1. VCE IT Theory Slideshows - ITA By Mark Kelly mark@vceit.com Vceit.com Entity Relationship Diagrams (ERD)

2. This is the VCAA sample ERD, published at http://www.vcaa.vic.edu.au/vcaa/v ce/studies/infotech/it- applications/publications/IT-Apps- samp.pdf http://www.vcaa.vic.edu.au/vcaa/v ce/studies/infotech/it- applications/publications/IT-Apps- samp.pdf

4. ENTITY

5. RELATIONSHIP

6. ENTITY ATTRIBUTE RELATIONSHIP

7. ENTITY ATTRIBUTE RELATIONSHIP KEY FIELD

8. In the real world, primary field names are often underlined.

9. Reading the ERD A Teacher

10. Reading the ERD supervises

11. Reading the ERD subjects

12. Reading the ERD Each subject has a name attribute

13. Reading the ERD The primary key for each subject is the SubjectID field

14. Reading the ERD A teacher also supervises classes

15. Reading the ERD Students belong to classes

16. But … The VCAA sample ERD has NO CARDINALITY i.e.no indications of – 1:1 – 1:many – many:1 – many:many relationships

17. We have to assume… No cardinality is shown. Can one teacher supervise more than one subject or class?

18. We have to assume… Can one subject or class be supervised by more than teacher? This ERD does not say. Use the case study and common sense.

19. Unofficial additions… The ‘many’ end of a relationship can be shown as “N” or “  ” … N  1

20. Unofficial additions… “Crow’s feet” notation of “many” “Crow’s feet” notation of “1”

21. Optional Advanced Crowsfeet If we wished to indicate that a school may enrol many students, or may not enrol any students at all. We also wish to indicate that each student attends exactly one school. This diagram indicates this optionality and cardinality:

22. The VCAA sample exam question List the three basic elements of entity relationship diagrams and provide an example of each from the diagram. (3 marks)

23. ENTITY ATTRIBUTE RELATIONSHIP KEY FIELD The answer

24. By Mark Kelly mark@vceit.com vceit.com These slideshows may be freely used, modified or distributed by teachers and students anywhere on the planet (but not elsewhere). They may NOT be sold. They must NOT be redistributed if you modify them. VCE IT THEORY SLIDESHOWS