Database Design (3) IS 240 – Database Management Lecture #12 – 2004-03-30 Prof. M. E. Kabay,...

Database Design (3)

IS 240 – Database ManagementLecture #12 – 2004-03-30

Prof. M. E. Kabay, PhD, CISSP

Norwich University

mkabay@norwich.edu

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Topics

SubtypesTreesNetworksBill of MaterialsSurrogate KeysNull Values

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Subtypes (1)

Consider a currency-trading operationThere’s 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 transactionsIn addition to info above, need name,

customer_ID, address etc.Corporate customers have a corporate_ID and a

list of authorized usersTheir monthly report includes a total_fees field

that gives the aggregated total for all users

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Subtypes (2)

Show diagrams for the relations above as subtypes of customer

Why not lump all the customers into a single relation?

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Subtypes (3)

Show the tables you could create to represent the situation described above

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Trees

Many systems involve hierarchical 1:N relationsAdministrative hierarchies

Department, directorate, division, unit…Polychotomous classifications of various

kindsKingdom, phylum, class, order, family…

Every element in tree is a nodeTop level of classification: rootParent nodes have child nodes in a branchA child node can have one and only one

parent

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Trees (cont’d)How can you represent the relationships in a

tree? Consider a system for tracking students in college dormitories:

College

Dormitory

Floor

Room

Student

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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

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Network: Doctors, Nurses, Patients

Draw the health-care example you have developed

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Bill of Materials

In manufacturing systems, many components may contain many partsTrying 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 tableEvery relationship is described by the

container and the contained elementsStructure thus very simple – now draw it!

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Bill of Materials Design

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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?

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Surrogate Keys (cont’d)

DBMS can supply a system-assigned unique identifierTypically a simple numberTakes little roomEasy to use in indexingPrevents 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?

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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 initioChange blank to N/A or UNKNOWN if

necessaryMust handle these values in code (why?)

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HOMEWORKReview 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)

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HOMEWORK (cont’d)

For a 2 extra points each, complete any or all of the FiredUp projects A through D on pp 180-181. To submit B you must first do ATo submit D you must first do C

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DISCUSSION