entity/relationship modellinglsir · 2. entity / relationship diagrams objects entities classes...

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Entity/RelationshipModelling

Lecture 4

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Outline

• E/R model (Chapter 5)• From E/R diagrams to relational schemas

(Chapter 5)• Constraints in SQL (Chapter 4)

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1. Database Design

• Modelling– Decide which part of reality is represented in a

database– Agree on structure of the database before

deciding on a particular implementation• Conceptual Modelling

– Oriented towards application and users– Independent of technology and implementation

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Some Terminology

Classes of similarobjects (and theirrelationships)

students

Instances

Student-name-firstname-birthdate-section-year

Type (of table)

Database

Student

(relational) Schema

Real world perception Abstraction / Description

E/R Diagram

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Conceptual Modelling

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

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Database Design Formalisms1. Entity/Relationship model (E/R):

– More relational in nature.– Very widely used

2. Object Definition Language (ODL):– Closer in spirit to object-oriented models (e.g. Java)– Will not be covered

Both can be translated (semi-automatically) torelational schemas

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2. Entity / Relationship Diagrams

Objects entitiesClasses entity sets

Attributes are like in Java.

Relationships: like object references in Java except

- first class citizens (not associated with classes)

- bidirectional

Product

address

buys

8address name ssn

Person

buys

makes

employs

CompanyProduct

name category

stockprice

name

price

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Keys in E/R Diagrams

• Every entity set must have a key– a key is an attribute that has a different value

for every entity

Product

name category

price

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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:

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a

b

c

d

A=

B=

makes CompanyProduct

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Multiplicity of E/R Relations

• one-one:

• many-one

• many-many

123

abcd

123

abcd

123

abcd

12address name ssn

Person

buys

makes

employs

CompanyProduct

name category

stockprice

name

price

What doesthis say ?

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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 ?)

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Q: what does the arrow mean ?

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

Rental

VideoStore

Person

Movie

Invoice

Arrows in Multiway Relationships

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Q: what do these arrows mean ?

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

Rental

VideoStore

Person

Movie

Invoice

Arrows in Multiway Relationships

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Q: how do I say: “invoice determines store” ?A: no good way; best approximation:

Rental

VideoStore

Person

Movie

Invoice

Arrows in Multiway Relationships

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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-wayRelationships to Binary

Purchase

Person

Store

Product

StoreOf

ProductOf

BuyerOf

date

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4. From E/R Diagramsto Relational Schema

• Entity set relation• Relationship relation

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Entity Set to Relation

Product

name category

price

Product(name, category, price)

name category price

gizmo gadgets $19.99

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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 Yearprice

(watch out for attribute name conflicts)

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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 Yearprice

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Multi-way Relationships toRelations

Purchase

Product

Person

Storename price

ssn name

name address

Purchase(prodName,stName,ssn)

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4. 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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Design Principles

PurchaseProduct

Person

Store

price date

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Information Loss

SellsProduct Store

price

date

Buys BuysAtPerson

Moral: avoid "navigation traps"

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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 E/R

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Product

name category

price

isa isa

Educational ProductSoftware Product

Age Groupplatforms

Subclasses

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Understanding Subclasses

• Think in terms of records:– Product

– SoftwareProduct

– EducationalProduct

field1field2

field1field2

field1field2

field3

field4field5

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Subclasses to Relations

Product

name category

price

isa isa

Educational ProductSoftware Product

Age Groupplatforms

gadget39Toy

photo49Camera

gadget99Gizmo

CategoryPriceName

unixGizmo

platformsName

retiredToy

todlerGizmo

Age GroupName

Product

Sw.Product

Ed.Product

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Modeling Subclass Structure

Product

Educational Product

SoftwareProduct

Educ-softwareProduct

isa

isa

isa

isa

Platforms

required memory

Educational-method

ageGroup

topic

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Modeling UnionTypes WithSubclasses

FurniturePiece

Person Company

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

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Modeling Union Types withSubclasses

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

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Modeling Union Types withSubclasses

Solution 2: better, more laborious

isa

FurniturePiece

Person CompanyownedBy

Owner

isa

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5. Constraints in E/R DiagramsFinding 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.

