Database Systems
(資料庫系統)
September 26, 2005
Lecture #2
1
Course Administration
• Office Hours: TA (336/338)
–
–
–
–
Mine: Mon 2-3
張耿豪 : Tue: 2~3
田知本: Wed 1~2
陳奕超 : Thu 9~11
• Can everyone get the textbook?
– There are 2 books for loan in the TA’s office (336/338)
• HW #1 will be on the course homepage later today
– It is due 3 weeks from today.
• Next week reading:
– R&G Chapters 3 & 4.1~4.2
2
Ubicomp Project of the Week:
Hyperdragging (SONY CSL, 1999)
• How to bring human computer interaction (desktop PC) into the physical
environment (just a bit)?
– “Disappearing computing”
3
Chapter 2
Introduction to Database Design
4
Scenario
• Say if you are hired by the bCool Beer retailer as a computer consultant.
• bCool wants you to design its database system.
• How to design it?
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Database Design
• Step 1: Requirements Analysis
– What data to store in the database?
– What application (e.g., queries, updates, ..) needs from the database?
• Step 2: Conceptual Database Design
– Come up with the design: Entity-Relation (ER) model
– Sketch the design using pictures called entity-relationship diagrams.
• Step 3: Logical Database Design
– Implement the design: relational data model
– Easy to map ER diagrams into the relational data model (CH 3).
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Requirement Analysis
• Requirement analysis:
– The Beer retailer wants to keep track of
• Beers on shelves
• Beer manufacturers: [name & address]
name
name
addr
• Conceptual database design
Beers
– ER diagram
ManfBy
Manfs
• Logical database design:
– Relational model
Manufacturer’s
names
Manufacturer’s
addresses
Beer names
Manufacturer's
names
台北市南昌路一
段4號
台灣啤酒
台灣菸酒公賣局
青島啤酒
台灣菸酒公賣
局
台灣菸酒公賣局
台灣菸酒公賣局
台灣生啤酒
青島啤酒廠
??
青島啤酒
青島啤酒廠
Beer names
台灣啤酒
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ER Model: Entity
(123: integer)
ssn
name
Employees
(‘Joe’: string)
(Joe, Alice, ..)
• Proposed by Peter Chen (BS NTU EE ‘68) in 1976.
• Entity: Real-world object distinguishable from other objects (e.g., Joe).
• An entity is described by a set of attributes.
– Each attribute has a domain of possible values.
• Entity Set: a collection of similar entities
• Each entity in an entity set is uniquely identified by a key attribute.
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ER Model: Relationship
(5/2001)
since
dname
name
(Joe)
Employees
budget
did
ssn
Works_In
Departments
(finance dept)
• Relationship: Association among two or more entities
– Joe works in finance department.
• A relationship can have description attributes.
– Joe has worked in finance department since 5/2001.
• Relationship Set: Collection of similar relationships.
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Ternary Relationship
name
since
budget
did
ssn
(Joe)
dname
Employees
address
Works_In
Locations
Departments
(finance dept)
capacity
(Taipei)
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Roles in Relationship
name
ssn
(Roles)
Employees
supervisor
subordinate
Reports_To
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Key Constraints
• Describe at most once (entitity) relationship
– Manages relationship: each department has at most one manager (okay to have none).
– One department can appear at most once in Manages relationship set, also called one-tomany relation.
name
dname
since
did
ssn
Employees
Mary
Joe
Alice
Peter
Manages
3/3/93
budget
Departments
Finance
Accounting
2/2/92
3/1/92
Research
Legal
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More Key Constraints
Women
Give Birth
1-to-1
Men
Married
Babies
1-to Many
Women
Many-to-Many
Men
Befriends
Women
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Participation Constraints
• Describe all (entitity) participation relationship
– Must every department have a manager?
•
–
If yes, this is a participation constraint
All Departments entities must participate in the Manages relationship set (total participation).
since
name
ssn
did
lot
Employees
dname
Manages
budget
Departments
Works_In
since
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Weak Entities
• A weak entity can be identified uniquely only by considering the key of another
(owner) entity.
–
–
–
Pname = partial key
Owner entity set and weak entity set must participate in a one-to-many relationship set (one
owner, many weak entities).
Weak entity set must have total participation in this identifying relationship set.
(Alicia)
name
cost
ssn
(Hao)
Employees
Policy
(2)
pname
age
Dependents
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ISA (`is a’) Hierarchies
name
ssn
superclass entity
Employees
hourly_wages
hours_worked
ISA
contractid
subclass entities Hourly_Emps
•
•
Contract_Emps
As in C++ and OO languages, attributes are inherited from superclass.
A ISA B, every A entity is also considered to be a B entity.
•
Reason for using ISA:
•
•
Add descriptive attributes specific (make sense) to a subclass.
Identify entities that make sense to a relationship (policy).
