Activity 2
End
Activity 3
Activity 4Object B
PFD vs. DFD vs. STD vs. ERD
Function XObject A Object B
Objects C
Object C Object C
Control Flow 1
ERD
ERD
STD turns on DFD,Causes PFD to execute
ENMA 6010: Entity Relationship Diagrams 14
Identifying Entities -
Generally speaking, entities are nouns, so lets find some nouns in the text description of the hospital…
• Patients are assigned to a ward, and are treated by doctors.
• Wards contain beds occupied by patients.
• Nurses work in a ward, where they attend to patients.
• Patients consume medications dispensed by nurses.
That was easy!
What next?
ENMA 6010: Entity Relationship Diagrams 15
Attributes:
We said that entities are elements of our system for which we need to store information.
An entity’s attributes are the individual items of information that we need to store.
For example, a patient in the hospital has:• Name• Birth date• Gender• Height• Weight
Note that a patient also has a particular eye color, but this hospital system doesn’t need to store this information, so it is not an attribute of this entity.
ENMA 6010: Entity Relationship Diagrams 16
Key Attributes
•Often, different entities will have the same types of attributes.
•So, both patients and doctors have birth days,And a patient may even have the same birthday as their doctor.
•But every entity must have one attribute that uniquely identifies the entity.
•So, both patients and doctors may have social security numbers,But they never have the same social security number.
•Thus, for this system, social security number is a key attribute.
It can’t be that simple!
What else do we need?
ENMA 6010: Entity Relationship Diagrams 17
Relationships:
• Patients are assigned to a ward, and are treated by doctors.
• Wards contain beds occupied by patients.
• Nurses work in a ward, where they attend to patients.
• Patients consume medications dispensed by nurses.
Relationships describe how entities are related to one another, e.g. a doctor examines a patient, and a patient is examined by a doctor.
To find relationships, just look for the verbs in your text description…
ENMA 6010: Entity Relationship Diagrams 18
Entities are the things that flow through the process we are modeling.• They are the things that get worked on by the process.• Different types of entities can be flowing through the process.
Attributes are characteristics that are common to all entities of the same type.• Entities may have many different characteristics, but attributes are just the
ones that we care about in the model.• For now, our entities have just two attributes: arrival time and service time.
Resources are the things that do the work on the entities.• Just as there can be multiple types of entities in a model, there can be
multiple types of resources.• More than one resource or one type of resource may be needed to work on
an entity.
Note: Arena simulation software also uses the concepts of entity and attribute.Also, Arena resources are a kind of entity in ER diagrams.
Don’t worry about this stuff for now, let’s just keep going…
ENMA 6010: Entity Relationship Diagrams 19
So what we know so far is that entity relationship models consist of:
•Entities, which have attributes,
•Relationships, which describe how entities are related to one another.
All we need to do now is to convert all this into a diagram.
So let’s do it!
ENMA 6010: Entity Relationship Diagrams 20
You may have already seen a simple form of ER diagram…
Assembly Sub-Assembly 1
Sub-Assembly 2
Component A
Component B
Component C
Structured (Indented) Bill-Of-Materials:
• Assembly uses Sub-Assemblies 1, 2….
• Sub-Assembly 1 uses Components A, B, C…
This diagram simply describes the “uses” relationship between assemblies,sub-assemblies, and components.
ENMA 6010: Entity Relationship Diagrams 21
Here’s another simple form of ER diagram…
CEO Vice President of Marketing
Vice President of Engineering
Northeast Regional Sales Manager
Southeast Regional Sales Manger
Org Chart:
• VPs report to CEO
• Sales Managers report to VP of Marketing
The org chart shows the “report to” relationships among staff positions.
ENMA 6010: Entity Relationship Diagrams 22
Here’s What Goes Into an ER Diagram:
Attribute
Entity(Object)
Relationship
Noun, “things” often grouped into sixclasses: Roles, events, locations, physical items, devices, organizational units. May be termed “object”.
Verb (verb phrase) which describes how entities (objects) are related to one another, e.g. a doctor examines a patient, and a patient is examined by a doctor.
Noun, which characterizes an entity, e.g. a doctor has a name, and examines a patient with a name.
ENMA 6010: Entity Relationship Diagrams 23
Here is the General Form of an ER Diagram:
Attribute 1.1
Attribute 1.2
Entity 1 Attribute 2.1
Attribute 2.2
Entity 2RelationshipA
ER Diagrams have Entities, Relationships, and Attributes.
