1. Mr C Johnston Sidney Stringer Academy
BTEC NCF IT Unit 02 Creating Systems to Manage Information Lesson 03 – Entity Relationships
Mr C Johnston
Sidney Stringer Academy

2. Learning Outcome Understand basic entity relationships
Understand how entity relationships can be used to keep data integral
Able to model and apply entity relationship diagrams to situations

3. STARTER
Look at the database table below… Are there any issues?

4. Problems with the Database
Repetition of data:
Supplier name and phone number entered in each record,
Wastes space
Inconsistency in data:
Same supplier spelt differently,
Same phone number has mistakes

5. The Solution To spilt the database up into different entities!
In this model we would create two entities
One for the products
One for the suppliers
So that the database works we need to join the two entities together to form a relationship.

6. Entity Relationships
A relationship is a link or association between entities
It represents the real life relationship between entities e.g.
Normally one dentist has many patients
There are three types of relationships in database land
One – to – One
A husband has one wife (at a time) AND a wife has one husband (as a time)
One – to – Many
A ward holds many patients
A landlord has many houses
Many – to – Many
One album can feature many artists BUT one artist can feature on many albums
A chocolate bas has many ingredients BUT An ingredient can be used in many different chocolate bars

7. QUESTION What is the relationship between these two entities?
One supplier has many products
One supplier has many products BUT one product can come from many suppliers

8. Entity Relationships Diagrams
This is a way of showing relationships between entities,
Entity names are always written in capitals and appear in rectangles,
Lines join entities together and have:
Symbol to show the degree of the relationship,
A description to show the name of the relationship,

9. Entity Relationships Diagrams
HUSBAND
WIFE
has
WARD
PATIENT
holds
ALBUM
SINGERS
features

10. Relationship Rules
Within database land it is not feasible or sensible for all three relationships to exist
ONE-to-ONE relationships normally get collapsed and one entity becomes an attribute of the other
MANY-to-MANY relationships are very common BUT there needs to be a way the DBMS knows which data is which

11. ONE-to-ONE Collapse CUSTOMER DETAIL has custno lastname firstname 0001
henry
john
0002
smith
adam
0003
johnson
mary
0004
bailey
harry
0005
morgan
alex
custno
height
weight
dateofbirth
0001
181 75
03/05/1960
0002
179 82
06/08/1974
0003
171 65
04/01/1955
0004
185 93
05/05/1980
0005
168 71
09/09/1975
custno
lastname
firstname
height
weight
dateofbirth
0001
henry
lohn
181 75
03/05/1960
0002
smith
adam
179 82
06/08/1974
0003
johnson
mary
171 65
04/01/1955
0004
bailey
harry
185 93
05/05/1980
0005
morgan
alex
168 71
09/09/1975

12. Many–to–Many Resolve
Many - to – Many relationships are not allowed so we have to resolve them some how.
To do this we introduce an additional entity between them
Thus creating 2 one-to– many relationships.

13. ACTIVITY
Draw an entity relationship diagram to show the relationship between
A product and its components
A house and street
A pet owner and their pets
Draw an entity relationship diagram providing a many – to – many resolve to show the relationship between
Ferries and their routes
Customers and videos in a rental shop
Students and courses in a school
Patients and medicine in a hospital

14. EXTENSION ACTIVITY
List a further three situations where a many-to-many relationship exists between two entities – give them to somebody else to draw the required ERD
Carry out some research to discover how we link the entities together within a DBMS