Monica Farrow EM G30 [email protected] Material on Vision Er 1 F28DM: Database Management Systems...

Monica Farrow EM [email protected]

Material on Vision

Er 1

F28DM: Database Management Systems

The Entity Relationship Model

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Topics today

• Conceptual database design• ER diagrams

• Notation• Definitions• Entities• Relationships• Cardinality constraints• Participation constraints• Limitations (other constraints)• Fan and chasm traps• Extended ER diagram

• Examples – Departmental, Hospital

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Database DesignDatabase design goes through several stages.

Physical Database Design

Logical Database Design

Data Analysis, Conceptual Design

Problem

Data Requirements

Conceptual Data Model

Storage structure and File Organisation

Logical Schema

Requirements Analysis

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Conceptual Database Design• Process of constructing a model of the data used

in an enterprise, independent of all physical considerations.

• E.g. ER diagram

• Conceptual database design is top-down design as you start by specifying entities (real-world objects) then build up the model by defining new entities, attributes, and relationships.

• The data model is built using the information in the users’ requirements specification.

• The conceptual data model is the source of information for logical design phase.

© Pearson Education Limited 1995, 2005

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Conceptual Design with the ER Model

The questions we need to ask are:

1. What are the entities (e.g. objects, individuals) in

the organisation?

2. Which relationships exist among these entities?

3. What information do we want to store about

these entities and relationships?

4. What are the business rules of the organisation?

5. Which integrity constraints do arise from them?

The answers are mostly represented in an Entity

Relationship diagram

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ER Diagrams

• There are quite a few different notations for ER diagrams• In year one, we used UML notation• Chen and Crow’s Foot notations are also

commonly used• There are others as well.....

• We’ll look at these 2 notations and some variations as we look at the components of an ER diagram

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No RDBMS design decisions!

• At this stage, we are ONLY concerned with modelling entities in the real world, and the connections between them, as we need it for our database requirements

• We are NOT thinking about how to store the data in the tables• So you will see some representations of data

that will have to be changed when converting into tables

• Don’t include foreign keys at this stage

• We COULD use the ER diagram as a basis for an Object Oriented database

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UML notation

• In this diagram• 2 Entities (SpyMaster, Spy) are connected by a relationship• One SpyMaster manages at least one and maybe multiple

Spies• One Spy is managed by exactly one master.• A Spy has attributes (name, date of birth etc)• The codeName uniquely identifies the Spy, and is used as a

primary key.

SpycodeName {PK}firstNamedateOfBirthgendermarkamountDue

SpyMaster

mCodeName {PK}contact

1..*

1

manages

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Other notations

• This shows the same information as the previous slide, using Chen notation

• Crow’s feet notation is similar to UML notation, with different symbols on the relationships

1SpySpyMaster

mCodeName

contactcodeName

firstName

etc

managesn

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Example ER Diagram for a Company database

• A company has several departments. Each department has a unique number and a unique name, and may be based in several locations (e.g. Hillhead, Overton, MillEnd). A department has a manager.

• A department controls various projects. Projects also have a unique ID and name, and several deadlines (e.g. startup, intermediate, final).

• Information required about the employee is a unique ssn (social security number), his name, date of birth, age, gender, salary. We don’t need to know about which employee works at which location.

• An employee works for one department and may work on several projects. The number of hours that they are allocated for each project must be recorded. Most employees are supervised by another employee.

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Company ER Diagram

Assumes we only want to know about which depts in which locations, not which employee in which location etc

Departmentdept number {PK}dept name {AK}location [1..5]

Employee

ssn {PK}name first names last namesdate of birthgendersalary/age

1..* 1

works for

1 0..1manages

Projectproj number {PK}project name {AK}

1..*

0..*

controlshours

works on

1

1..*

has

10..*

supervises

0..*

0..1

Deadlinesdate type

start date

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Entity

• Entity: a thing or object of importance, about which data must be captured• A entity is described by a set of attributes and

may have relationships with other entities

• Entities can be objects with a physical existence• E.g. Employee, Department

• Or with a conceptual or abstract existence• E.g. deadline

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Entity Type, Entity occurrence

• Entity Type (or Set): A group of objects with the same properties • E.g. all employees in the system

• Entity Occurrence: A uniquely identifiable object of an entity type• E.g a particular employee “John Smith”

• An entity set consists of entity occurrences and can be shown in a semantic net like this:

John Smith

TimGrey

AmyWhite

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Attributes 1

• An attribute is a property of an entity or a relationship type• E.g Employee name, staff number, date of birth

• The attribute domain is the set of allowable values E.g.:• 6 digits for the staff number• Up to 20 characters for a name• A set of building names (Earl Mountbatten,

David Brewster etc)

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Simple or composite

• Attributes can be simple or composite• Staff number is just 6 digits – simple• Name could be divided into first names and last name.• Date of birth consists of day, month, year

• Decide whether to model name as a simple attribute or to subdivide it, depending on how it will be used in the application

• As individual components, or not

• Show composite attributes indented Employeessn {PK}name first names last namedate of birthgender

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Single and multi-valued attributes

• Single and multi-valued• Most attributes are single-valued (date of birth,

name)• A company may have 3 telephone numbers –

multi-valued• A department may have different locations

• Show permitted number of attributes in square brackets

RoomroomID {PK}tel [1..3]

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Derived attribute

• A derived attribute is an important attribute, but one that should be calculated from other data

• E.g. we often need to know a person’s age, but this changes over time. It is best just to work it out when we need it .

