Transcript Database Design - Normalization and SQL

Lecture 14: Database Design
SIMS 202:
Information Organization
and Retrieval
Prof. Ray Larson & Prof. Marc Davis
UC Berkeley SIMS
Tuesday and Thursday 10:30 am - 12:00 pm
Fall 2002
http://www.sims.berkeley.edu/academics/courses/is202/f02/
IS 202 – FALL 2002
2002.10.17 - SLIDE 1
Lecture Overview
• Review
– Databases and Database Design
– Database Life Cycle
– ER Diagrams
• Database Design
• Normalization
• Web-Enabled Databases
IS 202 – FALL 2002
2002.10.17 - SLIDE 2
Lecture Overview
• Review
– Databases and Database Design
– Database Life Cycle
– ER Diagrams
• Database Design
• Normalization
• Web-Enabled Databases
IS 202 – FALL 2002
2002.10.17 - SLIDE 3
Models (1)
Application 1
External
Model
Application 2
Application 3
Application 4
External
Model
External
Model
External
Model
Application 1
Conceptual
requirements
Application 2
Conceptual
requirements
Application 3
Conceptual
requirements
Conceptual
Model
Logical
Model
Internal
Model
Application 4
Conceptual
requirements
IS 202 – FALL 2002
2002.10.17 - SLIDE 4
Database System Life Cycle
Physical
Creation
2
Conversion
3
Design
1
Growth,
Change, &
Maintenance
6
Integration
4
Operations
5
IS 202 – FALL 2002
2002.10.17 - SLIDE 5
Another View of the Life Cycle
Integration
4
Operations
5
Design
Physical
1
Creation Conversion Growth,
2
Change
3
6
IS 202 – FALL 2002
2002.10.17 - SLIDE 6
Database Design Process
Application 1
External
Model
Application 2
Application 3
Application 4
External
Model
External
Model
External
Model
Application 1
Conceptual
requirements
Application 2
Conceptual
requirements
Application 3
Conceptual
requirements
Conceptual
Model
Logical
Model
Internal
Model
Application 4
Conceptual
requirements
IS 202 – FALL 2002
2002.10.17 - SLIDE 7
Entity
• An Entity is an object in the real world (or
even imaginary worlds) about which we
want or need to maintain information
– Persons (e.g.: customers in a business,
employees, authors)
– Things (e.g.: purchase orders, meetings,
parts, companies)
Employee
IS 202 – FALL 2002
2002.10.17 - SLIDE 8
Attributes
• Attributes are the significant properties or
characteristics of an entity that help identify it
and provide the information needed to
interact with it or use it (This is the Metadata
for the entities)
Birthdate
First
Middle
Last
IS 202 – FALL 2002
Age
Name
Employee
SSN
Projects
2002.10.17 - SLIDE 9
Relationships
• Relationships are the associations
between entities
• They can involve one or more entities and
belong to particular relationship types
– One to One
– One to Many
– Many to Many
IS 202 – FALL 2002
2002.10.17 - SLIDE 10
Relationships
Student
Attends
Class
Project
Supplier
IS 202 – FALL 2002
Supplies
project
parts
Part
2002.10.17 - SLIDE 11
Types of Relationships
• Concerned only with cardinality of
relationship
Employee
Employee
Employee
1 Assigned
n
Assigned
1
1
m Assigned n
Truck
Project
Project
Chen ER notation
IS 202 – FALL 2002
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More Complex Relationships
Manager
1/1/1
Employee
1/n/n Evaluation n/n/1
Project
SSN
Date
Project
Employee
4(2-10)
Assigned
1
Manages
Employee
Is Managed By
Project
1
Manages
n
IS 202 – FALL 2002
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Weak Entities
• Owe existence entirely to another entity
Part#
Invoice #
Order
Invoice#
Contains
Quantity
Order-line
Rep#
IS 202 – FALL 2002
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Supertype and Subtype Entities
Employee
Sales-rep
Is one of
Manages
Clerk
Sold
Other
Invoice
IS 202 – FALL 2002
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Many to Many Relationships
SSN
Proj#
Proj#
Hours
Project
Assignment
Is
Assigned
Project
Assigned
Employee
IS 202 – FALL 2002
SSN
2002.10.17 - SLIDE 16
Lecture Overview
• Review
– Databases and Database Design
– Database Life Cycle
– ER Diagrams
• Database Design
• Normalization
• Web-Enabled Databases
IS 202 – FALL 2002
2002.10.17 - SLIDE 17
Database Design Process
Application 1
External
Model
Application 2
Application 3
Application 4
External
Model
External
Model
External
Model
Application 1
Conceptual
requirements
Application 2
Conceptual
requirements
Application 3
Conceptual
requirements
Conceptual
Model
Logical
Model
Internal
Model
Application 4
Conceptual
requirements
IS 202 – FALL 2002
2002.10.17 - SLIDE 18
Database Design Process
Application 1
External
Model
Application 2
Application 3
Application 4
External
Model
External
Model
External
Model
Application 1
Conceptual
requirements
Application 2
Conceptual
requirements
Application 3
Conceptual
requirements
Conceptual
Model
Logical
Model
Internal
Model
Application 4
Conceptual
requirements
IS 202 – FALL 2002
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Requirements Analysis
• Conceptual Requirements
– Systems Analysis Process
• Examine all of the information sources used in
existing applications
• Identify the characteristics of each data element
–
–
–
–
Numeric
Text
Date/time
Etc.
