1. Database Systems (資料庫系統)
9/27 10/1, 2007
Lecture #2

2. Course Administration
Office Hour:
Thur 2-3
Can everyone get the textbook?
HW #1 will be on the course homepage later today
It is due 2 weeks from today.
Next week reading:
R&G Chapters 3 & 4.1~4.2

3. Chapter 2 Introduction to Database Design

4. Scenario
Say if you are hired by iBeer Retailer as a computer consultant.
iBeer wants you to design its database system.
How to design it?

5. 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).

6. Requirement Analysis Requirement analysis: Conceptual database design
The Beer retailer wants to keep track of
Beers on shelves
Beer manufacturers: [name & address]
Conceptual database design
ER diagram
Logical database design:
Relational model
Beers
Manfs
ManfBy
name
addr
Beer names
Manufacturer's names
台灣啤酒
台灣菸酒公賣局
台灣生啤酒
青島啤酒
青島啤酒廠
Beer names
台灣啤酒
青島啤酒
台灣生啤酒
Manufacturer’s names
Manufacturer’s addresses
台灣菸酒公賣局
台北市南昌路一段4號
青島啤酒廠 ??

7. ER Model: Entity (‘Joe’: string) (123: integer) (Joe, Alice, ..)
name
(123: integer)
ssn
Employees
(Joe, Alice, ..)
Proposed by Peter Chen (BS NTU EE ‘68) in 1976.
Entity: A 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.

8. ER Model: Relationship
(5/2001)
since
name
dname
ssn
did
budget
(Joe)
Employees
Works_In
Departments
(finance dept)
Relationship: Association among two or more entities
Joe works in finance department.
A relationship can have descriptive attributes.
Joe has worked in finance department since 5/2001.
Relationship Set: Collection of similar relationships. 4

9. Ternary Relationship (Joe) (finance dept) (Taipei) since name dname
ssn
did
budget
Employees
Works_In
Departments
(Joe)
(finance dept)
address
capacity
Locations
(Taipei) 4

10. Roles in Relationship (Roles) name ssn Employees supervisor
subordinate
Reports_To

11. 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-to-many relation.
dname
name
since
ssn
did
budget
Employees
Manages
Departments
at most once = zero time?
Finance
Mary
3/3/93
Accounting
Joe
2/2/92
Research
Alice
3/1/92
Legal
Peter 6

12. More Key Constraints Women Give Birth Babies 1-to-1 1-to Many
IP address and hostnames
Many-to-Many
Marry
Women
Men
Men
Befriend
Women 6

13. 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
since
name
name
dname
dname
ssn
lot
did
did
budget
budget
one and only one
Employees
Manages
Departments
Works_In
since 8

14. 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)
(2)
name
cost
ssn
pname
age
(Hao)
Employees
Policy
Dependents 10

15. ISA (`is a’) Hierarchies
name
ssn
Employees
superclass entity
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.
Why using ISA?
Add descriptive attributes specific (make sense) to a subclass.
Identify entities that make sense to a relationship (policy). 12

16. ISA (`is a’) Constraints
name
ssn
Employees
hourly_wages
hours_worked
ISA
contractid
Hourly_Emps
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) 12

17. Aggregation Create relationship set from relationship sets.
name
ssn
Create relationship set from relationship sets.
Aggregation: relationship set turns into an entity set
So that they can participate in (other) relationships.
Employees
Monitors
until
Can we add a relationship set between an relationship set and an entity
started_on
since
dname
pid
pbudget
did
budget
Projects
Sponsors
Departments 2

18. Design Guideline Avoid redundancy.
Don’t use an entity set when an attribute will do.
Limit the use of weak entity sets.

19. 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.

20. Redundancy Example name name addr ManfBy Beers Manfs manf
This design states the manufacturer of a beer twice: as an attribute and as a related entity.

21. Fix Redundancy name name addr ManfBy Beers Manfs
This design gives the address of each manufacturer exactly once.

22. 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.

23. 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.

24. 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.

25. 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.

26. 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

27. 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

28. 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.

29. 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 in by one or more professors (known as Co-PIs)
Each project is worked on by one or more graduate students (known as RAs)

30. 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 project.
Departments have a department number, a department name, and a main office.
Department has a professor (known as Chairman) who runs the department.

31. 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.

32. 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 11

33. 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”

34. Entity Sets Versus Attributes
Modeling a concept with a new 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.

35. Example: Okay name name addr ManfBy Beers 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.

36. Example: Beers Entity not Needed
name
name
addr
ManfBy
Beers
Manfs
Beers can be an attribute rather than an entity.

37. 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.

38. Example: Bad name name ManfBy Beers 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.