44220: Database Design & Implementation ER Diagramming Ian Perry Room: C41C Tel Ext.: 7287 E-mail: [email protected]@hull.ac.uk

44220: Database Design & Implementation

ER Diagramming

Ian PerryRoom: C41C Tel Ext.: 7287

E-mail: [email protected]

http://itsy.co.uk/ac/0809/sem2/44220_DDI/

Ian Perry Slide 244220: Database Design & Implementation: ER Diagramming

Conceptual Data Modelling Process

1. Identify ALL of the relevant Entities.• must play a necessary role in the business

system.2. Identify those Attributes that adequately

describe each Entity.• remember to choose ‘key’ attribute(s).

3. Identify the Relationships between Entities.

• determine the Degree of each Relationship:• determine the Type of each Relationship.• attempt to decompose any many-to-many

Relationships that you have identified.


Hospital Example Let us suppose that you have been asked to

build a Conceptual Data Model for a Hospital. The questions you must ask (and answer?)

are as follows: What are the major objects of interest?

Entities What details adequately describe each of these

objects? Attributes

What associations are valid between these objects?

Relationships


What are the Entities? i.e. the major objects of interest.

Doctor Nurse Ward Patient Medication Operation Clinic etc.


What are the Attributes? i.e. the details that adequately describe

each of the Entities. Ward

Name, Location, Number of Beds, … Patient

Name, Address, Date-of-Birth, Gender, … Doctor

Name, Room, Extension, Speciality, … Nurse

Name, Room, Extension, Speciality, … NB.

Perhaps ‘people’ who work in the Hospital could be described using ONE Entity, i.e. Staff?


Entity Definition Syntax:

EntityName (key attribute(s), attribute, ...)

For Example: Staff (FirstName, FamilyName, Role, Room,

Extension, Speciality, …) Ward (WardName, Location, Number-of-Beds,

…) Medication (DrugCode, Type, Dosage, …) Patient (FirstName, FamilyName, DOB, Gender,

Street, Town, Post Code, Tel. No., …)

Always Remember: to indicate the ‘key’ Attribute(s).


‘Type’ of Relationships? i.e. associations which are valid, in BOTH

directions, between pairs of Entities. Patient has Operation.

Operation performed on Patient Staff work in Ward

Ward employs Staff Ward has beds for Patient

Patient stays in Ward Patient attends Clinic

Clinic treats Patient Etc.


‘Degree’ of Relationships? One-to-One (1:1)

Very rare! One-to-Many (1:M)

Quite rare as a ‘real world’ relationship; however, what we require for both Logical and Physical Modelling.

Many-to-Many (M:M) Most common as a ‘real world’ relationship;

however, impossible to implement Logically or Physically.

MUST be decomposed into two One-to-Many relationships linked by an ‘artificial’ Entity.


Occurrence Diagrams? Use these (with values for Key

Attributes) to discover how many occurrences of each Entity are actually on either side of a Relationship.

Staff

Fred Smith

Jane Bloggs

Arthur Jones

Angela Oust

WardWard 1

Ward 2

Ward 3

1 1M 1M M


Participation in a Relationship?

Mandatory: compulsory attachment on both sides (very

rare). Contingent:

compulsion on one side only (most common). Optional:

no compulsory attachment for either Entity (rare).

Inclusive: attachment only occurs when another

relationship exists (very rare). Exclusive:

only one attachment from a set of possible relationships is permitted (quite rare).


Degree, Type & Participation - 1

• One-to-Many, Contingent (compulsion one side)

Ward Patienthas beds for =>1 M

<= stays in

Clinic Patienttreats =>1 M

<= attends

• One-to-Many, Contingent (compulsion one side)

NB. a Patient can NOT both be staying in a Ward AND attending the Clinic, so these two Relationships are also Exclusive.


Degree, Type & Participation - 2

• Many-to-Many, Contingent (compulsion one side)

Patient Operationhas =>M M

<= performed on

Staff Wardwork in =>M M

<= employs

• Many-to-Many, Contingent (compulsion one side)


Decompose Complex Relationships

WardNameFirstName

FamilyName

FirstName

FamilyName

WardName

Staff Wardwork in =>M M

<= employs

• Can’t have any Many-to-Many Relationships; for example this one:

• MUST be decomposed into 2 x One-to-Many Relationships, linked by an ‘artificial’ Entity, i.e.:

Staff WardTeamM 1M1

<= employswork in =>


Rules for Drawing ER Diagrams

Need to look good: So, DON’T draw them by hand!

Need to be well laid out, so that: Entities with several Relationships are in

the centre of the diagram. AND related Entities are adjacent to

each other.

AND Relationship lines do not cross.


The final Degree, Type & Participation Diags.

Ward Patienthas beds for =>1 M

<= stays in

Clinic Patienttreats =>1 M

<= attends

NB. the above Relationships are also Exclusive.

Staff WardTeamM 1M1

<= employswork in =>

Patient OperationPat/OpM 1M1

<= performed onhas =>

i.e. having now ‘solved’ BOTH M:M Relationship ‘problems’.


Plan BEFORE you begin! ‘Patient’ has Relationships with THREE

other Entities: so, place ‘Patient’ in centre of the Diagram.

‘Ward’, ‘Team’ & ‘Pat/Op’ have Relationships with TWO other Entities: so, place these a little further out.

The other 3 Entities (i.e. ‘Staff’, ‘Operation’ & ‘Clinic’ only have ONE Relationship: so, they should be placed on the outside of the

digaram, close to the Entity that they have a Relationship with.

Join all of the Entities: indicating the Degree, Type and Participation

of each Relationship.


ER Diagram for a Hospital

Patient

Clinic

treats =>

1

M

<=

attend

s

Ward

has b

eds fo

r =>

1

M

<=

stays in

#==========#

Operation

M

Pat/Op <=

perfo

rmed

on1

has =>

1 M

Staff

TeamM

M1

<=

wo

rk in

has =>

1


This Week’s Workshop The purpose of this week’s Workshop it to check

that you can create good looking ER Diagrams, using a suitable software tool.

If you haven’t already got a favourite drawing application I suggest you use PowerPoint.

Your ER Diagrams MUST follow the diagram conventions given in this lecture.

The same is true for Assignment 1. Part 1 - Draw Degree, Type & Participation

diagrams, which: depict 4, relatively simple, ‘real-world’ situations.

Part 2 - Begin working on Assignment 1, by: identifying the Entities from the Case Study. then, drawing Occurrence diagrams for each Pair

of Entities.

Documents

44220: Database Design & Implementation ER Diagramming Ian Perry Room: C41C Tel Ext.: 7287 E-mail: [email protected]@hull.ac.uk