1. Data Modelling I

2. Plan Introduction Structured Methods –Data Flow Modelling –Data Modelling –Relational Data Analysis Feasibility Maintenance

3. Plan Introduction Structured Methods –Data Flow Modelling –Data Modelling –Relational Data Analysis Feasibility Maintenance

4. Data Model Requirements Designers, end-users and programmers tend to view data in different ways We need a communications tool that will enable models to be understood by all –Non-technical –Un-ambiguous (Connolly & Begg, 2000)

5. Entity Relationship Diagrams Entity Relationship Diagrams are used to represent the data model graphically They show entities in the system and the relationships between them They may also show more advanced concepts Also called the Logical Data Structure There are (again) different notations

6. What will we know? How to identify Entities and Attributes Why we need a Key Attribute How to describe the Relationships between Entities How to draw an Entity Relationship Diagram

7. Business Data We already know a lot about a business’ data from our Data Flow Diagrams We find this from our clearly documented descriptions of: –Data Flows (the arrows in DFDs) –Data Stores (again from our DFDs)

8. Data Dictionary Entry (Mason and Wilcocks, 1994)

9. Structured Analysis + Design Existing System ERD Repository Existing Physical System DFD New System ERD Repository New Logical System DFD Convert to Logical View User Requirements Technical Options + Constraints New Physical System

10. Structured Analysis + Design Existing System ERD Repository Existing Physical System DFD New System ERD Repository New Logical System DFD Convert to Logical View User Requirements Technical Options + Constraints New Physical System

11. Data Modelling There are various things that an organisation needs to store information about In data modelling these things are called Entities (These are NOT the same as external entities in DFDS)

12. Entity A thing that occurs more than once in the system about which we need to store information …occurs more than once… …about which we need to store information… (These are NOT the same as external entities in DFDS)

13. If an Entity = –Student Occurrence = –Wesley Snoopes –Carmen Dizi –Bard Pith Instance If an Entity = –Course Occurrence = –BSc Info Systems –BA Basket Weaving –PhD Surfing Studies Each occurrence of an entity is called an instance or Entity Occurrence

14. Attribute We need to store information about each entity Example: What information does a University need to store about a student? –Name –Address –Phone Number –Course

15. Example Entity - Student –Name –Address –Phone Number –Course Occurrence –Wesley Snoops –13 Manchester Blvd –0161 111 112 –BA Basket Weaving

16. So… Entity –A thing that occurs more than once in the system about which we need to store information Attribute –Information we need to store about each entity

17. Identifying Entities One way of identifying possible entities is to find all the nouns in a description of a system But… –Some of these might be attributes –Some of these might be neither entities or attributes

18. Exercise Of the following, which are Entities? –A member of staff in a personnel system –A book in a library system –A customer in a sales system –A customer’s address in a sales system –A store in the UHS system –The shop in a greengrocer’s shop system –The author of a book in a library system Of the following, which are Entities? –A member of staff in a personnel system –A book in a library system –A customer in a sales system –A customer’s address in a sales system –A store in the UHS system –The shop in a greengrocer’s shop system –The author of a book in a library system

19. Exercise Of the following, which are Entities? –A member of staff in a personnel system –A book in a library system –A customer in a sales system –A customer’s address in a sales system –A store in the UHS system –The shop in a greengrocer’s shop system –The author of a book in a library system

20. Exercise Some of the following are Attributes? –A member of staff in a personnel system –A book in a library system –A customer in a sales system –A customer’s address in a sales system –A store in the UHS system –The shop in a greengrocer’s shop system –The author of a book in a library system

21. Exercise Some of the following are Neither? –A member of staff in a personnel system –A book in a library system –A customer in a sales system –A customer’s address in a sales system –A store in the UHS system –The shop in a greengrocer’s shop system –The author of a book in a library system

22. Entities and Attributes Often an entity in one system will be an attribute in another system and might be neither in another system This depends on the context of the system we are studying We need to be clear about the system and its data and processing requirements