• Examine the tasks carried out using the
information
• Examine results or reports created using the
information
IS 202 – FALL 2002
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Database Design Process
Application 1
External
Model
Application 2
Application 3
Application 4
External
Model
External
Model
External
Model
Application 1
Conceptual
requirements
Application 2
Conceptual
requirements
Application 3
Conceptual
requirements
Conceptual
Model
Logical
Model
Internal
Model
Application 4
Conceptual
requirements
IS 202 – FALL 2002
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Conceptual Design
• Conceptual Model
– Merge the collective needs of all applications
– Determine what Entities are being used
• Some object about which information is to
maintained
– What are the Attributes of those entities?
• Properties or characteristics of the entity
• What attributes uniquely identify the entity
– What are the Relationships between entities
• How the entities interact with each other?
IS 202 – FALL 2002
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Developing a Conceptual Model
• Overall view of the database that integrates all
the needed information discovered during the
requirements analysis
• Elements of the Conceptual Model are
represented by diagrams, Entity-Relationship or
ER Diagrams, that show the meanings and
relationships of those elements independent of
any particular database systems or
implementation details
• Can also be represented using other modeling
tools (such as UML)
IS 202 – FALL 2002
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Database Design Process
Application 1
External
Model
Application 2
Application 3
Application 4
External
Model
External
Model
External
Model
Application 1
Conceptual
requirements
Application 2
Conceptual
requirements
Application 3
Conceptual
requirements
Conceptual
Model
Logical
Model
Internal
Model
Application 4
Conceptual
requirements
IS 202 – FALL 2002
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Logical Design
• Logical Model
– How is each entity and relationship
represented in the Data Model of the DBMS
•
•
•
•
Hierarchic?
Network?
Relational?
Object-Oriented?
IS 202 – FALL 2002
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Database Design Process
Application 1
External
Model
Application 2
Application 3
Application 4
External
Model
External
Model
External
Model
Application 1
Conceptual
requirements
Application 2
Conceptual
requirements
Application 3
Conceptual
requirements
Conceptual
Model
Logical
Model
Internal
Model
Application 4
Conceptual
requirements
IS 202 – FALL 2002
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Physical Design
• Internal Model
– Choices of index file structure
– Choices of data storage formats
– Choices of disk layout
IS 202 – FALL 2002
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Database Design Process
Application 1
External
Model
Application 2
Application 3
Application 4
External
Model
External
Model
External
Model
Application 1
Conceptual
requirements
Application 2
Conceptual
requirements
Application 3
Conceptual
requirements
Conceptual
Model
Logical
Model
Internal
Model
Application 4
Conceptual
requirements
IS 202 – FALL 2002
2002.10.17 - SLIDE 28
Database Application Design
• External Model
– User views of the integrated database
– Making the old (or updated) applications work
with the new database design
IS 202 – FALL 2002
2002.10.17 - SLIDE 29
Lecture Overview
• Review
– Databases and Database Design
– Database Life Cycle
– ER Diagrams
• Database Design
• Normalization
• Web-Enabled Databases
IS 202 – FALL 2002
2002.10.17 - SLIDE 30
Normalization
• Normalization theory is based on the
observation that relations with certain
properties are more effective in inserting,
updating and deleting data than other sets
of relations containing the same data
• Normalization is a multi-step process
beginning with an “unnormalized” relation
– Hospital example from Atre, S. Data Base:
Structured Techniques for Design,
Performance, and Management
IS 202 – FALL 2002
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Normal Forms
•
•
•
•
•
•
First Normal Form (1NF)
Second Normal Form (2NF)
Third Normal Form (3NF)
Boyce-Codd Normal Form (BCNF)
Fourth Normal Form (4NF)
Fifth Normal Form (5NF)
IS 202 – FALL 2002
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Normalization
No transitive
dependency
between
nonkey
attributes
All
determinants
are candidate
keys - Single
multivalued
dependency
IS 202 – FALL 2002
BoyceCodd and
Higher
Functional
dependency
of nonkey
attributes on
the primary
key - Atomic
values only
Full
Functional
dependency
of nonkey
attributes on
the primary
key
2002.10.17 - SLIDE 33
Unnormalized Relations
• First step in normalization is to convert the
data into a two-dimensional table
• In unnormalized relations data can repeat
within a column
IS 202 – FALL 2002
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Unnormalized Relations
Patient #
Surgeon #
145
1111 311
Surg. date
Patient Name
Jan 1,
1995; June
12, 1995
John White
Patient Addr Surgeon
15 New St.
New York,
NY
2345 189
Jan 8,
1996
4876 145
Nov 5,
1995
Hal Kane
5123 145
May 10,
1995
Paul Kosher
6845 243
Apr 5,
1994 Dec
15, 1984
Ann Hood
Hilton Road
Larchmont, Charles
NY
Field
IS 202 – FALL 2002
Apr 5,
1994 May
10, 1995
Mary Jones
Charles Brown
Postop drug
Drug side effects
Gallstone
s removal;
