Database Design (3) IS 240 – Database Management Lecture #12 – Prof. M. E. Kabay, PhD, CISSP Norwich University

2 Topics Subtypes Trees Networks Bill of Materials Surrogate Keys Null Values

3 Subtypes (1) Consider a currency-trading operation Theres a counter for walk-in anonymous clients Need only date, time, currency_in, rate, currency_out, fee; customer_ID is set to 0. People who use the service often can become regular customers and receive a monthly statement of all transactions In addition to info above, need name, customer_ID, address etc. Corporate customers have a corporate_ID and a list of authorized users Their monthly report includes a total_fees field that gives the aggregated total for all users

4 Subtypes (2) Show diagrams for the relations above as subtypes of customer Why not lump all the customers into a single relation?

5 Subtypes (2*) Show diagrams for the relations above as subtypes of customer Why not lump all the customers into a single relation? Customers Walk-inRegular IndividualCorporate

6 Subtypes (3) Show the tables you could create to represent the situation described above

7 Subtypes (3*) Show the tables you would create to represent the situation described above Cust_IDCust_info 0Walk-in 1name1 Individual Customers Cust_IDDate_etc 0… 1… Individual Transactions Corp_IDCorp_info 100corp1 200corp2 Corporate Customers Corp_IDEmpno 100emp1 200emp2 Corporate Employees Corp_IDEmpnoDate_etc 100emp1… 200emp2… Corporate Transactions

8 Trees Many systems involve hierarchical 1:N relations Administrative hierarchies Department, directorate, division, unit… Polychotomous classifications of various kinds Kingdom, phylum, class, order, family… Every element in tree is a node Top level of classification: root Parent nodes have child nodes in a branch A child node can have one and only one parent

9 Trees (contd) How can you represent the relationships in a tree? Consider a system for tracking students in college dormitories: College Dormitory Floor Room Student

10 Networks In simple networks an entity may have more than one parent The parent entities have M:N relations to the child entities Give an example of a network using doctors, nurses and patients in a health-care system

11 Network: Doctors, Nurses, Patients Draw the health-care example you have developed

12 Network: Doctors, Nurses, Patients* Draw the health-care example you have developed DoctorNursePatient Doctor-PatientNurse-Patient

13 Bill of Materials In manufacturing systems, many components may contain many parts Trying to model these relations caused headaches in the early years of DBMS design Why? Solution: treat the relation as recursive Every element is described in its own table Every relationship is described by the container and the contained elements Structure thus very simple – now draw it!

14 Bill of Materials Design

15 Bill of Materials Design* Kroenke p. 173

16 Surrogate Keys To link two relations in a relationship table, we need unique IDs What if the key we use when designing a relation is not in fact unique? What happens to our tables? What if the keys we use are very long? What about efficiency?

17 Surrogate Keys (contd) DBMS can supply a system-assigned unique identifier Typically a simple number Takes little room Easy to use in indexing Prevents duplication of keys How do we use these unique identifiers in our relationship table? Show an example with doctor/patient What is the main disadvantage of this surrogate key technique?

18 Null Values What are the possible meanings of a field that has not been filled in by a user (a null or empty field)? For inappropriate values (e.g., gender-specific attributes), should you always use subtypes? Can also assign a blank value ab initio Change blank to N/A or UNKNOWN if necessary Must handle these values in code (why?)

19 HOMEWORK Review Chapter 6 of Kroenke Prepare for next class by applying SQ3R to chapter 8 of Kroenke. REQUIRED: By Tuesday 6 April, complete and hand in answers to review questions 6.25 through 6.36 for 24 points. OPTIONAL: Also by Tuesday 6 April, for an extra 2 points each, complete any or all of Projects 6A, 6B or 6C. (More homework possibilities for extra credit on next page)

20 HOMEWORK (contd) For a 2 extra points each, complete any or all of the FiredUp projects A through D on pp To submit B you must first do A To submit D you must first do C

21 DISCUSSION