Database design and implementation CMPSCI 645. Lecture 06: Constraints and E/R model

Transcription

1 Database design and implementation CMPSCI 645 Lecture 06: Constraints and E/R model

2 Constraints } Constraint: a property that we want our data to sa2sfy } Enforce by taking ac2ons: } Forbid an update } Or perform compensa2ng updates } Two approaches: } Declara2ve constraints } Triggers 2

3 Constraints in SQL } Keys, foreign keys } ACribute-level constraints } Tuple-level constraints } Global constraints: asser2ons simple complex } The more complex the constraint, the harder it is to check and enforce 3

4 Keys Product(name, category) CREATE TABLE Product (!!name CHAR(30) PRIMARY KEY,!!category VARCHAR(20))! OR: CREATE TABLE Product (!!name CHAR(30),!!category VARCHAR(20)!!PRIMARY KEY (name))! 4

5 Keys with Multiple Attributes Product(name, category, price) CREATE TABLE Product (!!name CHAR(30),!!category VARCHAR(20),!!price INT,! PRIMARY KEY (name, category))! Name Category Price Gizmo Gadget 10 Camera Photo 20 Gizmo Photo 30 Gizmo Gadget 40 5

6 Other Keys CREATE TABLE Product (!!productid CHAR(10),!!name CHAR(30),!!category VARCHAR(20),!!price INT,!PRIMARY KEY (productid),!!unique (name, category))! There is at most one PRIMARY KEY; there can be many UNIQUE 6

7 Foreign Key Constraints CREATE TABLE Purchase (!!prodname CHAR(30)!!!REFERENCES Product(name),!!date DATETIME)! may write just Product prodname is a foreign key to Product(name) name must be a key in Product (primary key or unique) Product Purchase Name Gizmo Camera OneClick Category gadget Photo Photo ProdName Gizmo Camera Camera Store Wiz Ritz Wiz 7

8 Foreign Key Constraints CREATE TABLE Purchase (!!prodname CHAR(30),!!category VARCHAR(20),!!date DATETIME,!!store VARCHAR(30),!!FOREIGN KEY (prodname, category)! REFERENCES Product(name, category)! Product(name, category, price) Purchase(proName, category, date 8

9 What happens during updates? Types of updates: } In Purchase: insert/update } In Product: delete/update Product Name Category Purchase ProdName Store Gizmo gadget Gizmo Wiz Camera Photo Camera Ritz OneClick Photo Camera Wiz 9

10 What happens during updates? } SQL has three policies for maintaining referen2al integrity: } Reject viola2ng modifica2ons (default) } Cascade: a_er a delete/update do a delete/update } Set-null set foreign-key field to NULL CREATE TABLE Purchase (!!prodname CHAR(30)!!!REFERENCES Product(name)!!!ON DELETE SET NULL!!!ON UPDATE CASCADE)! 10

11 Constraints on Attributes and Tuples } Constraints on acributes: NOT NULL -- obvious meaning... CHECK condi2on -- any condi2on! CREATE TABLE Purchase (!!store VARCHAR(30) NOT NULL, )! CREATE TABLE Product(!!price INT CHECK(price>0 and price <999))! } Constraints on tuples CHECK condi2on CHECK(price*quantity < 1000)! 11

12 Constraint Examples CREATE TABLE Purchase (!!prodname CHAR(30)!!!CHECK (prodname IN SELECT Product.name FROM Product),!!date DATETIME NOT NULL)! O_en not implemented in DBMSs! 12

13 General Assertions CREATE ASSERTION myassert CHECK NOT EXISTS(!!SELECT Product.name!FROM Product, Purchase!WHERE Product.name = Purchase.prodName!GROUP BY Product.name!HAVING count(*) > 200)! 13

14 Semantic Optimization with Constraints Product(name, price) Purchase(buyer, seller, prodname, store) SELECT Purchase.store FROM Product, Purchase WHERE Product.name = Purchase.prodName!? SELECT Purchase.store FROM Purchase! 14

15 Semantic Optimization with Constraints Product(name, price) Purchase(buyer, seller, prodname, store) SELECT Purchase.store FROM Product, Purchase WHERE Product.name = Purchase.prodName! SELECT Purchase.store FROM Purchase! Yes, if Purchase.prodName is a foreign key, and not null 15

16 Database Design } Why do we need it? } Need a way to model real world en22es in terms of rela2ons } Not easy to go from real-world en22es to a database schema } Consider issues such as: } What en22es to model } How en22es are related } What constraints exist in the domain } How to achieve good designs } Several formalisms exists } We discuss E/R diagrams 16

17 Database Design Process Today Data Modeling Next lecture Refinement SQL Tables Files Conceptual Schema E/R diagrams Rela2ons Physical Schema 17

18 Conceptual Schema Design name Conceptual Model: Pa2ent pa2ent_of Doctor zip name dno Rela2onal Model: plus FDs (FD = Func2onal Dependency) Normaliza2on: Eliminates anomalies 18

