CSET 3300CSET 3300

Databases & ER Data ModelsDatabases & ER Data ModelsDatabases & ER Data ModelsDatabases & ER Data Models

DatabasesDatabases

• A database is a collection of data (information).• A DataBase Management System (DBMS) is a software system

that manages execution of users applications to access and modify database data.– Manages very large amounts of data – Supports efficient access to very large amounts of

data – Provides data security and data reliability– Supports multi-user concurrent access– Safety from system failures – Recovery– High throughput (thousands ~ millions transactions

per minute)

Relational Database Management System Relational Database Management System

Attributes(columnheaders)

Tuples(rows)

Relation name

Name Manufacturer

Winterbrew Pete’s

Bud Lite Anheuser-Busch

Beers

Data organized as tables called relations

DataBase Management System DataBase Management System

High-levelHigh-levelQuery QQuery Q

DBMS

Answer

Data

TeraBytes PetaBytes<CD> <TITLE>Empire B.</TITLE> <ARTIST>Bob Dylan</ARTIST> <COUNTRY>USA</COUNTRY> <COMPANY>Columbia</COMPANY><PRICE>10.90</PRICE></CD>

Translates Q intobest execution plan

for current conditions,runs plan

Keeps data safe and correct

despite failures, concurrent

updates, online processing, etc.

Example: Car Sales DBExample: Car Sales DB

ID Name Age

12 Nemo 22

34 Ray 42

89 Gill 36

156 Dory 21

… … …

Make Model OwnerID

Honda Accord 12

Toyota Camry 34

Mini Cooper 89

Honda Accord 156

… … …

Cars Owners

Filter (Make = Honda andModel = Accord)

Join (Cars.OwnerID = Owners.ID)

Make Model OwnerID ID Name Age

Honda Accord 12 12 Nemo 22

Honda Accord 156 156 Dory 21

Owners ofHonda Accords

who are <=23 years old

Filter (Age <= 23)

DB ApplicationsDB Applications

• It used to be about boring stuff: employee records, bank records, etc.

• Today, the field covers all the largest sources of data, with many new ideas.– Web search.– Scientific and medical databases.– Market transaction databases (sales data)

• You may not notice it, but databases are behind almost everything you do on the Web.– Google searches.– Queries at Amazon, eBay, etc.

Database SystemsDatabase Systems• The big commercial database vendors:

– Oracle– IBM (with DB2) bought Informix recently– Microsoft (SQL Server)– Sybase

• Some free database systems (Unix) :– Postgres– MySQL– Predator

• Here we use MySQL. You may use something else, but then you are on your own.

Three Aspects to Studying DBMS'sThree Aspects to Studying DBMS's

• Three broad categories– Modeling and design of databases.

• Allows exploration of issues before committing to an implementation.

– Database programming: queries and DB operations like update.

• SQL

– DBMS implementation.• How does one build a DBMS (query processing, organizing

storage, etc.)

Database DesignDatabase Design

• The process of designing the general structure of the database:– Logical Design – Deciding on the database schema.

• Business decision – What properties should we record in the database?

• Computer Science decision – What relation schemas should we have and how should the attributes be distributed among the various relation schemas?

– Physical Design – Deciding on the physical layout of the database

Entity-Relationship ModelEntity-Relationship Model• The E/R model allows us to sketch the

design of a database informally.• Designs are pictures called entity-

relationship diagrams.• Fairly mechanical ways to convert E/R

diagrams to real implementations like relational databases exist.

Entity SetsEntity Sets• Entity = “thing” or object.• Entity set = collection of similar entities.

– Similar to a class in object-oriented languages.• Attribute = property of an entity set.

– Generally, all entities in a set have the same properties.

– Attributes are simple values, e.g. integers or character strings.

E/R DiagramsE/R Diagrams• In an entity-relationship diagram, each entity set is

represented by a rectangle.• Each attribute of an entity set is represented by an oval,

with a line to the rectangle representing its entity set.

Beers

name manufacturer

Entity set Beers has two attributes, name and manufacturer.

Each Beer entity has values for these two attributes, e.g. (Bud, Anheuser-Busch)

RelationshipsRelationships• A relationship connects two or more

entity sets.• It is represented by a diamond, with

lines to each of the entity sets involved.

ExampleExample

Drinkers addrname

Beers

name

Bars

name

license

addr

Note:license =beer, full,none

Sells Bars sell somebeers.

Likes

Drinkers likesome beers.

Frequents

Drinkers frequentsome bars.

manufacturer

Relationship SetRelationship Set• The current “value” of an entity set is the set of

entities that belong to it.– Example: the set of all bars in our database.