Beth Little Kidney
Michael
stones
Penicillin,
Diamond removal
none-
Charles
Field
10 Main St. Patricia
Rye, NY
Gold
Dogwood
Lane
Harrison,
David
NY
Rosen
55 Boston
Post Road,
Chester,
CN
Beth Little
Blind Brook
Mamaronec
k, NY
Beth Little
243
1234 467
Surgery
rash
none
Eye
Cataract
removal
Thrombos Tetracyclin Fever
is removal e none
none
Open
Heart
Surgery
Cholecyst
ectomy
Gallstone
s
Removal
Eye
Cornea
Replacem
ent Eye
cataract
removal
Cephalosp
orin
none
Demicillin
none
none
none
Tetracyclin
e
Fever
2002.10.17 - SLIDE 35
First Normal Form
• To move to First Normal Form a relation
must contain only atomic values at each
row and column
– No repeating groups
– A column or set of columns is called a
Candidate Key when its values can uniquely
identify the row in the relation
IS 202 – FALL 2002
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First Normal Form
Patient #
Surgeon # Surgery DatePatient Name Patient Addr Surgeon Name
1111
145
01-Jan-95 John White
1111
311
12-Jun-95 John White
15 New St.
New York,
NY
15 New St.
New York,
NY
1234
243
05-Apr-94 Mary Jones
10 Main St.
Rye, NY
1234
467
10-May-95 Mary Jones
2345
4876
5123
6845
6845
IS 202 – FALL 2002
189
145
145
243
243
Charles
08-Jan-96 Brown
10 Main St.
Rye, NY
Dogwood
Lane
Harrison,
NY
05-Nov-95 Hal Kane
55 Boston
Post Road,
Chester,
CN
05-Apr-94 Ann Hood
15-Dec-84 Ann Hood
Hilton Road
Larchmont,
NY
Drug adminSide Effects
Charles Field
Gallstone
s removal
Kidney
stones
removal
Eye
Cataract
removal
Patricia Gold
Thrombos
is removal none
none
David Rosen
Open
Heart
Surgery
none
Beth Little
Cholecyst
Demicillin
ectomy
Beth Little
Michael
Diamond
Penicillin
rash
none
none
Tetracyclin
e
Fever
Cephalosp
orin
Charles Field
Gallstone
s
none
Removal
Eye
Cornea
Replacem Tetracyclin
e
ent
Charles Field
Eye
cataract
removal
Blind Brook
Mamaronec
Beth Little
10-May-95 Paul Kosher k, NY
Hilton Road
Larchmont,
NY
Surgery
none
none
none
Fever
none
2002.10.17 - SLIDE 37
1NF Storage Anomalies
• Insertion: A new patient has not yet undergone
surgery -- hence no surgeon # -- Since surgeon
# is part of the key we can’t insert
• Insertion: If a surgeon is newly hired and hasn’t
operated yet -- there will be no way to include
that person in the database
• Update: If a patient comes in for a new
procedure, and has moved, we need to change
multiple address entries
• Deletion (type 1): Deleting a patient record may
also delete all info about a surgeon
• Deletion (type 2): When there are functional
dependencies (like side effects and drug)
changing one item eliminates other information
IS 202 – FALL 2002
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Second Normal Form
• A relation is said to be in Second Normal
Form when every nonkey attribute is fully
functionally dependent on the primary key
– That is, every nonkey attribute needs the full
primary key for unique identification
IS 202 – FALL 2002
2002.10.17 - SLIDE 39
Second Normal Form
Patient #
1111
1234
2345
4876
5123
6845
IS 202 – FALL 2002
Patient Name Patient Address
15 New St. New
John White York, NY
10 Main St. Rye,
Mary Jones NY
Charles
Dogwood Lane
Brown
Harrison, NY
55 Boston Post
Hal Kane
Road, Chester,
Blind Brook
Paul Kosher Mamaroneck, NY
Hilton Road
Ann Hood
Larchmont, NY
2002.10.17 - SLIDE 40
Second Normal Form
Surgeon #
Surgeon Name
145 Beth Little
189 David Rosen
243 Charles Field
311 Michael Diamond
467 Patricia Gold
IS 202 – FALL 2002
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Second Normal Form
Patient # Surgeon # Surgery Date
1111
1111
1234
1234
2345
4876
Drug Admin Side Effects
