Lecture 9:Conceptual Database Design

January 27th, 2003

Building an Application with a DBMS

• Requirements modeling (conceptual, pictures)– Decide what entities should be part of the application and

how they should be linked.

• Schema design and implementation– Decide on a set of tables, attributes.

– Define the tables in the database system.

– Populate database (insert tuples).

• Write application programs using the DBMS– way easier now that the data management is taken care of.

Database Design

• Why do we need it?– Agree on structure of the database before

deciding on a particular implementation.

• Consider issues such as:– What entities to model– How entities are related– What constraints exist in the domain– How to achieve good designs

Database Design Formalisms1. Object Definition Language (ODL):

– Closer in spirit to object-oriented models

– I don’t teach it anymore.

2. Entity/Relationship model (E/R):– More relational in nature.

• Both can be translated (semi-automatically) to relational schemas

• ODL to OO-schema: direct transformation (C++ or Smalltalk based system).

2. Entity / Relationship Diagrams

Entities

Attributes

Relationships between entities

Product

address

buys

Keys in E/R Diagrams

• Every entity set must have a key

Product

name category

price

address name ssn

Person

buys

makes

employs

CompanyProduct

name category

stockprice

name

price

What is a Relation ?

• A mathematical definition:– if A, B are sets, then a relation R is a subset of

A x B

• A={1,2,3}, B={a,b,c,d}, R = {(1,a), (1,c), (3,b)}

- makes is a subset of Product x Company:

1

2

3

a

b

c

d

A=

B=

makes CompanyProduct

Multiplicity of E/R Relations

• one-one:

• many-one

• many-many

123

abcd

123

abcd

123

abcd

address name ssn

Person

buys

makes

employs

CompanyProduct

name category

stockprice

name

price

What doesthis say ?

Multi-way RelationshipsHow do we model a purchase relationship between buyers,products and stores?

Purchase

Product

Person

Store

Can still model as a mathematical set (how ?)

Q: what does the arrow mean ?

A: if I know the store, person, invoice, I know the movie too

Rental

VideoStore

Person

Movie

Invoice

Arrows in Multiway Relationships

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

Q: how do I say: “invoice determines store” ?

A: no good way; best approximation:

Q: Why is this incomplete ?

Rental

VideoStore

Person

Movie

Invoice

Arrows in Multiway Relationships

Roles in Relationships

Purchase

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

Product

Person

Store

salesperson buyer

Attributes on Relationships

Purchase

Product

Person

Store

date

Converting Multi-way Relationships to Binary

Purchase

Person

Store

Product

StoreOf

ProductOf

BuyerOf

date

From E/R Diagramsto Relational Schema

• Entity set relation

• Relationship relation

Entity Set to Relation

Product

name category

price

Product(name, category, price)

name category price

gizmo gadgets $19.99

Relationships to Relations

makes CompanyProduct

name category

Stock price

name

Makes(product-name, product-category, company-name, year) Product-name Product-Category Company-name Starting-year

gizmo gadgets gizmoWorks 1963

Start Year

price

(watch out for attribute name conflicts)

Relationships to Relations

makes CompanyProduct

name category

Stock price

name

No need for Makes. Modify Product:

name category price StartYear companyName

gizmo gadgets 19.99 1963 gizmoWorks

Start Year

price

Multi-way Relationships to Relations

Purchase

Product

Person

Storename price

ssn name

name address

Purchase( , , )

3. Design Principles

PurchaseProduct Person

What’s wrong?

President PersonCountry

Moral: be faithful!

Design Principles:What’s Wrong?

Purchase

Product

Store

date

personNamepersonAddr

Moral: pick the right kind of entities.

Design Principles:What’s Wrong?

Purchase

Product

Person

Store

dateDates

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

Modeling Subclasses

The world is inherently hierarchical. Some entities are special cases of others

• We need a notion of subclass.• This is supported naturally in object-oriented formalisms.

Products

Software products

Educational products

Product

name category

price

isa isa

Educational ProductSoftware Product

Age Groupplatforms

Subclasses in E/R Diagrams

Understanding Subclasses

• Think in terms of records:– Product

– SoftwareProduct

– EducationalProduct

field1

field2

field1

field2

field1

field2

field3

field4field5

Subclasses to Relations

Product

name category

price

isa isa

Educational ProductSoftware Product

Age Groupplatforms

Name Price Category

Gizmo 99 gadget

Camera 49 photo

Toy 39 gadget

Name platforms

Gizmo unix

Name Age Group

Gizmo todler

Toy retired

Product

Sw.Product

Ed.Product

Modeling UnionTypes With Subclasses

FurniturePiece

Person Company

Say: each piece of furniture is owned either by a person, or by a company

Modeling Union Types with Subclasses

Say: each piece of furniture is owned either by a person, or by a company

Solution 1. Acceptable, imperfect (What’s wrong ?)

FurniturePiecePerson Company

ownedByPerson ownedByPerson

Modeling Union Types with Subclasses

Solution 2: better, more laborious

isa

FurniturePiece

Person CompanyownedBy

Owner

isa

Constraints in E/R Diagrams

Finding 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.

Other constraints: peoples’ ages are between 0 and 150.

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:

Single Value Constraints

makes

makes

v. s.

Referential Integrity Constraints

CompanyProduct makes

CompanyProduct makes

Other Constraints

CompanyProduct makes<100

What does this mean ?

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

UniversityTeam affiliation

numbersport name

Handling Weak Entity Sets

UniversityTeam affiliation

numbersport name

Convert to a relational schema (in class)