1 Database Systems ( 資料庫系統 ) 9/23/2009 Lecture #2

2 Course Administration 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

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: –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 Beer names 台灣啤酒 青島啤酒 台灣生啤酒 BeersManfs ManfBy name addr 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 ?? Beer namesManufacturer's names 台灣啤酒台灣菸酒公賣局 台灣生啤酒台灣菸酒公賣局 青島啤酒青島啤酒廠

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

8 ER Model: Relationship 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. dname budget did since name Works_In DepartmentsEmployees ssn (5/2001) (finance dept) (Joe)

9 Ternary Relationship dname budget did since name Works_In DepartmentsEmployees ssn capacity Locations address (Joe)(finance dept) (Taipei)

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

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 budgetdid since name ssn Employees Departments Manages Joe Alice Mary Peter Finance Accounting Research Legal 3/3/93 2/2/92 3/1/92

12 More Key Constraints 1-to-11-to Many Many-to-Many WomenGive Birth Babies Marry Women Men Befriend Women Men

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). lot name dname budgetdid since name dname budgetdid since Manages since Departments Employees ssn Works_In

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

15 ISA (`is a ’ ) Hierarchies 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). subclass entities superclass entity Contract_Emps name ssn Employees hourly_wages ISA Hourly_Emps contractid hours_worked

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

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

18 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.

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 BeersManfs ManfBy name This design states the manufacturer of a beer twice: as an attribute and as a related entity. name manf addr

21 Fix Redundancy BeersManfs ManfBy name This design gives the address of each manufacturer exactly once. nameaddr

22 Example: Bad Beers name 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. manfmanfAddr

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

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 BeersManfs ManfBy name 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. nameaddr

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

37 Example: Okay Beers name There is no need to make the manufacturer an entity set, because we record nothing about manufacturers besides their name. manf

38 Example: Bad BeersManfs ManfBy name 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. name