145
Gallstones
01-Jan-95 removal
Kidney
Penicillin
rash
311
stones
12-Jun-95 removal
none
none
243
Eye Cataract
05-Apr-94 removal
Tetracycline Fever
467
Thrombosis
10-May-95 removal
189
Open Heart
08-Jan-96 Surgery
Cephalospori
n
none
145
Cholecystect
05-Nov-95 omy
Demicillin
none
none
none
none
none
5123
145
6845
243
6845
243
IS 202 – FALL 2002
Surgery
Gallstones
10-May-95 Removal
Eye cataract
15-Dec-84 removal
Eye Cornea
05-Apr-94 Replacement
none
none
Tetracycline Fever
2002.10.17 - SLIDE 42
1NF Storage Anomalies Removed
• Insertion: Can now enter new patients without
surgery
• Insertion: Can now enter Surgeons who haven’t
operated
• Deletion (type 1): If Charles Brown dies the
corresponding tuples from Patient and Surgery
tables can be deleted without losing information
on David Rosen
• Update: If John White comes in for third time,
and has moved, we only need to change the
Patient table
IS 202 – FALL 2002
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2NF Storage Anomalies
• Insertion: Cannot enter the fact that a particular
drug has a particular side effect unless it is given
to a patient
• Deletion: If John White receives some other drug
because of the penicillin rash, and a new drug
and side effect are entered, we lose the
information that penicillin can cause a rash
• Update: If drug side effects change (a new
formula) we have to update multiple occurrences
of side effects
IS 202 – FALL 2002
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Third Normal Form
• A relation is said to be in Third Normal Form if
there is no transitive functional dependency
between nonkey attributes
– When one nonkey attribute can be determined with
one or more nonkey attributes there is said to be a
transitive functional dependency
• The side effect column in the Surgery table is
determined by the drug administered
– Side effect is transitively functionally dependent on
drug so Surgery is not 3NF
IS 202 – FALL 2002
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Third Normal Form
Patient # Surgeon # Surgery Date
Surgery
Drug Admin
1111
145
1111
311
01-Jan-95 Gallstones removal
Kidney stones
12-Jun-95 removal
1234
243
05-Apr-94 Eye Cataract removal Tetracycline
1234
467
10-May-95 Thrombosis removal
2345
189
08-Jan-96 Open Heart Surgery
Cephalosporin
4876
145
05-Nov-95 Cholecystectomy
Demicillin
5123
145
10-May-95 Gallstones Removal
none
6845
243
none
6845
243
15-Dec-84 Eye cataract removal
Eye Cornea
05-Apr-94 Replacement
IS 202 – FALL 2002
Penicillin
none
none
Tetracycline
2002.10.17 - SLIDE 46
Third Normal Form
Drug Admin
IS 202 – FALL 2002
Side Effects
Cephalosporin
none
Demicillin
none
none
none
Penicillin
rash
Tetracycline
Fever
2002.10.17 - SLIDE 47
2NF Storage Anomalies Removed
• Insertion: We can now enter the fact that a
particular drug has a particular side effect
in the Drug relation
• Deletion: If John White recieves some
other drug as a result of the rash from
penicillin, but the information on penicillin
and rash is maintained
• Update: The side effects for each drug
appear only once
IS 202 – FALL 2002
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Boyce-Codd Normal Form
• Most 3NF relations are also BCNF
relations
• A 3NF relation is NOT in BCNF if:
– Candidate keys in the relation are composite
keys (they are not single attributes)
– There is more than one candidate key in the
relation, and
– The keys are not disjoint, that is, some
attributes in the keys are common
IS 202 – FALL 2002
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Most 3NF Relations Are Also BCNF – Is This One?
