The Object Data Model

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Laureen Wilson
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1 The Object Data Model 1

2 What is different: -- object identity -- complex structure -- classes: attributes, methods, encapsulation -- inheritance --more complex query languages 2

3 Object identity -- built-in notion, similar to entities in ER model -- independent of value (unlike in relational dbs) Example parent age child age Peter 40 John 15 Sue 41 John Same child? With object identity: Peter Sue John 3

4 Object identity -- built-in notion, similar to entities in ER model -- independent of value (unlike in relational dbs) Example parent age child age Peter 40 John 15 Sue 41 John Same child? With object identity: Peter Sue John John 4

5 Class: type + extent --extent: set of objects in class (like database instance) --type: structure + methods encapsulation: separate interface from implementation Pseudo-code example: class employee { string name; int annual salary(); string address; string get-name(); int salary; int employment-length() }; attributes methods 5

6 Inheritance: class is a special case of given class --what is inherited: structure + methods Example class person { string name; string address; } class customer isa person { int credit-rating; } class employee isa person { date start-date; int salary; } class officer isa employee { int office-number; int expense-account-number; } customer person employee officer Multiple inheritance: when one class is subclass of several classes -- potential ambiguity 6

7 Relationships Beside attributes, an object may have some relationships with other objects. These can be 1-1, 1-n or m-n relationships. Let us take an example: 7

8 Consider these classes: Person Course Address Student Employee Professor 8

9 Attributes and Methods Person Student Employee Professor Course Address name, birth_date, age() takes, activities() number, activities() grade subject, notes() street, city 9

10 Relationships spouse Professor Person children parents teaches taught_by address Course taken_by takes Student Address prerequisite 10

11 Graphic representation of more complex structure: travel agency example has-name name person lives-at speaks goes-to home set language address street city zip tuple is-a set destination located-at is-a works-for employer is-a business tourist set enjoys Business traveler works-as occupation has-name bname activity 11

12 Another complex structure (nested relation) Director Hitchcock Title The Birds Psycho Movies Actors Tippi Hedren Rod Taylor Alfred Hitchcock Anthony Perkins Janet Leigh Alfred Hitchcock Cinema: set ( tuple ( Director: char, Movies: set( tuple ( Title: char, Actors: set (char))))) Other structures: list, bag Coppola Title Apocalypse Godfather Actors Robert Duvall Marlon Brando Marlon Brando Al Pacino Diane Keaton Example query: find all movies whose director is not an actor in the movie select m.title from f in Cinema, m in f.movies where f.director not in m.actors 12

13 Complex structure Director Title Movies Actors Another example query: find all directors that have not acted in any Hitchcock movie Hitchcock The Birds Psycho Title Tippi Hedren Rod Taylor Alfred Hitchcock Anthony Perkins Janet Leigh Alfred Hitchcock Actors select f.director from f in Cinema where f.director not in flatten (select m.actors from g in Cinema, m in g.movies where g.director = Hitchcock ) Coppola Apocalypse Robert Duvall Marlon Brando flatten operator: constructs the union of sets of actors Godfather Marlon Brando Al Pacino Diane Keaton there are more operators to deal with complex structures 13

14 Another example: building complex answers Menu Restaurant name: string menu: list <Menu > address: {street: string, city: City} rate: int contents: list<string> City name: string select struct(restaurant: r.name, choices: select menu.contents from r.menu as menu where menu.rate < 40) from Restaurant as r where r.address.city.name = «San Francisco» and (exists menu in r.menu: menu.rate < 40) 14

SQL:Union, Intersection, Difference. SQL: Natural Join

SQL:Union, Intersection, Difference Union UNION Intersection INTERSECT Difference EXCEPT Find all actors or directors