Often times, Entities are termed Objects.
ENMA 6010: Entity Relationship Diagrams 24
Condition
Name*
Patient Specialty
Name*
DoctorExamined ByExamines
Simple model of a patient-doctor relationship:
Here is an Example:
* Indicates a key attribute
ENMA 6010: Entity Relationship Diagrams 25
Condition
Name
Patient Specialty
Name
DoctorExamined ByExamines
Relationships are always bi-directional:
Patient is examined by a Doctor
Doctor examines a Patient
There is a simpler way to show this…
ENMA 6010: Entity Relationship Diagrams 26
Condition
Name
Patient Specialty
Name
DoctorExamined By
Simplified notation:
Patient is examined by a Doctor
Arrow shows how to read relationship
Arrow shows how to read relationship
The arrow simply shows how to read the relationship.
The arrow has nothing to do with flow,since nothing “flows” in an ER diagram.
ENMA 6010: Entity Relationship Diagrams 27
Condition
Name
Patient Specialty
Name
DoctorExamines
Simplified notation:
Doctor examines Patient
Arrow shows how to read relationship
Arrow shows how to read relationship
This diagram is exactly equivalent to the diagram on the previous slide.
ENMA 6010: Entity Relationship Diagrams 28
Here is an even more simplified ER notation that we will use:
Patient - Name* - Condition
Doctor - Name* - Specialty
Examines
Direction
Entity Relationship
Attributes
* Indicates a key attribute
ENMA 6010: Entity Relationship Diagrams 29
In Visio, go here for ER diagrams
It’s convenient to draw simplified ER diagrams
in Visio
ENMA 6010: Entity Relationship Diagrams 30
My favoriteER shapes…
Entity 1
Attribute 1Attribute 2
Entity 2
Attribute 1Attribute 2
Relationship
ENMA 6010: Entity Relationship Diagrams 31
So now we know what the basic elements of an ER model are,
And also the basic components of an ER diagram.
Let’s walk through the construction of a diagramusing our hospital example…
ENMA 6010: Entity Relationship Diagrams 32
Identifying Entities -
• Patients are assigned to a ward, and are treated by doctors.
• Wards contain beds occupied by patients.
• Nurses work in a ward, where they attend to patients.
• Patients consume medications dispensed by nurses.
To help keep track of things, let’s put this into a matrix format…
Noun, “things” often grouped into six classes: Roles, events, locations, physical items, devices, organizational units. May be termed “object”.
ENMA 6010: Entity Relationship Diagrams 33
Entity-Entity Matrix – Step 1: Entities
All we need to do here is go back to the narrative, pull out all the entities,and put them on the axes of the matrix.
Patients Doctors Nurses Wards Beds MedsPatientsDoctorsNursesWardsBedsMeds
ENMA 6010: Entity Relationship Diagrams 34
Identifying Relationships:
• Patients are assigned to a ward, and are treated by doctors.
• Wards contain beds occupied by patients.
• Nurses work in a ward, where they attend to patients.
• Patients consume medications dispensed by nurses.
Relationships describe how entities are related to one another.
To find relationships, just look for the verbs in your text description…
ENMA 6010: Entity Relationship Diagrams 35
Now, we go back to the narrative,pull out all the verbs, and put themin the appropriate cells in the matrix.
Patients Doctors Nurses Wards Beds MedsPatientsDoctors Treated byNurses Attended byWards Assigned to Work inBeds Occupy ContainMeds Consume Dispense
Entity-Entity Matrix – Step 2: Relationships
Note: We are just showing the relationships in one
direction, so only half the matrix needs to be filled in
ENMA 6010: Entity Relationship Diagrams 36
Here’s our basic ER diagram in matrix form with entities and relationships:
• Patients are assigned to a ward, and are treated by doctors.
• Wards contain beds occupied by patients.
• Nurses work in a ward, where they attend to patients.
• Patients consume medications dispensed by nurses.
Patients Doctors Nurses Wards Beds MedsPatientsDoctors Treated byNurses Attended byWards Assigned to Work inBeds Occupy ContainMeds Consume Dispense
ENMA 6010: Entity Relationship Diagrams 37
Here’s our basic ER diagram in matrix form with entities and relationships:
Are Doctors related to Meds?
Are Wards related to Meds?
What about all the other blank cells?
