Database Design

Sections 9 & 10Modeling Historical Data, conditional nontranferability, time-related constraints,Database conventions, generic modeling

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Modeling Over Time

Any relationship that changes through time needs special consideration.

An attribute such as status may change over time.

Examples of time sensitive attributes are: rental status, fee payment, country name, or price

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Change and Time

1. Every attribute update means loss of information.

2. Time in your model makes the model more complex.

3. There are often complex join conditions.

4. Users can work in advance.

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Why Modeling Time examples

Track trends in products City police department tracks crimes

in each neighborhood frequency of crimes during certain times

of the year holidays or during really hot or cold

weather

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Entity DAY or Attribute Date Single attribute

entity without M:1 relationships is usually replaced by attribute

PURCHASE* date

PURCHASE

DAY #date

on

for

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What is wrong with revision?

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Entity Day vs. Attribute Date

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Examples

When there is an interest in a particular day not a date. Is it a holiday? Is it a work day or weekend? Is it leap year? Month end?

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Entity DAY

TASK ASSIGNMENT*duration in hours

TASK#id

DAY #date

* public holiday indicator

EMPLOYEE#name

first day of

starts on

for

of

in

with

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Time-related Contstraints

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Modeling time constraint example 1

Be aware of constraints that can result from the time dimension. Here is an example:

Consider a school fair that features several booths. The manager signs up volunteers to work different shifts at different booths. Some volunteers can work for several hours; others can work fewer hours depending on their free time. The schedule has to be determined in advance, so that the manager knows which times are not covered by any volunteers.

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Example 2

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Review example 1-

What are some other rules?13

Modeling Change

ASSIGNMENT#start dateo end date

EMPLOYEE#id

COUNTRY#name

of

for

in

as

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Modeling Change

ASSIGNMENT#start dateo end date

EMPLOYEE#id

COUNTRY#name

#start time*end time

of

for

in

as

life cycle attributes

Even a Country has a life cycle

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Modeling change: Price

When do we need to track price? Determine best price for item Sale price vs. original price Grade change Value of home Asking price vs. sale price Stock, bond, or investment Others

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Mapping Historical Price

PRICE = PRICED PRODUCT = HISTORICAL PRICE

PRODUCT#id

*name

PRICE#start date

*price $o end date

have

for

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Slide 11

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Revised previous ERD

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Price Changes

Stock Market – example Price changes with time (seconds) Need to know to buy/sell Trends in price effects decisions

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Price with time

Price may fluctuate with time “The Good Old Days” Examples:

Value of Gold or Silver Real Estate Value of currency Gasoline

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Journaling

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Example

When a student’s grade is changed, we need to record information on the teacher who changed the grade and the reason for the change.

Start with the ENROLLMENT entity, which is the resolution of the M:M between STUDENT and CLASS.

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Solution 10.2.13

STUDENT

CLASS

ENROLLMENT#date*grade

GRADE CHANGE*old grade

*changed by#date changed

o reason for change

Student / ClassM:M

Enrollment is the intersection entity

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Adding the Time element to ERD

When does a school need to keep time related information?

Could these be similar to issues in a business

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Drawing Conventions

Improve readability Examples from “DJs on Demand”

scenario Dividing a big ERD up into functional

areas

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Marge Hohly 28

Conventions Review Crows feet

Crows fly East and South Divide complex ERD’s into functional areas Place Highest volume entities in upper left

corner Improve readability

avoid criss-crossing lines increase white spaces so relationships don’t

overlap be consistent with font type, size, and styles

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Marge Hohly 36

Generic Modeling Can reduce number of entities in diagram Can provide more flexibility in unstable

situations (where business requirements change often)

Use a more distant perspective See next slide. What would happen to the

generic model if we had to add 10 new ARTICLE types, each with their own attributes?

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Marge Hohly 38

Generic Modeling Have more attributes in fewer entities Many mandatory requirements/attributes

become optional Structural rules become procedural rules Example: PANTS waist size was mandatory,

with ARTICLE waist size becomes optional What other businesses would be good

candidates for generic modeling?

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Generic Model - Benefits

Benefits Flexibility – can prevent need to change

data structure Can reduce the number of entities

dramatically

Disadvantages Increased complexity is both data model

and application programs

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Marge Hohly 45

Relational Database Concepts

Conceptual model transforms into a relational database

A relational database is a database that is perceived by the user as a collection of relations or two-dimensional tables.

Table, each employee (instances), and each column (attribute)

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Conventions Review

Crows feet Crows fly East and South Divide complex ERD’s into functional areas Place highest volume entities in upper left

corner Improve readability

Avoid criss-crossing lines Increase white spaces so relationships don’t

overlab Be consistent with font type, size, and styles

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