• Precede with a slash

• Likely usage - fairly rare• Eg. result of calculation

such as duration, age, actual miles per gallon of car

Employeessn {PK}name first names last namedate of birth/age

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Keys

• A minimal set of attributes which uniquely identifies each occurrence of an entity type is called a candidate key

• E.g. Employee could be identified by staff number or NHS number

• Names are a poor choice of key, they might not be unique

• Select one candidate key to uniquely identify the entity in your system – the primary key

• E.g. staff number.• Often there is only one candidate key

• Show primary keys with {PK}• Alternative or candidate keys {AK}

Employeessn {PK}nhsNo {AK}name first names last namedate of birthgender

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Problems with keys

• Sometimes it might be hard to choose a key• An Address entity

• Street number and postcode?• Do all addresses fit this pattern?

• A Volunteer for a charity• May not be assigned any sort of id number• Name might not be unique• Invent one? E.g. initials and sequence number• Use an ‘autonumber’ – meaningless but unique

• This problem should be noted but decisions can be delayed until later in the design.

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Composite key

• A candidate key that consists of more than one attribute is called a composite key• i.e. more than one attribute is needed to make

the entity unique• E.g. Keep a list of company contacts. Some

companies have branches in various towns. So the company name and the town is needed to identify the company.

CompanyContactcompany name {PK}town {PK}contact namecontact phone

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Strong and weak entity types

• A strong entity is not existence-dependent upon other entities • Employee and Department entities both exist in

a company

• A weak entity is existence-dependent upon some other entity. Details about this weak entity would not be kept in the database without the other entity, and it cannot be uniquely identified without knowing about the associated entity.• E.g. the details of one item in an order

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• Weak entities are quite confusing, and there can quite often be a debate as to whether an entity is weak or not

• It’s not possible to identify candidate keys for a weak entity

• Think about possible OrderItem entities• Order 5 could contain an order for 10 of item number 51• Order 6 could also contain an entry for 10 of item number

51• We need to include the order number to make the entry

unique – but don’t do this in the ER diagram, leave as a weak entity

Orderorder number {PK}customer numberdate of order

OrderItemItem numberquantity

1

has

1..*

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• At this stage, another way of thinking about the weak entity is to consider it as a composite multivalued attribute:

Orderorder number {PK}customer numberdate of orderorder item [1..*] item number quantity

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Relationships• Relationship: An association among 2 or

more entities. (example on next slide)• In UML notation, the relationship should be

labelled, and a black triangle indicates the direction that the label should be read

• E.g. Employee works on Project

• Relationships may have attributes• E.g. Employee John Smith works on Project X for

10 hours

• We can also show how many entities participate in the relationships, shown here using 1..*

• More about this in a later slide

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Relationships example

Projectproj number {PK}project name

Employee

ssn {PK}name first names last namesdate of birth

1..* 1..*works on

hours

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Relationships• Relationship Type or Set: A collection of similar

relationships• The Set is the set of all lines in the semantic net below

• Relationship occurrence: a uniquely identifiable association, which includes one occurrence from each particpating entity type

• E.g. John Smith works on project X• Each line below is an occurrence

John Smith

TimGrey

AmyWhite

X

A

RR

X

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Relationships• There may be more than one

relationship between 2 entity types

Departmentdept number {PK}Dept name

Employee


1..* 1works for

11manages

Er

28

Complex relationships• Introduce a diamond (as in Chen notation) to

represent relationships between more than two entity types

• For a particular course and programme, there can be many students. It’s possible that no students choose this course for a programme.

• For a particular student and course, they are only taking one programme

• For a particular student and programme, there should be 8 courses – but allow less in case of exemptions

Programme

Student

Coursestudies

0..*

1 1..8

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Exceptions to the rule

• Never impose ‘normal’ constraints on a database system if there could be exceptions to the rule!