Patients Doctors Nurses Wards Beds MedsPatientsDoctors Treated byNurses Attended byWards Assigned to Work inBeds Occupy ContainMeds Consume Dispense
Hey! What’s with these blank cells?!
ENMA 6010: Entity Relationship Diagrams 38
Missed in original description!
Missed in original verbal description!
Patients Doctors Nurses Wards Beds MedsPatientsDoctors Treated byNurses Attended byWards Assigned to Work inBeds Occupy ContainMeds Consume Perscribe Dispense Stock
Here are two relationships missed in the initial system description.
An advantage of the matrix approach is that it allows thorough examination of all potential relationships.
Now let’s build the diagram!
Entity-Entity Matrix – Step 3: Validation
ENMA 6010: Entity Relationship Diagrams 39
Building the Diagram:
Just for fun, let’s pretend the doctor-patient relationship is the most important part of our system, so let’s start there…
Patient DoctorTreated by
Patients Doctors Nurses Wards Beds MedsPatientsDoctors Treated byNurses Attended byWards Assigned to Work inBeds Occupy ContainMeds Consume Perscribe Dispense Stock
ENMA 6010: Entity Relationship Diagrams 40
Building the Diagram:
Nurses are pretty important, too, so let’s add them next…
Patient DoctorTreated by
Nurse
Attended by
Patients Doctors Nurses Wards Beds MedsPatientsDoctors Treated byNurses Attended byWards Assigned to Work inBeds Occupy ContainMeds Consume Perscribe Dispense Stock
ENMA 6010: Entity Relationship Diagrams 41
Now let’s throw in Wards…
Ward
Assigned toPatient Doctor
Treated by
Nurse
Attended by
Works in
Patients Doctors Nurses Wards Beds MedsPatientsDoctors Treated byNurses Attended byWards Assigned to Work inBeds Occupy ContainMeds Consume Perscribe Dispense Stock
ENMA 6010: Entity Relationship Diagrams 42
Now add Beds…
Ward
Assigned toPatient Doctor
Treated by
Nurse
Attended by
Works in
Bed
Contain
Occupies
Patients Doctors Nurses Wards Beds MedsPatientsDoctors Treated byNurses Attended byWards Assigned to Work inBeds Occupy ContainMeds Consume Perscribe Dispense Stock
ENMA 6010: Entity Relationship Diagrams 43
Now add Medications…
Ward
Assigned toPatient Doctor
Treated by
Nurse
Attended by
Works in
Bed
Contains
MedicationStocks
Dispenses
Consumes
Prescribes
So, that completes our basic ER diagram for our hospital system analysis.
Occupies
ENMA 6010: Entity Relationship Diagrams 44
This is the basic idea behind ER diagrams, now let’s add some detail:
• Entity dictionary,
• Cardinality,
• Logical model vs. physical model,
• ER diagram rules,
• Many-to-many relationships.
ENMA 6010: Entity Relationship Diagrams 45
Entity Dictionary:
Doctor - S.S. No* - Name - Specialty
Earlier, we introduced a matrix notation to keep track of entities and relationships.
You can see that showing attributes on an ER diagram can get quite cumbersome, so you may choose to show an entities attributes in a separate entity dictionary:
Hospital System Entity DictionaryDoctor
S.S.No.*NameSpecialty
PatientS.S.No.*NameCondition
ENMA 6010: Entity Relationship Diagrams 46
Cardinality
Ward
Assigned toPatient Doctor
Treated by
Nurse
Attended by
Works in
Bed
Contains
MedicationStocks
Dispenses
Consumes
Prescribes
OccupiesPatient occupies only 1 bed at a
time.
Bed is occupied byonly 1 patient at a
time.
We need to show cardinality
of relationships
ENMA 6010: Entity Relationship Diagrams 47
Cardinality:
In order to keep things clear and well-defined, a detailed ERdiagram will include “cardinality” notation –
Entity
Entity
Entity
Entity
Exactly 1 (mandatory)
One or more (mandatory)
Zero or more (optional)
Zero or one (optional)
Here’s an example…
ENMA 6010: Entity Relationship Diagrams 48
Example of Cardinality:
Department SupervisorRuns
Employee ProjectWorks on
Staffed by
• Supervisors run departments.
• Departments are staffed by employees.
• Employees work on projects.
Now lets add cardinality…
ENMA 6010: Entity Relationship Diagrams 49
Here is an Example:
Department SupervisorRuns
Employee ProjectWorks on
Staffed by
• One and only one supervisor runs one and only one department.
