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Lecture 3: Database Modeling (continued)

April 5, 2002

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Q: what do these arrow mean ?

A: store, person, invoice determines movie and store, invoice, movie determines person

Rental

VideoStore

Person

Movie

Invoice

Arrows in Multiway Relationships: Correction

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Roles in Relationships

Purchase

What if we need an entity set twice in one relationship?

Product

Person

Store

salesperson buyer

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Attributes on Relationships

Purchase

Product

Person

Store

date

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Converting Multi-way Relationships to Binary

Purchase

Person

Store

Product

StoreOf

ProductOf

BuyerOf

date

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Design Principles

PurchaseProduct Person

What’s wrong?

President PersonCountry

Moral: be faithful!

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Design Principles:What’s Wrong?

Purchase

Product

Store

date

personNamepersonAddr

Moral: pick the right kind of entities.

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Design Principles:What’s Wrong?

Purchase

Product

Person

Store

dateDates

Moral: don’t complicate life more than it already is.

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Next Attactions

• Subclasses (2.4)

• Constraints: (2.5)

• Weak entity sets (2.6)

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Modeling Subclasses

Some objects in a class may be special• define a new class• better: define a subclass

Products

Software products

Educational products

So --- we define subclasses (in ODL and in E/R).

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Subclasses in ODL

interface SoftwareProduct: Product{ attribute string platform; attribute integer requiredMemory;}

interface EducationalProduct: Product{ attribute struct Interval {integer begin, integer end} ageGroup; attribute string topic;}

interface SoftwareProduct: Product{ attribute string platform; attribute integer requiredMemory;}

interface EducationalProduct: Product{ attribute struct Interval {integer begin, integer end} ageGroup; attribute string topic;}

The two classes inherit all the properties of Product.

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Understanding Subclasses

• Think in terms of records:– Product

– SoftwareProduct

– EducationalProduct

field1

field2

field1

field2

field1

field2

field3

field4field5

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Multiple Inheritance in ODL

Product

Educational Product

Educ-softwareProduct

Software Product

ageGrouptopic

Platformsrequired memory

Educational-method

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interface EducSoftwareProduct:

SoftwareProduct, EducationalProduct {

attribute string educational-method;

}

interface EducSoftwareProduct:

SoftwareProduct, EducationalProduct {

attribute string educational-method;

}

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Understanding Multiple Inheritance

• Think in terms of records:– EducSoftwareProduct

field1

field2field3

field4field5

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How do we resolve conflicts?

Product

Educational Product

Educ-softwareProduct

Software Product

ageGrouptopic

Platformsrequired memory

Educational-method

Rating(ATA)

Rating(ASA) Rating?

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Product

name category

price

isa isa

Educational ProductSoftware Product

Age Groupplatforms

Subclasses in E/R Diagrams

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• ODL: classes are disjoint

p1 p2

p3sp1

sp2

ep1

ep2

ep3

Difference between ODL and E/R inheritance

Product

SoftwareProductEducationalProduct

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• E/R: entity sets overlap

Difference between ODL and E/R inheritance

SoftwareProduct

EducationalProduct

p1 p2

p3sp1

sp2

ep1

ep2

ep3

Product

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• No need for multiple inheritance in E/R

• we have three entity sets, but four different kinds of objects

SoftwareProduct

EducationalProduct

p1 p2

p3sp1

sp2

ep1

ep2

ep3

Product

esp1 esp2

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Constraints

• A constraint = an assertion about the database that must be true at all times

• part of the db schema

• types in programming languages do not have anything similar

• correspond to invariants in programming languages

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Modeling ConstraintsFinding constraints is part of the modeling process. Commonly used constraints:

Keys: social security number uniquely identifies a person.

Single-value constraints: a person can have only one father.

Referential integrity constraints: if you work for a company, it must exist in the database.

Domain constraints: peoples’ ages are between 0 and 150. General constraints: all others (at most 50 students enroll in a class)

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KeysA set of attributes that uniquely identify an object or entity:

Person: social-security-number name name + address name + address + age

Perfect keys are often hard to find, so organizations usuallyinvent something.

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Keys

• Multi-attribute keys:– E.g. name + address

• Multiple keys:– E.g social-security-number, name + address

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Keys in ODL

Defining multiple keys:

(key ssn employeID (name address age))

interface Person (key ssn) { attribute string ssn; attribute string name; ... }

interface Person (key ssn) { attribute string ssn; attribute string name; ... }

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Keys in E/R Diagrams

address name ssn

Person

Product

name category

price

No formal way to specify multiple keys in E/R diagrams

Underline:

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Single Value Constraints

• Sometimes we can choose to allow one or more values

• ODL: – attributes are always single value– relationships have single or multiple values

relationship person president;

relationship set<person> presidents;

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Single Value Constraints

• E/R:

offers offers

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Single Value Constraints

Single Value Constraint:

• we explicitly require one value

• two flavors:– allow nulls– do not allow nulls

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Referential Integrity Constraints

• In some formalisms we may refer to other object but get garbage instead– e.g. a dangling pointer in C/C++

• the Referential Integrity Constraint explicitly requires a reference to exists

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Referential Integrity Constraints

• In ODL:– means that a relationship cannot be NULL

• In E/R:

CompanyProduct makes

CompanyProduct makes

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Weak Entity SetsEntity sets are weak when their key attributes come from otherclasses to which they are related.

This happens if:

- part-of hierarchies - splitting n-ary relations to binary.

UniversityTeam affiliation

numbersport name