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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

makes

makes

v. s.

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Referential Integrity Constraints

CompanyProduct makes

CompanyProduct makes

Each product made by at most one company.Some products made by no company

Each product made by exactly one company.

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Other Constraints

CompanyProduct makes<100

What does this mean ?

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

UniversityTeam affiliation

numbersport name

Notice: we encountered this when convertingmultiway relationships to binary relationships

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Handling Weak Entity Sets

UniversityTeam affiliation

numbersport name

Convert to a relational schema (in class)

Team(number,sport,university-name)

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6. Constraints in SQL

• A constraint = a property that we’d like ourdatabase to hold

• The system will enforce the constraint bytaking some actions:– forbid an update– or perform compensating updates

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Constraints in SQLConstraints in SQL:• Keys, foreign keys• Attribute-level constraints• Tuple-level constraints• Global constraints: assertions

The more complex the constraint, the harder it is tocheck and to enforce

simplest

Mostcomplex

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Keys

OR:

CREATE TABLE Product (name CHAR(30) PRIMARY KEY,category VARCHAR(20))

CREATE TABLE Product (name CHAR(30),category VARCHAR(20)

PRIMARY KEY (name))

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Keys with Multiple Attributes

CREATE TABLE Product (name CHAR(30),category VARCHAR(20),price INT,

PRIMARY KEY (name, category))

40GadgetGizmo

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20

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Price

PhotoGizmo

PhotoCamera

GadgetGizmo

CategoryName

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Other Keys

CREATE TABLE Product ( productID CHAR(10),

name CHAR(30),category VARCHAR(20),price INT,

PRIMARY KEY (productID), UNIQUE (name, category))

There is at most one PRIMARY KEY;there can be many UNIQUE

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Foreign Key Constraints

CREATE TABLE Purchase (prodName CHAR(30)

REFERENCES Product(name), date DATETIME)

prodName is a foreign key to Product(name)name must be a key in Product

Referentialintegrity

constraints

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PhotoOneClick

PhotoCamera

gadgetGizmo

CategoryName

WizCamera

RitzCamera

WizGizmo

StoreProdName

Product Purchase

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Foreign Key ConstraintsOR

(name, category) must be aPRIMARY KEY

CREATE TABLE Purchase (prodName CHAR(30),category VARCHAR(20),

date DATETIME, FOREIGN KEY (prodName, category) REFERENCES Product(name, category)

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PhotoCamera

gadgetGizmo

CategoryName

WizCamera

RitzCamera

WizGizmo

StoreProdName

Product Purchase

What happens during updates ?

Types of updates:• In Purchase: insert/update• In Product: delete/update

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What happens during updates ?• SQL has three policies for maintaining

referential integrity:• Reject violating modifications (default)• Cascade: after a delete/update do a

delete/update• Set-null set foreign-key field to NULL

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Constraints on Attributes andTuples

• Constraints on attributes:NOT NULL -- obvious meaning...CHECK condition -- any condition !

• Constraints on tuplesCHECK condition

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CREATE TABLE Purchase (prodName CHAR(30)

CHECK (prodName IN SELECT Product.name FROM Product), date DATETIME NOT NULL)

Whatis the difference from

Foreign-Key ?

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General Assertions

CREATE ASSERTION myAssert CHECK NOT EXISTS(

SELECT Product.nameFROM Product, PurchaseWHERE Product.name = Purchase.prodNameGROUP BY Product.nameHAVING count(*) > 200)

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Final Comments on Constraints

• Can give them names, and alter later• We need to understand exactly when they

are checked• We need to understand exactly what

actions are taken if they fail

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Summary

• E/R Models to model the real world• Adhere to certain design recommendations