[PDF] - More Design Issues 1. Roles. 2. Sub classes. 3. Keys. 4. W PDF Document

SLIDE 1 More Design Issues 1. Roles. 2. Sub classes. 3. Keys. 4. W eak en tit y sets. 5. Design principles and some hard examples. 1

SLIDE 2 Roles Sometimes an E.S. participates more than

nce

in a relationship.

Lab

el edges with r

les

to distinguish. Married Drink ers wife h usband Husband Wife d 1 d 2 d 3 d 4

2

SLIDE 3 Drink ers 1 2 Buddies Buddy1 Buddy2 d 1 d 2 d 1 d 3 d 2 d 1 d 2 d 4

Notice

Buddies is symmetric, Marrie d not.

✦

No w a y to sa y \symmetric" in E/R.

✦

But in ODL, symmetric relations are their

wn

in v erse. 3

SLIDE 4 Roles in ODL No problem; names

f

relationships handle \roles." interface Drinkers { attribute string name; attribute Struct Bars::Addr address; relationship Set<Beers> likes inverse Beers::fans; relationship Set<Bars> frequents inverse Bars::customers; relationship Drinkers husband inverse wife; relationship Drinkers wife inverse husband; relationship Set<Drinkers> buddies inverse buddies; }

Notice

that Drinkers:: is

ptional

when the in v erse is a relationship

f

the same class. Design Issue Should w e replace husband and wife b y

ne

relationship spouse? 4

SLIDE 5 Sub classes Sub class = sp ecial case = few er en tities/ob jects = more prop erties.

Example:

Ales are a kind

f

b eer. In addition to the prop erties (= attributes, relationships, metho ds)

f

b eers, there is a \color" attribute for ales. ODL Sub classes F

llo

w name

f

sub class b y colon and its sup erclass. interface Ales:Beers { attribute String color; }

Ob

jects

f

the Ales class acquire all the attributes and relationships

f

the Beers class. 5

SLIDE 6 E/R Sub classes

isa

triangles indicate the sub class relation. isa Beers Ales name manf color 6

SLIDE 7 Dierence in Sub class Viewp

in

ts

In

ODL, an en tit y is in exactly

ne

class.

✦

It inherits prop erties

f

its sup erclass(es).

In

E/R, an en tit y has \represen tation" in all the sub classes to whic h it logicall y b elongs.

✦

Its prop erties are the union

f

the prop erties

f

these classes.

The

distincti

n

matters later, when w e con v ert ODL and E/R to relations. isa Beers Ales name manf color P ete's Ale 7

SLIDE 8 Multiple Inheritance Theoreticall y , a class/E.S. could b e a sub class

f

sev eral classes. isa isa name manf name manf Beers Wines Grap e Beers 8

SLIDE 9 Problems Ho w should conicts b e resolv ed?

Example:

manf means gro w er for wines, b

ttler

for b eers. What do es manf mean for \grap e b eers"?

E/R

m ute

n

m ultiple inheritance; ODL lea v es it to implemen ter. 9

SLIDE 10 Keys = set

f

attributes whose v alues can b elong to at most

ne

en tit y

r
b

ject.

In

E/R: underline all k ey attributes.

In

E/R, eac h E.S. m ust ha v e a k ey .

Example:

Supp

se

name is k ey for Be ers. isa Beers Ales manf color name

Beer

name is also k ey for ales.

✦

In general, k ey at ro

t

(no m ultiple inheritance!) is k ey for all. 10

SLIDE 11 Keys in ODL Indicate with key(s) follo wing the class name, and a list

f

attributes forming the k ey .

✦

Sev eral lists ma y b e used to indicate sev eral alternativ e k eys.

✦

P aren theses group mem b ers

f

a k ey , and also group key to the declared k eys.

✦

Th us, (key(a 1 ; a 2 ; : : : ; a n )) = \one k ey consisting

f

all n attributes." (key a 1 ; a 2 ; : : : ; a n ) = \eac h a i is a k ey b y itself.

Example:

interface Beers (key name) { attribute string name ... 11

SLIDE 12 A Multiattribute Key Courses dept n um b er hours ro

m

interface Courses (key (dept, number), (room, hours)) { ...

E/R

requires exactly

ne

k ey , but ODL allo ws zero

r

more k eys.

V

ery Imp

rtan

t: In ODL, \ob ject iden tit y" serv es to distinguish

b

jects, so no key at al l is ne c essary. 12

SLIDE 13 W eak En tit y Sets Sometimes an E.S.'s k ey comes not (completely) from its

wn

attributes, but from the k eys

f
ne
r

more E.S's to whic h the rst is link ed b y a man y-one relationship.

Called

a we ak E.S.

Represen

ted b y putting double rectangle around the w eak E.S. and a double diamond around eac h relationship to whic h the w eak E.S. is link ed to an E.S. that pro vides part

f

its k ey .

Use
f

man y-one relationship (includes 1-1) essen tial.

✦

With a man y-man y , w e w

uldn't

kno w whic h en tit y pro vided the k ey v alue.

V

ery Imp

rtan

t: There is no suc h thing as a \w eak class" in ODL.

✦

Because

b

jects ha v e

b

ject-iden tit y , classes don't need k eys, and therefore they don't need to \b

rro

w" k eys from related classes. 13

SLIDE 14 Example: Logins Login name = user name + host name, e.g., ullman@shalmaneser .stan ford. edu.

A

\login" en tit y corresp

nds

to a user name

n

a particular host, but the passwd table do esn't record the host, just the user name, e.g. ullman.

Key

for a login = the user name at the host (whic h is unique for that host

nly)

+ the name

f

the host (whic h is unique globally) . @ Logins Hosts name name 14

SLIDE 15 Example: Chain

f

\W eakness" Consider IP addresses consisting

f

a primary domain (e.g., edu) sub domain (e.g., stanford), and host (e.g. shalmaneser). name name name Primary Domains In1 In2 Hosts Domains 2ndary

Key

for primary domain = its name.

Key

for secondary domain = its name + name

f

primary domain.

Key

for host = its name + k ey

f

secondary domain = its name + name

f

secondary domain + name

f

primary domain. 15

SLIDE 16 All \Connecting" En tit y Sets Are W eak Beers Bars The- Beer Prices Bar The- The- Price BBP name addr name manf price

In

this sp ecial case, where bar and b eer determine a price, w e can

mit

price from the k ey , and remo v e the double diamond from ThePrice. 16