• E.g. The normal number of courses for a student to take is 8, but occasionally students may take more or less.• More if repeating a course from previous year• Less if some sort of part-time arrangement

Er 30

Recursive relationships• A recursive relationship is when an

entity has a relationship with itself• E.g. • A member of staff may supervise one or more

other members of staff• A member of staff may be supervised by

another member of staff

supervises

Staff0..*

0..1

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Cardinality Constraints (1)• Cardinality = how many

• When reading, ignore the number nearest to the first entity mentioned

• Start sentence with ‘A’ or ‘An’• End sentence with the details in the numbers

nearest the second entity• A department is worked for by many employees • An employee works for one department

Departmentdept number {PK}dept name

Employee


1..* 1works for

11manages

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Cardinality Constraints (2)• Cardinality = how many• 1:1

• An employee manages one department• A department is managed by one employee

• 1:Many• An employee works for one department• A department has many employees

Departmentdept number {PK}dept name

Employee


1..* 1works for

11manages

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Cardinality Constraints (2)• Many : Many

• An employee works on many projects• A project has many employees working on it

Projectproj number {PK}project name

Employee


1..* 1..*works on

hours

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Cardinality Constraints (3)

One-to-One One-to-Many Many-to-One Many-to-Many

• Cardinality constraints can be shown by a semantic net

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Participation Constraints• The participation constraint determines

whether all or only some entity occurrences participate in a relationship• Every department must have a manager• Not all employees are managers

• In UML notation, the multiplicity determines optional or mandatory participation• 0..* means optional• 1..* means mandatory

• In other notations, participation may be shown by vertical bar on the relationship near the entity, or by open and closed circles, or thicker relationship lines

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Other constraints

• There are various constraints that can’t be shown on an ER diagram, for example:• Domain constraints

• E.g. gender can only be M or F• Multi-table constraints other than the

relationships• E.g. Employees aged under 21 cannot work on

Project X• E.g. The time ranges allocated to a module

cannot overlap

Er 37

Conceptual Design using the ER Model

• Design Choices:• Should a concept be modelled as an entity or an

attribute?• Should a concept be modelled as an entity or a

relationship?• Identifying relationships: binary or ternary

• Constraints in the ER Model:• A lot of data semantics can (and should) be captured.• Keep the initial ER model like the real world

• E.g. don’t show foreign keys• When converting to tables, various relational database

rules may change some attributes to entities

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Fan traps

• These obscure the relationship between two entities

• In the following, each student is attached to exactly one department as is each course, but you cannot tell which courses each student attends.

• Fan traps occur when two supposedly related entity types are connected only through many-to-one relationships with some third entity type

Course StudentDepartment1 11..* 1..*

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Fan traps

• The solution is to change which entity type is at the centre or maybe to add a new relationship • Depending on whether there is a significant link

between student and department, or whether the link is just through the course

Student DepartmentCourse1 11..* 1..*

Student Department

Course111..*

1..*

11..*

Er 40

Chasm traps

• These imply relationships between entity types which do not hold

• In the following, it has been decided to model the student information using the intuition that a student’s department can be inferred through a supervisor.

Student DepartmentSupervisor0..1 11..* 1..*

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Chasm traps

• However, not all students have supervisors (partial participation), so what is the department of such students?

• The solution is to put in a direct relationship between student and department

Student DepartmentSupervisor0..1 11..* 1..*

11..*

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Limitations to ER diagrams

• We’ve covered the basic ER diagrams• An extended version (brief details on next

slide) uses generalisation and specialisation to cover more complex situations

• In your coursework, if you find some of your constraints cannot be captured in your ER diagram, make notes of these and hand in as text.

Er 43

Extended ER Diagrams

• See diagram on the next slide• Uses generalisation and specialisation• ALL Employees have the attributes in the

generalised superclass Employee • Extra attributes must be kept according to

the role of the employee (e.g. manager or sales personnel) and according to their employment category (salaried or hourly paid)

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Employee


Manager

mgrStartDate

Salaried

salary scale

HourlyPaid

rate

SalesPersonnel

salesArea

(Optional, And) (Mandatory, Or)

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Company ER Diagram

Departmentdept number {PK}dept name {AK}location [1..5]

Employee

ssn {PK}name first names last namesdate of birth/age

1..* 1works for

1

0..1manages

supervises

0..*

0..1

ManagermgrStartDate

SalesPersonnelsalesArea

(Optional, And)

SalessaleDatesalePrice

1..*1

Fragment of ER diagram showing that relationships can be shown from both the generalised and the specialised entities – as appropriate

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• Participation constraints are shown with the labels (Mandatory or Optional)• Employees do not have to be managers or sales

personnel• Employees must be salaried or hourly paid

• Disjoint constraints are shown with the labels (And or Or)• Employees could be both a Manager AND

SalesPersonnel• Employees must be either Salaried OR

HourlyPaid, not both

Er 47

Conceptual Design: Summary• Conceptual design

• Follows requirements analysis • Is performed at a high level of abstraction*

involving entities, relationships and attributes• Yields a high-level description of data to be stored

• The Entity Relationship Model is popular for conceptual design• But can be very subjective

• * Abstraction = hiding implementation details

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Conceptual Design: Summary• Include integrity constraints in the ER

model, • key constraints, participation constraints,

overlap/disjoint, etc

• *** Foreign key constraints are implicit ***• i.e. shown by the relationship lines ONLY, not

described as attributes• In an ER diagram, each attribute occurs only

once, in the most logical place

• Constraints are important to determine the best DB design for an organisation

Monica Farrow EM G30 [email protected] Material on Vision Er 1 F28DM: Database Management Systems...

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