23. Exercise Of the following, which are Entities? –A member of staff in a personnel system –A book in a library system –A customer in a sales system –A customer’s address in a sales system –A store in the UHS system –The shop in a greengrocer’s shop system –The author of a book in a library system Of the following, which are Entities? –A member of staff in a personnel system –A book in a library system –A customer in a sales system –A customer’s address in a sales system –A store in the UHS system –The shop in a greengrocer’s shop system –The author of a book in a library system

24. Exercise Of the following, which are Entities? –A member of staff in a personnel system –A book in a library system –A customer in a sales system –A customer’s address in a sales system –A store in the UHS system –The shop in a greengrocer’s shop system –The author of a book in a library system Of the following, which are Entities? –A member of staff in a personnel system –A book in a library system –A customer in a sales system –A customer’s address in a sales system –A store in the UHS system –The shop in a greengrocer’s shop system –The author of a book in a library system

25. Big Jim’s Bikes Big Jim sells Bikes on the Internet. Customers email Big Jim with their details and those of the bike that they want to buy Bike details are taken from the bike list posted on Big Jim’s Website, where every bike is listed with a unique stock number On receipt of an email Big Jim checks the bike details and then requests payment in full On receipt of payment Big Jim wraps the bike in brown paper and posts it to the customer using Royal Mail Big Jim stores customer details in case they buy another bike. No details of payments are stored.

26. Identify Noun Phrases Big Jim sells Bikes on the Internet. Customers email Big Jim with their details and those of the bike that they want to buy Bike details are taken from the bike list posted on Big Jim’s Website, where every bike is listed with a unique stock number On receipt of an email Big Jim checks the bike details and then requests payment in full On receipt of payment Big Jim wraps the bike in brown paper and posts it to the customer using Royal Mail Big Jim stores customer details in case they buy another bike. No details of payments are stored.

27. Identify Noun Phrases Big Jim sells Bikes on the Internet. Customers email Big Jim with their details and those of the bike that they want to buy Bike details are taken from the bike list posted on Big Jim’s Website, where every bike is listed with a unique stock number On receipt of an email Big Jim checks the bike details and then requests payment in full On receipt of payment Big Jim wraps the bike in brown paper and posts it to the customer using Royal Mail Big Jim stores customer details in case they buy another bike. No details of payments are stored.

28. Identify Entities Big Jim sells Bikes on the Internet. Customers email Big Jim with their details and those of the bike that they want to buy Bike details are taken from the bike list posted on Big Jim’s Website, where every bike is listed with a unique stock number On receipt of an email Big Jim checks the bike details and then requests payment in full On receipt of payment Big Jim wraps the bike in brown paper and posts it to the customer using Royal Mail Big Jim stores customer details in case they buy another bike. No details of payments are stored.

29. Key Attributes How do we identify an individual occurrence? –E.g. we might be searching for a student’s address By adding a Key Attribute –E.g. we add a student number to uniquely identify each student

30. Identifying Entities List some attributes for each entity in Big Jim’s Bikes Identify the Key Attribute for each entity

31. Entity Relationship Diagrams Entity Relationship Diagrams are used to represent the data model graphically They show entities in the system and the relationships between them

32. ERD Elements E.G. In a student enrolment system Student Student Number Name Address Phone Number Course Entity Name Key Attribute Non-Key Attributes

33. ERD Elements Entities - in Big John’s store system Customer Customer Number Name Address Phone Number Product Product ID Description Colour Weight

34. Relationships Entities in a system will be related to one another For example –A student is related to a course because a student is enrolled on a course –A supplier is related to a product because they may supply that product

35. The Matrix Sometimes the relationships are marked on a matrix This is especially useful for large systems

36. Degree/Multiplicity Relationships are always two way Example System Description – Mulchester University –Each student may only enrol on one course –Each course typically has 150 students although this could be higher or lower These describe the same SINGLE relationship

37. Degree/Multiplicity Example System Description – Dulchester College –A student may enrol on any number of courses –Each course typically has 30 students although this could be higher or lower –Here the relationship between course and student is different

38. Degree/Multiplicity Mulchester University –Each student may only enrol on one course –Each course has many students enrolled on it Dulchester College –Each student may enrol on many courses –Each course has many students enrolled on it They have a different degree/multiplicity