• The “value” of a relationship is a set of lists of currently related entities, one from each of the related entity sets.

For the relationship Sells, we might have a relationship set like:

Bar BeerJoe’s Bar BudJoe’s Bar MillerSue’s Bar BudSue’s Bar Pete’s AleSue’s Bar Bud Lite

Binary vs. Multiway RelationshipsBinary vs. Multiway Relationships• Sometimes, we need a relationship that

connects more than two entity sets.• Suppose that drinkers will only drink certain

beers at certain bars.– Our three binary relationships Likes, Sells, and

Frequents do not allow us to make this distinction.

– But a 3-way relationship would.

ExampleExample

Bars Beers

Drinkers

name nameaddr

name addr

license

Preferences

manufacturer

Bar Drinker BeerJoe’s Bar Ann MillerSue’s Bar Ann BudJoe’s Bar Bob BudJoe’s Bar Bob MillerJoe’s Bar Cal MillerSue’s Bar Cal Bud

One-One RelationshipsOne-One Relationships• In a one-one relationship, each entity of

either entity set is related to at most one entity of the other set.

Example: Relationship Best-seller between entity sets manufacturer and Beers. A beer cannot be made by more than

one manufacturer, and no manufacturer can have more than one best-seller (assume no ties).

Many-One RelationshipsMany-One Relationships• Some binary relationships are many -one from one entity set to

another.• Each entity of the first set is connected to at most one entity of

the second set.• But an entity of the second set can be connected to zero, one, or

many entities of the first set.

E.g. Favorite, from Drinkers to Beers is many-one.

A drinker has at most one favorite beer.

But a beer can be the favorite of any number of drinkers, including zero.

Many-Many RelationshipsMany-Many Relationships• Think of a relationship between two entity sets,

such as Sells between Bars and Beers.• In a many-many relationship, an entity of either set

can be connected to many entities of the other set.– E.g., a bar sells many beers; a beer is sold by many bars.

Representing “Multiplicity”Representing “Multiplicity”• Show a many-one relationship by an arrow

entering the “one” side.• Show a one-one relationship by arrows entering

both entity sets.

Drinkers BeersLikes

Favorite

Attributes on RelationshipsAttributes on Relationships• Sometimes it is useful to attach an attribute to a

relationship.• Think of this attribute as a property of tuples in

the relationship set.

Bars BeersSells

price

Price is a function of both the bar and the beer,not of one alone.

Diagrams Without Attributes on RelationsDiagrams Without Attributes on Relations

• Create an entity set representing values of the attribute.

• Make that entity set participate in the relationship.

Bars BeersSells

price

PricesNote convention:arrow from multiwayrelationship = “all otherentity sets determine aunique one of these.”

RolesRoles• Sometimes an entity set appears more

than once in a relationship.• Label the edges between the relationship

and the entity set with names called roles.

Drinkers

Married

husband wife

Relationship Set

Husband WifeBob AnnJoe Sue… …

DB Design ExampleDB Design Example• A university registrar’s office maintains data about the following

entities: a) courses, including number, title, credits, syllabus, and

prerequisitesprerequisites; b) course offerings, including course numbercourse number, year, semester, section

number, instructorinstructor(s), timings, and classroom; c) students, including student-id, name, and program;d) instructors, including identification number, name, department,

and title. – Further, the enrollment of students in courses and grades awarded

to students in each course they are enrolled for must be appropriately modeled.

EntitiesEntities

course

course-offerings

student instructor

CourseCourse• “courses including number, title, credits,

syllabus, and prerequisitesprerequisites”

Attribute?

Relationship?

prerequisite

maincourse

course_no

title

requires course

syllabus

credits

Course OfferingCourse Offering• “course offerings, including course numbercourse number,

year, semester, section number, instructorinstructor(s), timings, and classroom”

time

roomcourse

section_no

semester

year

course- offeringsis offered

course_no

Student, InstructorStudent, Instructor• “students, including student-id, name, and

program”• “instructors, including identification

number, name, department, and title”

student

namesid

program

instructor

titledept

nameiid

Enrollment RelationshipEnrollment Relationship• “Further, the enrollment of students in courses

and grades awarded to students in each course they are enrolled for must be appropriately modeled.”

studentcourse-

offeringsenrolls

grade

Teaching RelationshipTeaching Relationship

• Instructor teaches course…..

course-offerings

instructorteaches

titledept

nameiid

prerequisite

maincourse

course_no

requires course

syllabus

credits

time room

section_no

semester

year

course- offerings

is offered

enrolls

grade

namesid

program

student instructorteaches

title

Complete ER Diagram for UniversityComplete ER Diagram for UniversityComplete ER Diagram for UniversityComplete ER Diagram for University