19 Entity / Relationship Diagrams Product en2ty set address All en22es in the same en2ty set have the same acributes buys Associa2on between 2 or more en22es A rela2onship may have acributes too! 19

20 name category stockprice name Product makes Company price since buys employs Person address name ssn 20

21 Keys in E/R Diagrams } Every en2ty set must have a key price name category Product 21

22 What is a Relationship? } if A, B are sets, then a rela2onship R is a subset of A B } A={1,2,3}, B={a,b,c,d}, } A B = {(1,a),(1,b),..., (3,d)} } R = {(1,a), (1,c), (3,b)} } makes is a subset of Product Company: A a b c d B Product makes Company 22

23 Multiplicity of E/R Relations } one-one: a b c d } one-many: a b c d } many-many: a b c d 23

24 name category stockprice name Product makes Company price What does this mean? since buys employs Person address name ssn 24

25 Multi-way Relationships How do we model a work rela2onship between employees, departments and loca2ons? Employees WorksIn Departments Loca2ons 25

26 Key Constraints in Multi-way Relationships Q: What does the arrow mean? Employees WorksIn Departments Loca2ons A: A given employee works at a single department and loca2on 26

27 Participation Constraints Q: Every employee works in at least one department. Employees WorksIn Departments 27

28 Participation Constraints Q: Every employee works in at least one department, and every department has at least one employee Employees WorksIn Departments 28

29 Participation Constraints Q: What does this mean? Employees WorksIn Departments Manages A: previous + every department has exactly one manager 29

30 Constraints in E/R Diagrams } Finding constraints is part of the modeling process. } Commonly used constraints: } Keys: social security number uniquely iden2fies a person. } Single-value constraints: a person has only one (biological) mother. } Referen2al integrity constraints: if you work for a company, it must exist in the database. } Other constraints: peoples ages are between 0 and

31 Keys in E/R Diagrams price name category underline Product No formal way to specify mul2ple keys Person ssn name address 31

32 Single Value Constraints makes vs makes 32

33 Referential Integrity Constraints Product makes Company Each product made by at most one company. Some products made by no company Product makes Company Each product made by exactly one company. 33

34 Other Constraints Product makes <100 Company What does this mean? 34

35 Design Principles What s wrong? Product Purchase Person Country President Person Be faithful to the specifica2ons of the app! 35

36 Design Principles What s wrong? date Product Purchase Store personaddr personname Pick the right kind of en22es! 36

37 Design Principles What s wrong? Dates date Product Purchase Store Person Don t complicate your design more than necessary! 37

38 From E/R Diagrams to Relational Schema } En2ty set à rela2on } Rela2onship à rela2on 38

39 Entity Set to Relation price name category Product Product(name, category, price) Name Category Price Gizmo Gadgets $

40 Relationships to Relations name startyear price category stockprice name Product makes Company Watch out for acribute name conflicts Makes(product-name, product-category, companyname, year) ProductName ProductCategory CompanyName startyear Gizmo Gadgets GizmoWorks 1963 Foreign keys 40

41 Modeling Subclasses } Some objects in a class may be special } Define a new class } BeCer: define a subclass Products Software products Educational products So --- we define subclasses in E/R 41

42 Subclasses price name category Product platforms isa isa agegroup Software Product Educational Product 42

43 Subclasses to Relations price name category Product Name Price Category Product Gizmo 99 gadget platforms agegroup isa isa Software Product Educational Product So_ware Product Name plaqorms Gizmo unix Camera 49 photo Toy 39 gadget Educa2onal Product Name agegroup Gizmo toddler Camera adult 43

44 E/R Inheritance So_wareProduct sp1 sp2 p1 Product p2 p3 ep1 ep3 En2ty sets overlap Educa2onalProduct ep2 Product No need for mul2ple inheritance So_wareProduct sp1 sp2 p1 p2 p3 esp1 esp2 ep1 Educa2onalProduct ep2 ep3 44

45 Modeling Union Types With Subclasses FurniturePiece Person Company Say: each piece of furniture is owned either by a person, or by a company 45

46 Modeling Union Types with Subclasses } Solu2on 1: acceptable, but imperfect (why?) Person FurniturePiece Company ownedbyperson ownedbycomp. 46

47 Modeling Union Types with Subclasses } Solu2on 2: becer, more laborious Owner isa disjoint ownedby Person Company FurniturePiece In fact there is no formal way to represent disjoint vs overlapping subclasses. 47

48 Weak Entity Sets name ssn dept age name Employees policy Dependents En2ty sets are weak when their key comes from classes to which they re related 48

49 Handling Weak Entity Sets name ssn dept age name Employees policy Dependents Employee(ssn, name, dept) Dependents(ssn,name,age) We should have an arrow here, otherwise Dependents key would have NULL value! No need to represent policy separately 49