39. Degree/Multiplicity Three alternatives –One-to-Many (1:M) –Many-to-Many (M:N) –One-to-One (1:1)

40. One-to-Many Mulchester University Student Student Number Course Course Code Enrolled on Enrols Parent/Master Child/Detail

41. Many-to-Many Dulchester College –Note the different configuration Student Student Number Course Course Code Enrolled on Enrols

42. One-to-One Each course has one course leader and each course leader leads only one course Course Leader Staff Number Course Course Code Responsible for Responsibility of

43. Problem (1) Mulchester University creates a new Course (Origami) during the summer vacation The course details need to be put it on the system before any students can enrol –This means a course may exist with no students –We must be able to show this situation on our ERD

44. Solution (1) Optionality –Each course MAY be enrols students (A course enrols 0, 1 or many students) –Each student MUST be enrolled on a course Student Student Number Course Course Code Enrolled on Enrols Dotted line indicates optional relationship

45. Problem (2) CD Hotels may have Customers who have not yet booked a room They may have Rooms that have not yet been booked by anyone Customer Customer Number Name Address Phone Number Room Room Number Books Booked by

46. Solution (2) Each Customer MAY book rooms Each Room MAY be booked by Customers Customer Customer Number Name Address Phone Number Room Room Number Book Booked by

47. Examples 1 to many (1 or more) 1 to 1 (or 0) 1 to many (0, 1 or more) Many (1 or more) to Many (1 or more) Many (0, 1 or more) to 1 (or 0)

48. Exercise 1 Big Jim sells Bikes on the Internet. Customers email Big Jim with their details and those of the bike that they want to buy Bike details are taken from the bike list posted on Big Jim’s Website, where every bike is listed with a unique stock number On receipt of an email Big Jim checks the bike details and then requests payment in full On receipt of payment Big Jim wraps the bike in brown paper and posts it to the customer using Royal Mail Big Jim stores customer details in case they buy another bike. No details of payments are stored.

49. Identify Entities Big Jim sells Bikes on the Internet. Customers email Big Jim with their details and those of the bike that they want to buy Bike details are taken from the bike list posted on Big Jim’s Website, where every bike is listed with a unique stock number On receipt of an email Big Jim checks the bike details and then requests payment in full On receipt of payment Big Jim wraps the bike in brown paper and posts it to the customer using Royal Mail Big Jim stores customer details in case they buy another bike. No details of payments are stored.

50. Draw ERD Each Customer can buy many bikes Each bike is bought by one customer Big Jim holds customer details before they buy a bike Bike details are held before they are bought Bike Stock Number Customer Customer Number Buys Bought by

51. Exercise 2 Big Jim has decided that it is too much work to list every bike. He now lists bikes by model and customers buy one of that model rather than a specific bike. Each bike still has a unique stock number. Example models in Big Jim’s system are: –Raleigh Chopper –Penny Farthing –Dahon Espresso –Shogun XC200 How does this effect the data model?

52. Solution

53. What do we know? How to identify Entities and Attributes Why we need a Key Attribute How to describe the Relationships between Entities How to draw an Entity Relationship Diagram

54. What problems still exist? There are some problems associated with M:N relationships that need to be resolved We need to examine attributes in more detail We need to check our model against the required transactions There are additional relationships we could describe

55. References Whiteley, D. (2004) Introduction to Information Systems, Palgrave, 2004. Lejk, M. and D. Deeks (2002) Systems Analysis Techniques, Addison Wesley 2002 Mason, D. and L. Willcocks (1994), Systems Analysis, Systems Design, Alfred Waller, 1994.

56. References Yeates, D. and T. Wakefield (2004) Systems Analysis and Design, FT/Prentice Hall 2004 Gane, C. and T. Sarson (1979) Structured Systems Analysis, Prentice Hall, 1979 Eva, M (1994) SSADM Version 4: A users guide, McGraw hill, 1994

57. References DeMarco, T. (1979) Structured Analysis and System Specification, Yourdon, 1979 Royce, W. (1970) Managing the development of large software systems, In: Proceedings of IEEE WESCON, 1970 pp1-9. Connolly, T. and C. Begg (2000) Database Solutions, Addison-Wesley, 2000