Patient # Patient Name Patient Address
15 New St. New
1111 John White York, NY
10 Main St. Rye,
1234 Mary Jones NY
Charles
Dogwood Lane
2345 Brown
Harrison, NY
55 Boston Post
4876 Hal Kane
Road, Chester,
Blind Brook
5123 Paul Kosher Mamaroneck, NY
Hilton Road
6845 Ann Hood
Larchmont, NY
IS 202 – FALL 2002
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BCNF Relations
Patient # Patient Name
IS 202 – FALL 2002
Patient #
1111 John White
1111
1234 Mary Jones
Charles
2345 Brown
1234
4876 Hal Kane
4876
5123 Paul Kosher
5123
6845 Ann Hood
6845
2345
Patient Address
15 New St. New
York, NY
10 Main St. Rye,
NY
Dogwood Lane
Harrison, NY
55 Boston Post
Road, Chester,
Blind Brook
Mamaroneck, NY
Hilton Road
Larchmont, NY
2002.10.17 - SLIDE 51
Fourth Normal Form
• Any relation is in Fourth Normal Form if it
is BCNF and any multivalued
dependencies are trivial
• Eliminate non-trivial multivalued
dependencies by projecting into simpler
tables
IS 202 – FALL 2002
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Fifth Normal Form
• A relation is in 5NF if every join
dependency in the relation is implied by
the keys of the relation
• Implies that relations that have been
decomposed in previous NF can be
recombined via natural joins to recreate
the original relation
IS 202 – FALL 2002
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Normalizing to Death
• Normalization splits database information
across multiple tables
• To retrieve complete information from a
normalized database, the JOIN operation
must be used
• JOIN tends to be expensive in terms of
processing time, and very large joins are
very expensive
IS 202 – FALL 2002
2002.10.17 - SLIDE 54
Lecture Overview
• Review
– Databases and Database Design
– Database Life Cycle
– ER Diagrams
• Database Design
• Normalization
• Web-Enabled Databases
IS 202 – FALL 2002
2002.10.17 - SLIDE 55
Overview
• Why use a database system for Web
design and e-commerce?
• What systems are available?
• Pros and Cons of different Web database
systems?
• Text retrieval in database systems
• Search engines for Intranet and Intrasite
searching
IS 202 – FALL 2002
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Why Use a Database System?
• Simple Web sites with only a few pages
don’t need much more than static HTML
files
IS 202 – FALL 2002
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Simple Web Applications
Web
Server
Internet
Files
Server
Clients
IS 202 – FALL 2002
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Adding Dynamic Content to the Site
• Small sites can often use simple HTML
and CGI scripts accessing data files to
create dynamic content for small sites
IS 202 – FALL 2002
2002.10.17 - SLIDE 59
Dynamic Web Applications 1
Web
Server
Files
CGI
Internet
Server
Clients
IS 202 – FALL 2002
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Issues For Scaling Up Web Applications
•
•
•
•
•
Performance
Scalability
Maintenance
Data integrity
Transaction support
IS 202 – FALL 2002
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Why Use a Database System?