• Can’t have more than one supervisor running a department.
• Supervisor can’t run more than one department.
ENMA 6010: Entity Relationship Diagrams 50
Here is an Example:
Department SupervisorRuns
Employee ProjectWorks on
Staffed byWorks in
• One or more departments are staffed by one or more employees.
• There is at least one person in each department.
• A person can work in more than one department,but must be assigned to at least 1 department.
ENMA 6010: Entity Relationship Diagrams 51
Here is an Example:
Department SupervisorRuns
Employee ProjectWorks on
Staffed byWorks in
• One or more employees work on zero or more projects.
• Every project has at least one employee.
• Some employees don’t work on any projects.
ENMA 6010: Entity Relationship Diagrams 52
Logical Vs. Physical Models
Here is a good time to distinguish between logical and physical models.
•The logical model shows what could be,
•The physical model shows what actually is for a particular instance.
For ER diagrams, cardinality converts logical models into physical models.
What does that mean?!
ENMA 6010: Entity Relationship Diagrams 53
Cardinality is Implementation Specific:
Department SupervisorRuns
• One and only one supervisor runs one and only one department.
• Can’t have more than one supervisor running a department.
• Supervisor can’t run more than one department.
Here, the business rules of a particular organization dictate that a supervisor can not run more than one department, so the physical model must reflect this cardinality.
But in a logical model, the logical relationship between supervisors and departments doesn’t specify how many supervisors run how many departments., so cardinality is not included.
ENMA 6010: Entity Relationship Diagrams 54
Adding Cardinality to Hospital Model…
Ward
Assigned to
Patient DoctorTreated by
Nurse
Attended by
Works in
Bed
Contains
MedicationStocks
Dispenses
Consumes Prescribes
Occupies
ENMA 6010: Entity Relationship Diagrams 55
Cardinality Matrix:
Showing cardinality on an ER diagram can get confusing, but, again, a matrix representation can help.
Patients Doctors Nurses Wards Beds MedsPatients 0 or more 0 or more 0 or more 0 or 1 0 or moreDoctors 1 or more 0 or moreNurses 1 or more 1 or more 0 or moreWards only 1 only 1 only 1 0 or moreBeds only 1 1 or moreMeds 0 or more 0 or more 0 or more 0 or more
Note that we must show both “ends” of the relationship in this matrix:• A bed can be occupied by 0 or 1 patients.• A patient occupies only 1 bed at a time.
ENMA 6010: Entity Relationship Diagrams 56
Some Rules for ER Diagrams:
• The entities in an ER diagram are things that you must store information on in order for the system to work. If you don’t need the entity to make the system work, get rid of it. Do you really need to include information about janitorial services in your hospital ER?
• Entities can have many attributes, but you really only need to include the essential attributes relevant to entity-entity relationships. A patient’s driver’s license number is an attribute that probably doesn’t effect the doctor-examines-patient relationship.
• Every entity must have a unique key attribute, that allows individual cases (instances) of the entity to be identified. If you can’t tell individual cases apart, then whatever it is that you have identified is not an entity.
ENMA 6010: Entity Relationship Diagrams 57
Some Rules for ER Diagrams:
• If you only have one specific case (instance) of an entity, you don’t need to include it in the ER model. For example, if the hospital has only one doctor, you do not need to include Doctor in the ER model.
• If an entity has just one attribute, it may be that the entity can be included as an attribute of a different entity. Check all single-attribute entities for this possibility.
ENMA 6010: Entity Relationship Diagrams 58
Some Rules for ER Diagrams:
• When identifying entities, eliminate:• Synonyms (multiple names for the same entity)• Homonyms (same name used for different entities).
Synonym: Service Department calls them customers, and Help Desk calls them contacts, but they are the same people.
Homonym: “Market” can be a geographical area to Sales Department and a demographic designation to Marketing Department.
ENMA 6010: Entity Relationship Diagrams 59
Some Rules for ER Diagrams: Attributes and Relationships
A natural way to decomposeER diagrams is to look for
groups of entities which arenot related to each other…
Entity X
Entity Y Entity Z
Entity A Entity B
Entity C
ENMA 6010: Entity Relationship Diagrams 60
Patient - S.S. No.* - Name - Condition
Doctor - S.S. No.* - Name - Specialty
Examines
Some Rules for ER Diagrams: Self-Referencing Relationships
OK, well, what if a doctor examines themself?