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ISA (`is a’) Constraints
name
ssn
Employees
hourly_wages
hours_worked
ISA
Hourly_Emps
contractid
Contract_Emps
• Overlap constraints: Can Joe be an Hourly_Emps as well as a Contract_Emps entity?
(Allowed/disallowed)
• Covering constraints: Does every Employees entity also have to be an Hourly_Emps
or a Contract_Emps entity? (Yes/no)
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Aggregation
•
•
Create relationship set from
relationship sets.
Aggregation: relationship set turns into
an entity set
–
name
ssn
Employees
Monitors
until
So that they can participate in (other)
relationships.
since
started_on
pid
pbudget
Projects
dname
did
Sponsors
budget
Departments
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Design Guideline
1. Avoid redundancy.
2. Don’t use an entity set when an attribute will do.
3. Limit the use of weak entity sets.
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Avoiding Redundancy
• Redundancy occurs when we say the same thing in two different
ways.
• Redundancy is bad
– wastes space
– encourages inconsistency.
• The two instances of the same fact may become inconsistent if we change one
and forget to change the other instance.
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Redundancy Example
name
Beers
name
ManfBy
addr
Manfs
manf
This design states the manufacturer of a beer twice: as an attribute
and as a related entity.
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Fix Redundancy
name
Beers
name
ManfBy
addr
Manfs
This design gives the address of each manufacturer exactly once.
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Example: Bad
name
manf
manfAddr
Beers
This design repeats the manufacturer’s address once for each beer.
Why is it bad?
•Manf updates its address.
•Loses the address if there are temporarily no beers for a
manufacturer.
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Entity Sets Versus Attributes
•
An entity set should satisfy at least one of the
following conditions:
–
–
It is more than the name of something; it has at least one
nonkey attribute.
or
It is the “many” in a many-one or many-many relationship.
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Example: Okay
name
Beers
name
ManfBy
addr
Manfs
•Manfs deserves to be an entity set because of the nonkey attribute
addr.
•Beers deserves to be an entity set because it is the “many” of the
many-one relationship ManfBy.
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Example: Okay
name
manf
Beers
There is no need to make the manufacturer an entity set, because we
record nothing about manufacturers besides their name.
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Example: Bad
name
Beers
name
ManfBy
Manfs
Since the manufacturer is nothing but a name, and is not at the “many”
end of any relationship, it should not be an entity set.
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Don’t Overuse Weak Entity Sets
• Beginning database designers often doubt that anything could
be a key by itself.
– They make all entity sets weak, supported by all other entity sets to
which they are linked.
• In reality, we usually create unique ID’s for entity sets.
– Examples include social-security numbers, automobile VIN’s etc.
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When Do We Need Weak Entity Sets?
• The usual reason is that there is no global authority
capable of creating unique ID’s.
• Example: it is unlikely that there could be an agreement
to assign unique player numbers across all football teams
in the world.
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Exercise 2.2 (R-G Book)
A university database contains information about professors (identified by
social security number) and courses (identified by courseid). Professors
teach courses; each of the following situations concerns the Teaches
relationship set. For each situation, draw an ER diagram that describes it.
• Professors can teach the same course in several semesters, and each
offering must be recorded.
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• Professors can teach the same course in several semesters, and only the
most recent such offering needs to be recorded.
• Every professor must teach some courses
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• Every professor teaches exactly one course (no more, no less)
• Every professor teaches exactly one course (no more, no less), and every
course must be taught by some professor
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Exercise 2.3 (R-G Book)
• Professors have an SSN, a name, an age, a rank, and a research specialty.
• Projects have a project number, a sponsor name (e.g., NSF), a starting date, an ending
date, and a budget.
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•
•
•
•
Graduate students have an SSN, a name, an age, and a degree program
Each project is managed by exactly one professor (known as PI)
Each project is worked on by one or more professors (known as Co-PIs)
Each project is worked on by one or more graduate students (known as RAs)
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• When graduate students work on a project, a professor must supervise their work on the
project. Graduate students can work on multiple projects, in which case they will have a
potentially different supervisor for each one
• Departments have a department number, a department name, and a main office.
• Department has a professor (known as Chairman) who runs the department.
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• Professors work in one or more departments, and for each department that they work in,
a time percentage is associated with their job
• Graduate students have one major department in which they are working on their degree.
• Each graduate student must have another, more senior graduate student as an advisor.
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Summary
• ER model is popular for conceptual design
– Sketch the design of a database informally using pictures
• Basic constructs in ER model:
– entities, relationships, and attributes (of entities and relationships).
• Some additional constructs:
– weak entities, ISA hierarchies, and aggregation.
• Several kinds of integrity constraints:
– key constraints, participation constraints, and overlap/covering constraints for ISA
hierarchies.
• Design guideline in ER model
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