• Database systems have concentrated on
providing solutions for all of these issues
for scaling up Web applications
– Performance
– Scalability
– Maintenance
– Data integrity
– Transaction support
• While systems differ in their support, most
offer some support for all of these
IS 202 – FALL 2002
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Dynamic Web Applications 2
Web
Server
Internet
Files
CGI
DBMS
Server
database
database
database
Clients
IS 202 – FALL 2002
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Server Interfaces
SQL
HTML
DHTML
Web Server
JavaScript
Native
DB
Interfaces
Database
Web DB
CGI
App ODBC
Web Server
API’s
ColdFusion
Native DB
interfaces
JDBC
PhP Perl
Web Application
Server
Adapted from
John P. Ashenfelter,
Choosing a Database for Your Web Site
IS 202 – FALL 2002
Java
ASP
2002.10.17 - SLIDE 64
Web Application Server Software
•
•
•
•
ColdFusion
PHP
ASP
All of these are server-side scripting
languages that embed code in HTML
pages
IS 202 – FALL 2002
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ColdFusion
• Developing WWW sites typically involved
a lot of programming to build dynamic
sites
– E.g., pages generated as a result of catalog
searches, etc.
• ColdFusion was designed to permit the
construction of dynamic Web sites with
only minor extensions to HTML through a
DBMS interface
IS 202 – FALL 2002
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What ColdFusion Is Good For
• Putting up databases onto the Web
• Handling dynamic databases (frequent
updates, etc.)
• Making databases searchable and
updateable by users
IS 202 – FALL 2002
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CFML ColdFusion Markup Language
• Read data from and update data to databases
and tables
• Create dynamic data-driven pages
• Perform conditional processing
• Populate forms with live data
• Process form submissions
• Generate and retrieve email messages
• Perform HTTP and FTP function
• Perform credit card verification and authorization
• Read and write client-side cookies
IS 202 – FALL 2002
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Templates
• Assume we have a database named
contents_of_my_shopping_cart.mdb -single table called contents...
• Create an HTML page (uses extension
.cfm), before <HEAD>...
• <CFQUERY NAME= ”cart"
DATASOURCE=“contents_of_my_shoppin
g_cart"> SELECT * FROM contents ;
</CFQUERY>
IS 202 – FALL 2002
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Templates (cont.)
•
•
•
•
•
•
•
•
•
•
•
•
<HEAD>
<TITLE>Contents of My Shopping Cart</TITLE>
</HEAD>
<BODY>
<H1>Contents of My Shopping Cart</H1>
<CFOUTPUT QUERY= ”cart">
<B>#Item#</B> <BR>
#Date_of_item# <BR>
$#Price# <P>
</CFOUTPUT>
</BODY>
</HTML>
IS 202 – FALL 2002
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Templates (cont.)
Contents of My Shopping Cart
Bouncy Ball with Psychedelic Markings
12 December 1998
$0.25
Shiny Blue Widget
14 December 1998
$2.53
Large Orange Widget
14 December 1998
$3.75
IS 202 – FALL 2002
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CFIF and CFELSE
<CFOUTPUT QUERY= ”cart">
Item: #Item# <BR>
<CFIF #Picture# EQ"">
<IMG SRC=“generic_picture.jpg"> <BR>
<CFELSE>
<IMG SRC="#Picture#"> <BR>
</CFIF>
</CFOUTPUT>
IS 202 – FALL 2002
2002.10.17 - SLIDE 72
Photo Browser
• The current photo browser uses a
combination of
– Javascript for expandable hierarchies
– Database in MS Access
– ColdFusion to search the database when one
of the facets is selected
• The database design for the photo
database currently looks like…
IS 202 – FALL 2002
2002.10.17 - SLIDE 73
Photo Browser ER
IS 202 – FALL 2002
2002.10.17 - SLIDE 74
Photo Database
• Lets look at the photo database in the
Access interface
– Multi-Facet queries
– Queries for multiple descriptors in the same
facet (harder)
IS 202 – FALL 2002
2002.10.17 - SLIDE 75
Assignment 7 (Database Design)
• Involves
– Examining a Web Site (probably) using a
DBMS for E-commerce to sell books
– Inferring the structure and kinds of entities
and attributes used in that site (book info only)
– Creating your own design using ER diagrams
showing the entities and relationships that you
inferred
IS 202 – FALL 2002
2002.10.17 - SLIDE 76
Next Week
• Introduction to Information Retrieval
IS 202 – FALL 2002
2002.10.17 - SLIDE 77