Wouldn’t the doctor and patient have the same key attribute?
Here’s how to show self-referencing relationships:
ENMA 6010: Entity Relationship Diagrams 61
Some Rules for ER Diagrams: Multiple Relationships
Entities may have multiple relationships. Here is an example:
PatientDoctorOperates on
Examines
Many other rules for building good ER models exist. These can be found in the references and supplemental reading for this lecture.
We will discuss one additional complication of ER diagrams…
ENMA 6010: Entity Relationship Diagrams 62
Multiple Relationships Between Multiple Entities
Many other rules for building good ER models exist.
We will discuss one additional complication of ER diagrams…
Real Estate Agent
Buyer Seller
Buyer’s Attorney
Seller’s Attorney
Negotiatesprice
Negotiatesterms
Real Estate Agent negotiates price with Buyer and Seller
Real Estate Agent negotiates terms with Buyer’s and Seller’s Attorneys
ENMA 6010: Entity Relationship Diagrams 63
Critical Aspect of ER Diagrams: Many-to-Many Relationships
It is necessary to eliminate many-to-many relationships in the model.
A prime example is seen in our patient/doctor relationship in ourhospital ER model…
Patient - Name - Condition
Doctor - Name - Specialty
Examines
One or more doctors examine one or more patients.
So, who examines who?
ENMA 6010: Entity Relationship Diagrams 64
Resolving Many-To-Many Relationships
•The literature shows several ways to model many-to-many relationships, none of which is particularly elegant.
•But let’s just reason this out how we could resolve this.
•The problem is that if you just say many doctors examine many patients…
•…you don’t know which doctor examines which patient.
•One result of this is that the doctors don’t know which patient to invoice! (Or just invoice them all and let the insurance companies sort things out, since they’ll just deny coverage anyway).
So, what must we add to the model to determinewhich doctor is examining which patient.
ENMA 6010: Entity Relationship Diagrams 65
Resolving Many-To-Many Relationships:
Old: Many doctors examine many patients
New:One or more doctors perform one and only one examination on a particular date and time.One and only one patient receives one and only one examination on the date/time.
Patient - Name* - Condition
Doctor - Name* - Specialty
Examination - Date/Time* - Room
Perform Receives
Patient - Name* - Condition
Doctor - Name* - Specialty
Examine
Add an Examination
entity
ENMA 6010: Entity Relationship Diagrams 66
Resolving Many-To-Many Relationships:
Patient - Name* - Condition
Doctor - Name* - Specialty
Examination - Date/Time* - Room
Perform Receives
Patient - Name* - Condition
Doctor - Name* - Specialty
Examine
In essence, we turned a relationship into an entity by giving it attributes.
By adding the key attribute of Date/Time to the relationship Examine,we now know exactly which doctors examine which patients.
ENMA 6010: Entity Relationship Diagrams 67
Resolving Many-To-Many Relationships:
Patient - Name* - Condition
Doctor - Name* - Specialty
Examination - Date/Time* - Room
Perform Receivies
Note that doctors in the hospital probably do not randomly walk around the hospital examining any patient they run across (how could they bill for that?!).
Rather, they do probably examine patients at some specific time, but this was not shown in our original ER diagram.
The point is that the identification of a many-to-one relationship in our model alerted us to a shortcoming to the model, which we subsequently corrected.
ENMA 6010: Entity Relationship Diagrams 68
So why all the fuss over resolving many-to-many relationships?
Well, if you have them in your model, this means one of two things:
1.Your model isn’t complete and accurate, so you need to fix it.
2.The system you are modeling isn’t well-designed.
Consider the following…
ENMA 6010: Entity Relationship Diagrams 69
Many-To-Many Relationships Can Be Dangerous!
Ward
Assigned to
Patient DoctorTreated by
Nurse
Attended by
Works in
Bed
Contains
MedicationStocks
Dispenses
Consumes Prescribes
Occupies
What patients are consuming what medications?!
ENMA 6010: Entity Relationship Diagrams 70
So What?!
•Your initial introduction to the system that you are modeling will probably consist of interviews with system users.
•You need a way of systematically documenting this information and checking it for accuracy.
•ER diagrams provide one good way to accomplish these objectives,so constitute a tool that can assist in your early modeling efforts.
•That’s why you may choose to apply this tool first.