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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
44
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)
45
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
46
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