Transcript What is a Database Management System?

COEN 178
Database Systems
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JoAnne Holliday
Email: [email protected] (best way to reach me)
Office: Engineering 247, (408) 551-1941
Office Hours: Tu 1:30-2:30, Th 3:00-4:00 and by
appointment
• Class web page: http://www.cse.scu.edu/~jholliday/
• Two midterms, a project and a final exam
• T.A. – Annie Wu
Santa Clara University
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Textbooks and Meetings
Required:
• A First Course in Database Systems, by Ullman
and Widom (second edition), Prentice Hall, 2002.
Recommended:
• Oracle 8 Programming, A Primer, (fourth
edition), by R. Sunderraman, Addison-Wesley
Longman, 2000.
Class Meetings:
• Lecture TuTh 9:55-11:40am, Engr 106,.
• Lab: W 2:15-5:00 608B and Th 11:50-2:20 in 618
• You may attend either lab session
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Grading
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Lab work and instructor discretion: 10% of grade.
Project: 15% of grade.
2 Midterms for a total of 40% of grade.
Final: 35% of grade.
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Project
• You will build an application using a relational
database system (Oracle8i).
• The project has 2 parts, starting with design
and ending up with a complete application.
• The project should be written in Java and use
the JDBC or written in Perl or C.
• Java is the preferred language.
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Why take this class?
• This class may seem easy, but there is a
theoretical part which many students find
difficult – don’t be caught sleeping!!
• The benefits of accepting data
constraints.
• Of all courses you take at SCU, this may
be the one that gets you a job.
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Schedule
• Today: Sept. 24 (Tu)
Intro,
Entity-Relationship Model.
Read Chapter 1 and Sections 2.1-2.2.
• Sept 26 (Th)
Weak
Entity Sets, Entity-Relationship Design.
Read Sections 2.3-2.4.
• Oct. 1 (Tu)
Relational
Model, Functional Dependencies.
Read Sections 3.1-3.5.
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Syllabus
• Chapter 1. Overview of database management
systems. (1)
• Chapter 2. ERD’s, principles of modeling data and
constraints. (2)
• Chapter 3. The relational data model, functional
dependencies and normal forms. (4)
• Chapter 5 (5.1-5.2 only). Theoretical background
of the relational model. (1)
• Chapter 6. Queries and Updates. (3)
• Chapter 7. Constraints and Triggers (2)
• Chapter 8 (sec 6,7) Transactions and Security. (2)
• Indexing and the DBA (2)
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Definitions
• A database is a collection of (related) data.
• Database Management System (DBMS) is
the hardware/software package that
facilitates the creation and maintenance of a
computerized database.
• In a relational database, the tables are
relations, the rows are tuples and there are
mathematically well defined operations on
relations (relational algebra).
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Benefits of a DBMS
1. Manages very large amounts of data.
2. Supports efficient access to data through
indexing and a rich query language.
3. Supports concurrent access to data.
 Example:
bank and its ATM machines.
4. Supports secure, atomic access to very large
amounts of data.
 Contrast
two people editing the same UNIX file – last to write
“wins” – with the problem if two people deduct money from
the same account via ATM machines at the same time – new
balance is wrong whichever writes last.
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More Benefits of a DMBS
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Controls redundancy in data storage
Protects against unauthorized access of data
Enforces integrity constraints
Provides backup and recovery services
Enforces user-defined standards
Provides data abstraction through schemas.
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Relational Model
• Based on tables, like the Account table:
acct #
12345
34567
…
name
Sally
Sue
…
balance
1000.21
285.48
…
• Today used in most DBMS's.
• DB schema Account=(acct#,name,balance)
and DB instance
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The DBMS Marketplace
• Relational DBMS companies – Oracle, Sybase – are among the
largest software companies in the world.
• IBM offers its relational DB2 system. With IMS, a nonrelational
system, IBM is by some accounts the largest DBMS vendor in the
world.
