Entity Relationship Diagrams ERDs

1
Entity Relationship Diagrams (ERDs) (based on
EasiComp Case Study)

• An ERD is a representative data model

• If a student can enrol on many courses and a
• course can have many students, draw the ERD

2
Entity Relationship Diagrams (ERDs) (based on
EasiComp Case Study)

• Produce a simple ERD for these situations

• A student enrols on a degree course (think!)
• An SEN students receives private tuition

3
Entity Relationship Diagrams (ERDs) (based on
EasiComp Case Study)

• Priority reduce the mn to create 1n or 11

• Create a new entity to link the mn with 1n

• The entity enrolment relates to student course

4
Entity Relationship Diagrams (ERDs) (based on
EasiComp Case Study)
Relationships

• Each course has many teachers

• Each teacher teaches many courses

• Each course takes place in many rooms

• Each room hosts many courses

• BUT!

• Some teachers only teach one course

• Some courses only run in one room

• The mn relationship can cope with 1n

• The 1n cannot cope with mn relationship

5
Entity Relationship Diagrams (ERDs) (based on
EasiComp Case Study)

• So many courses in many rooms with many teachers

• We need to remove the mn and produce 1n

• We need to create a common entity linking

• Course, Room and Teacher

• which would be LESSON

• So

6
Entity Relationship Diagrams (ERDs) (based on
EasiComp Case Study)

• These must now be amalgamated

7
Entity Relationship Diagrams (ERDs) (based on
EasiComp Case Study)

• Amalgamated to reduce mn to 1n

• Now we can complete the ERD by including
• ENROLMENT added earlier

8
Entity Relationship Diagrams (ERDs) (based on
EasiComp Case Study)

• The completed ERD now looks like

• All mn relationships have been reduced to 1n

• All one entities are on upper level

• All many entities are on lower level