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44220: Database Design & Implementation ER Diagramming

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1 44220: Database Design & Implementation ER Diagramming
Ian Perry Room: C41C Tel Ext.: 7287 1 1 1 1

2 Conceptual Data Modelling Process
Identify ALL of the relevant Entities. must play a necessary role in the business system. Identify those Attributes that adequately describe each Entity. remember to choose ‘key’ attribute(s). 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. 21 20 22

3 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 21 20 22

4 What are the Entities? i.e. the major objects of interest. Doctor
Nurse Ward Patient Medication Operation Clinic etc.

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

6 Entity Definition Syntax: For Example: Always Remember:
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).

7 ‘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.

8 ‘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. 13 11 13

9 Occurrence Diagrams? Staff Ward
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 Ward Ward 1 Ward 2 Ward 3 M M 1 1

10 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).

11 Degree, Type & Participation - 1
One-to-Many, Contingent (compulsion one side) Ward Patient has beds for => 1 M <= stays in Clinic Patient treats => 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. 21 19

12 Degree, Type & Participation - 2
Many-to-Many, Contingent (compulsion one side) Patient Operation has => M <= performed on Staff Ward work in => M <= employs Many-to-Many, Contingent (compulsion one side) 20 20 18

13 Decompose Complex Relationships
Staff Ward work in => 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 Ward Team M 1 <= employs work in => FirstName FamilyName WardName WardName FirstName FamilyName 20 18 20

14 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.

15 The final Degree, Type & Participation Diags.
Ward Patient has beds for => 1 M <= stays in Clinic Patient treats => 1 M <= attends NB. the above Relationships are also Exclusive. Staff Ward Team M 1 <= employs work in => Patient Operation Pat/Op M 1 <= performed on has => i.e. having now ‘solved’ BOTH M:M Relationship ‘problems’.

16 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.

17 ER Diagram for a Hospital
Patient Clinic treats => 1 M <= attends Ward has beds for => <= stays in #==========# Operation Pat/Op <= performed on has => Staff Team <= work in

18 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.

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