2. E/R Design Considerations

Transcription

1 2. E/R Design Considerations 32

2 What you will learn in this section Relationships cont d: multiplicity, multi-way Design considerations Conversion to SQL 33

3 Multiplicity of E/R Relationships

4 Multiplicity of E/R Relationships One-to-one: Many-to-one: a b c d 1 a 2 b 3 c d Indicated using arrows One-to-many: a b c d Many-to-many: a b c d 35

5 Example: One to One Relationships (1:1) One-to-one: a b c d Def : Maximum one X for each Y and one Y for each X Manager_ID Branch_ID MANAGER manages BRANCH X Y An employee (manager) can manage only one branch at one time and each branch only has one manager. 36

6 Example: One to Many Relationships (1:M) One-to-many: a b c d Def : Maximum one X for each Y but possibly many Y for each X Faculty_Name Course_Code Faculty_Name FACULTY offer COURSE X Y At university, one faculty offers many courses for students but one course is offered by one faculty only. 37

7 Example: Many to Many Relationships (M:N) Many-to-many: a b c d Def : Possibly many X s for each Y and many Y s for each X ISDN Author_ID BOOK Written by AUTHOR A book may have more than one author and an author may write more than one book. 38

8 name category name price Product makes Company buys What does this say? employs stockprice Person address name ssn 39

9 Activity in the class For actors and directors, we want to store their name, a unique identification number, address and birthday (why not age?) For films, we want to store the title, year of production and type (thriller, comedy, etc.) We want to know who directed and who acted in each film. Every film has one director. We store the salary of each actor for each film For Studio we want to store name, address and name of its president. A movie can be produced by only one studio.

10 address id name dateofbirth title year type Actors actedin Film name president directs address produces Studio dateofbirth Director address name id 41

11 Self Relationships or Recursive Relationships

12 manager manages EMPLOYEE worker

13 Multi-way Relationships

14 Multi-way Relationships How do we model that a studio has made a contract with a particular actor to act in a particular movie? Actor Contract Film Studio NB: Can still model as a mathematical set (how?) 45

15 An n-ary relationship set R involves exactly n entity sets: E 1,, E n. Each relationship in R involves exactly n entities: Formally, R E 1 x x E n Formal mathematical definition e 1 in E 1,, e n in E n E 1. R E 2. E n

16 Arrows in Multiway Relationships Q: What does the arrow mean? Actor For a particular star and movie there is only one studio under the contract Contract Film A start may contract with a studio for several movies Studio A studio may contract with many stars for a movie 47

17 Attributes in Relationships Q: Suppose we want to add the signing money information. Where do we put? salary Actor salary Contract salary Film Studio 48

18 Attributes in Relationships: The correct way Q: Suppose we want to add the signing money information. Where do we put? Actor salary Contract Film Studio 49

19 Converting Multi-way Relationships to Binary Relations salary Contracts signedby formaking Actor Film From what we had on previous slide to this - what did we do? ownerof Studio 50

20 Converting Multi-way Relationships to New Entity + Binary Relationships salary signedby Actor Side note: What arrows should be added here? Are these correct? Contracts formaking ownerof Film Studio For a particular contract there can be only one star, one movie and one studio. But an actor or a movie or a studio may have several contracts 51

21 3. Design Principles

22 Design Principles What s wrong with these examples? Product Purchase Person Person ispresident Country Moral: be faithful! 53

23 Design Principles: What s Wrong? Movie OwnedBy Studio studioname name Moral: should not model the same information in multiple ways 54

24 Design Principles: What s Wrong? Product date Purchase Store personaddr personname Moral: pick the right kind of elements (entities/relationships). 55

25 Design Principles: What s Wrong? Dates date Product Purchase Store Moral: don t complicate life, it is already enough complicated. Person 56

26 Design Principles: Entity vs. Attribute Should address (A) be an attribute? How do we handle employees with multiple addresses here? Addr 1 Employee How do we handle addresses where internal structure of the address (e.g. postal code, city, division) is useful? 57

27 Design Principles: Entity vs. Attribute Should address (A) be an attribute? Or (B) be an entity? Street Addr Postal code Addr 1 Addr 2 Address AddrOf Employee Employee In general, when we want to record several values, we choose new entity 58

28 From E/R Diagrams to Relational Schema

29 From E/R Diagrams to Relational Schema Key concept: Both Entity sets and Relationships become relations (tables in RDBMS) 60