• Microsoft offers SQL-Server, plus Microsoft Access for the cheap
DBMS on the desktop, answered by “lite” systems from other
competitors.
• Relational companies also challenged by “object-oriented DB”
companies.
• But countered with “object-relational” systems, which retain the
relational core while allowing type extension as in OO systems.
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Three Aspects to Studying DBMS's
1. Modeling and design of databases.
 Allows
exploration of issues before committing to an
implementation.
2. Programming: queries and DB operations like
update.
 SQL =
“intergalactic dataspeak.”
3. DBMS implementation. (concurrency control2PL, backup and recovery- logging, query
optimization)
COEN178 = (1) + (2)
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Query Languages
Department
Schema is
Department = (Dept,Manager)
Dept
Manager
Data definition language (DDL) creates the schema and metadata
CREATE TABLE Account (…..);
Data Manipulation Language (DML)
Query (SELECT), UPDATE, DELETE
In the SQL query language:
SELECT Manager
FROM Department
WHERE Department.Dept = ‘accounting’
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Host Languages
C, C++, Java, Lisp
Application prog.
Calls to
DB
DBMS
Local Vars
(Memory)
(Storage)
Host language is completely general (Turing complete)
but gives you no support
Query language—less general "non procedural" and
optimizable
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Relational model is good for:
Large amounts of data —> simple operations
Navigate among small number of relations
Difficult Applications for relational model (complex objects and
relationships:
• VLSI Design (CAD in general)
• CASE
• Graphical Data
ALU
ADDER
A
FA
CPU
Adder
ALU
ADDER
Bill of Materials or
transitive closure
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Other Models
Where number of "relations" is large, relationships are complex
• Object Data Model
• Logic Data Model
OBJECT DATA MODEL
1.
2.
3.
4.
Complex Objects – Nested Structure (pointers or
references)
Encapsulation, set of Methods/Access functions
Object Identity
Inheritance – Defining new classes like old classes
Object model: usually find objects via explicit navigation
Also query language in some systems
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LOGIC (Horn Clause) DATA MODEL
• Prolog, Datalog
if A1 and A2 then B
prolog B:- A1 and A2
Functions s(5) = 6 (successor)
Predicates with Arguments sum(X,Y,Z)
X+Y=Z
sum(X,0,X) means X + 0 = X (always true for all X)
sum(X,s(Y),s(Z)):-sum(X,Y,Z)
means X+(Y+1) = (Z+1) if X + Y = Z
More power than relational model
Can Compute Transitive Closure
edge(X,Y)
path(X,Y) :- edge(X,Y)
path(X,Z) :- path(X,Y) & edge(Y,Z)
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Entity/Relationship Diagrams
A model where diagrams represent designs.
• Entity like object, = “thing.”
• Entity set like class = set of “similar”
entities/objects.
• Attribute = property of entities in an entity set,
similar to fields of a struct.
• In diagrams, entity set  rectangle;
attribute  oval.
ID
name
Students
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phone
major
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Relationships
• Connect two or more entity sets.
• Represented by diamonds.
Students
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Taking
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Courses
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Relationship Set
Think of the “value” of a relationship set as a table.
• One column for each of the connected entity sets.
• One row for each of the entities, one from each
set, that are connected by the relationship.
Students
Sally
Sally
Joe
…
Santa Clara University
Courses
COEN178
COEN111
COEN178
…
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Multiway Relationships
Usually binary relationships (connecting two E.S.) suffice.
• However, there are some cases where three or more E.S.
must be connected by one relationship.
• Example: relationship among students, courses, TA's
(and graders).
Students
Taking
Courses
Possibly, this E/R diagram is OK:
Assisting
TA/Graders
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Multiway Relationships
• Works in COEN178, because each TA (or
grader) is a TA of all students. Connection
student-TA is only via the course.
• But what if students were divided into
sections, with different TAs?
Then,
a student in COEN178 would be related
to only one of the TA's. Which one?
• Need a 3-way relationship to tell.
