IST 220 – Intro to Databases

Lecture 3Database Design Guidelines

Recap – Database & DBMS

A database is a collection of related dataA DBMS is a system that is designed for

two main purposesTo add, delete, and update the data in the

databaseTo provide various ways to view (on the

screen or in print) the data in the databaseIn a relational database, data is stored in

a number of tables

A Sample Application – A Library

DB In A Flat File

Problems for DB’s with one BIG tableUnnecessary repetition (or redundancy)

Name and phone number of the Big House publishers repeated six times

Data about different real-world entities is messed up all together

Hard to maintain and manipulateThe library table contains information about

Books Authors Publishers Book-author (relationship)

DB In Related Tables

A solution is to break the big table down to a number of tables (decomposition)BOOKS – 1 record for each of the 14 booksAUTHORS – 1 record for each of the 13 authorsPUBLISHERS – only three publishers!

BOOK_AUTHOR Some authors have published more than one book Some books are co-authored by more than one person

Tables For The Three EntitiesPubID PubName PubNumber

1 Big House 123-455-7890

2 Alpha Press 999-990-9999

3 Small House 714-000-0000

AuID AuName AuPhone

1 Austen 111-111-1111

12 Grumpy 321-321-0000

3 Homer 333-333-3333

10 Jones 123-333-3333

6 Joyce 066-666-6066

2 Melville 222-222-2222

8 Mill 858-688-8888

4 Roman 444-444-4444

5 Shakespeare 555-555-5555

13 Sleepy 321-321-1111

9 Smith 123-222-2222

11 Snoopy 321-321-2222

7 Spenser 777-777-7777

ISBN Title PubID Price

0-555-55555-9 Macbeth 2 $12.00

0-91-335678-7 Faerie Queene 1 $15.00

0-99-909999-9 Emma 1 $20.00

0-91-045678-5 Hamlet 2 $20.00

0-55-123456-9 Main Street 3 $22.95

1-22-233100-0 Visual Basic 1 $25.00

0-12-333433-3 On Liberty 1 $25.00

0-103-45678-9 Iliad 1 $25.00

1-1111-1111-1 C++ 1 $29.95

0-321-32132-1 Balloon 3 $34.00

0-123-45678-0 Ulysses 2 $3400

0-90-777777-7 King Lear 2 $49.00

0-12-345678-9 Jane Eyre 3 $49.00

0-12-345678-9 Moby-Dick 3 $49.00

The Table For The RelationshipISBN AuID

0-103-45678-9 3

0-11-345678-0 2

0-12-345678-9 8

0-12-345678-9 1

0-123-45678-0 6

0-321-32132-1 11

0-321-32132-1

12

0-321-32132-1

13

0-55-123456-9

9

0-55-123456-9

10

0-555-55555-9 5

0-91-045678-5 5

0-91-335678-7 7

0-99-777777-7 5

0-99-999999-9 1

1-1111-1111-1 4

1-22-233700-0 4

One book with three co-authors

One author with three books

Complications Of RDB Design

Avoiding data loss during decompositionEach attribute should be included in at least

one tableMaintaining relational integrity

Defining and implementing relationship between tables correctly

Some attributes will show up in more than one tables

Complications Of RDB Design

Creating views Gathering data from more than one table

when neededExample

Display a list of all publishers that publish books priced under $20.00 – need to access BOOKS and PUBLISHERS tables

Entity & Entity Set

Entities: real-world objects or conceptsEntity set: a set of entities sharing the

same properties (or attributes)Examples

Entity sets: books, authors, publishersAttributes: author id (AuID), author name

(AuName), and author phone (AuPhone)Entities: Austen, Grumpy, Homer, etc

Relationship & Relationship Set

Relationship – an association among entitiesRelationship set – a set of relationships of

the same type Example

Relationship set: book-authorRelationship: Macbeth-Shakespeare, Iliad-

Homer

Relationship Among Tables

PubID PubName PubNumber

AuID AuName AuPhone

ISBN Title PubID Price

ISBN AuID

BOOKS

PUBLISHERS

AUTHORS

BOOK-AUTHOR

An Example

0-555-55555-9 Macbeth 2 $12.00

5 Shakespeare 555-555-5555

0-555-55555-9 5

2 Alpha Press 999-990-9999

AUTHORS

BOOK-AUTHOR

BOOKS

PUBLISHERS

DB Design Rules – I

Compound attributesAn attribute which is made up by a few partsExamples

Name = first-name + last-nameAddress = st number, st name, city, state, zip

Rule 1: Field Uniqueness Avoid using compound attributes Instead, use a number of attributes, each for

one of those parts

DB Design Rules – II

Primary key A primary key is one or more attributes which

can uniquely identify records in a tableExamples

ISBN in the BOOKS tableISBN and AuID in the BOOK-AUTHOR table

Rule 2: Primary Keys Each table should have a primary key (PK)

Foreign Keys

An attribute is referred as a foreign key (FK) if it is used as a primary key in another tableExample: PubID in the BOOKS table – FKPubID in the PUBLISHERS table – PK

FK’s are critical in linking the tables together

An attribute can be a foreign key and a part of the primary key at the same time

PK & FK

Cardinality Constraints

PubID PubName PubNumber

AuID AuName AuPhone

ISBN Title PubID Price

ISBN AuID

BOOKS

PUBLISHERS

AUTHORS

BOOK-AUTHOR1

n

1

1

n

n

DB Design Rules – III

Functional dependenceA property that each attribute in a table is

determined by the PK Satisfy: a book name is known for a given ISBNViolate: in the flat table, a publisher’s phone may

change regardless what the ISBN is

Rule 3: Functional DependenceFor each unique PK value, other attributes

must be relevant to, and must completely describe the subject of the table

DB Design Rules – IV

Rule 4: Field IndependenceYou must be able to make a change to the

data in any field (other than a field in the PK) without affecting the data in any other field

Example In the flat table, if you’ve mistaken the publisher

for the book 0-103-45678-9 (Iliad). When you want to change the publisher id, you have to change the associated name and phone attributes as well.