30 From E/R Diagrams to Relational Schema An entity set becomes a relation (collection of tuples / table) name price Product category Each tuple is one entity Each tuple is composed of the entity s attributes, and has the same primary key Product name price category Gizmo Camera Gizmo Edible 61

31 From E/R Diagrams to Relational Schema price category name CREATE TABLE Product( name CHAR(50) PRIMARY KEY, price FLOAT, category VARCHAR(30) ) Product Product name price category Gizmo Camera Gizmo Edible 62

32 From E/R Diagrams to Relational Schema Converting Many-to-Many Relationships to Converting Many-to-Many Relationships to Relational Schema

33 From E/R Diagrams to Relational Schema A relation between entity sets A 1,, A N also becomes a set of tuples / a table price name Product category date Purchased firstname Person lastname Each row/tuple is one relation, i.e. one unique combination of entities (a 1,,a N ) Each row/tuple is composed of the union of the entity sets attributes has the entities primary keys as foreign keys has the union of the entity sets keys as primary key Purchased name firstname lastname date Gizmo1 Bob Joe 01/01/15 Gizmo2 Joe Bob 01/03/15 Gizmo1 JoeBob Smith 01/05/15 64

34 From E/R Diagrams to Relational Schema CREATE TABLE Purchased( name CHAR(50), firstname CHAR(50), lastname CHAR(50), date DATE, PRIMARY KEY (name, firstname, lastname), FOREIGN KEY (name) REFERENCES Product(name), FOREIGN KEY (firstname, lastname) REFERENCES Person(firstname, lastname) ) price name Product category date Purchased Purchased firstname Person name firstname lastname date lastname Gizmo1 Bob Joe 01/01/15 Gizmo2 Joe Bob 01/03/15 Gizmo1 JoeBob Smith 01/05/15 65

35 From E/R Diagram to Relational Schema How do we represent this as a relational schema? Purchased (pname, sname, firstname, lastname, date) name category date firstname lastname price Product Purchased Person name Store address Foreign key (1) pname Product (name) (2) sname Store(name) (3) firstname, lastname Person(firstname, lastname) 66

36 From E/R Diagrams to Relational Schema Converting One-to-Many Relationships to Converting One-to-Many Relationships to Relational Schema

37 From E/R Diagram to Relational Schema For many to one relations we have two options: Option 1: Same as without key constraints (3 tables), except that the primary key of Directed is now (title, year) (why?) id Director Film (title, year, type) Director (id, name) Directed (title, year, id, salary) type Directed Film title name salary year 68

38 From E/R Diagram to Relational Schema Film (title, year, type) CREATE TABLE Film( title CHAR(50), year INT, type CHAR(40), PRIMARY KEY (title, year) ) CREATE TABLE Directed( title CHAR(50), year INT, CREATE TABLE Director( id INT, id INT, salary FLOAT, name CHAR(40), PRIMARY KEY (title, year), PRIMARY KEY (id) FOREIGN KEY (title, year) ) REFERENCES Film (title, year), FOREIGN KEY (id) REFERENCES Director(id) ) Director (id, name) Directed (title, year, id, salary) 69

39 From E/R Diagram to Relational Schema Film (title, year, type, id, salary) Option 2: Do not create a table for the relationship Director (id, name) Add information columns that would have been in the relationship's relation to the relation of the entity which does not have the incoming arrow type id Director Directed Film title name salary year 70

40 From E/R Diagram to Relational Schema CREATE TABLE Film( title CHAR(50), year INT, type CHAR(40), id INT, salary FLOAT, PRIMARY KEY (title, year), FOREIGN KEY (id) REFERENCES Director(id) ) Film (title, year, type, id, salary) Director (id, name) CREATE TABLE Director( id INT, name CHAR(40), PRIMARY KEY (id) ) 71

41 From E/R Diagrams to Relational Schema Converting (1:1) Relationships to Relational Converting (1:1) Relationships to Relational Schema

42 address name Studio Studio (name, address) President (sname, name, address) leadby address name President CREATE TABLE Studio( name CHAR(40), address CHAR(40), PRIMARY KEY (name) ) CREATE TABLE President( sname CHAR(40), name CHAR(40), address VARCHAR(50), PRIMARY KEY (sname), FOREIGN KEY (sname) REFERENCES Studio(name) )

43 Translating Recursive Relationship Sets (without constraints) manager id name Employee worker Manages Employee (id, name, address) address Manages (wid, mid) What are all the relations created for this diagram?