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Courses
Students
Enrolls
TAs
Students
Ann
Sue
Bob
…
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Courses
CS180
CS180
CS180
…
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TAs
Jan
Pat
Jan
…
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Beers-Bars-Drinkers Example
• Example of complex relationships.
name
addr
license
Serves
Bars
Frequents
Beers
Likes
Drinkers
name
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manf
name
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addr
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Multiplicity of Relationships
Many-many
Many-one
One-one
Representation of Many-One
• E/R: arrow pointing to “one.”
Rounded
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arrow = “exactly one.”
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Example:
Drinkers Have Favorite Beers
name
Serves
addr
license
Bars
Frequents
Likes
Beers
name
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manf
Drinkers
Favorite
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name
addr
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Many-One Relationships
A bar has one best seller – many bars may
have the same one.
Bars
Bestseller
Beers
Is the rounded arrow justified?
Alternate notation: total participation
Bars
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Bestseller
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Beers
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Attributes on Relationships
price
Bars
Sells
Beers
• Shorthand for 3-way relationship:
price
Prices
Bars
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Sells
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Beers
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• A true 3-way relationship.
Price
depends jointly on beer and bar.
• Notice arrow convention for multiway
relationships: “all other E.S. determine one
of these.”
Not
sufficiently general to express any
possibility.
However, if price, say, depended only on the
beer, then we could use two 2-way
relationships: price-beer and beer-bar.
Or better: just make price an attribute of beer.
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Converting Multiway to 2-Way
• Necessary in certain “object-oriented” models.
• Create a new connecting E.S. to represent rows of a relationship
set.
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E.g., (Joe's Bar, Bud, $2.50) for the Sells relationship.
• Many-one relationships from the connecting E.S. to the others.
BBP
TheBar
TheBeer
ThePrice
Bars
Beers
Price
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Roles
Sometimes an E.S. participates more than
once in a relationship.
• Label edges with roles to distinguish.
Married
husband
wife
Husband
d1
d3
…
Wife
d2
d4
…
Drinkers
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Buddies
1
2
Drinkers
Buddy1
d1
d1
d2
d2
…
Buddy2
d2
d3
d1
d4
…
• Notice Buddies is symmetric, Married not.
No
way to say “symmetric” in E/R.
Design Question
Should we replace husband and wife by one
relationship spouse to make it symmetric?
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More Design Issues
1. Subclasses.
2. Keys.
3. Weak entity sets.
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Subclasses
Subclass = special case = fewer entities =
more properties.
• Example: A bank stores records of bank
accounts. Savings accounts are a kind of
bank account. In addition to the properties
(= attributes and relationships) of bank
accounts, there is a “interest rate” attribute
for savings accounts.
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E/R Subclasses
• Assume subclasses form a tree (no multiple
inheritance).
• isa triangles indicate the subclass relation.
name
Accounts
Acc#
isa
rate
Savings a/c
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Checking a/c
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Multiple Inheritance
Theoretically, an E.S. could be a subclass of
several other entity sets.
name
owner
Acc#
Bank account
Interest instrument
isa
isa
Savings
A/C
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Certificate
of deposit
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Problems
How to show?
• A customer can have several loans (or no
loans) but each loan is associated with only
one (exactly one) customer?
• A customer can have at most one loan and a
loan may be ‘owned’ by several customers?
Customer
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Borrows
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Loan
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Keys
A key is a set of attributes whose values can
belong to at most one entity.
• In E/R model, every E.S. must have a key.
It
could have more than one key, but one set of
attributes is the “designated” key.
• In E/R diagrams, you should underline all
attributes of the designated key.
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Example
• Suppose acc# is key for Account.
acc#
Account
name
isa
rate
S/A
• acc# is also key for S/A.
 In
general, key at root is key for all.
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Example: A Multiattribute Key
number
dept
hours
Courses
room
• Possibly, the combination of hours + room
also forms a key, but we have not
designated it as such.
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