Post on 17-Jan-2016
transcript
1
Database Design
Sections 10 & 11Modeling Historical Data, conditional nontranferability, time-related constraints, conventions, and generic modeling
2
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
3
Change and Time
1. Every 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.
4
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
5
Entity DAY or Attribute Date Single attribute
entity without M:1 relationships is usually replaced by attribute
PURCHASE* date
PURCHASE
DAY #date
on
for
6
What is wrong with revision?
7
Entity Day vs. Attribute Date
8
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?
9
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
10
Modeling Change
ASSIGNMENT#start dateo end date
EMPLOYEE#id
COUNTRY#name
of
for
in
as
11
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
12
Mapping Historical Price
PRICE = PRICED PRODUCT = HISTORICAL PRICE
PRODUCT#id
*name
PRICE#start date
*price $o end date
have
for
13
Revised previous ERD
14
Example Movie Star Jewelry rental
JEWELRY PIECE
#code*description*rental rate
*insured value
MOVIE STAR#id
* first name* last name
rented by
renting
15
Possible Solution
16
8.1.8
17
Review
18
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.
19
Review example 1-
What are some other rules?
20
Example 2
21
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
22
Review slide 4
23
24
Price with time
Price may fluctuate with time “The Good Old Days” Examples:
Value of Gold or Silver Real Estate Value of currency Gasoline
25
Price with Time
PRODUCT
#id
*name
PRICE
#start date
*price is $
o end date
with
of
Price =Priced Produce = Historical Price
26
Slide 11
27
Journaling
28
29
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.
30
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
31Marge Hohly 31
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
32Marge Hohly 32
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
33
34
35
36
37
38
39
40Marge Hohly 40
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 Review Slide 3 & 4 See next slide. What would happen to the
generic model if we had to add 10 new ARTICLE types, each with their own attributes?
41
42Marge Hohly 42
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?
43
44
45
46
47
48Marge Hohly 48
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)