database design, a practical guide

Database Design, a Practical Guide

Gus Björklund ([email protected])Wizard, Progress Software Corporation

© 2007 Progress Software Corporation2 Database Design: A Practical Guide

Ask questions as we goif I am not being clear.

Warning: there is a mistake in these slides.


To every rule, there is an exception!

Rules are made to be broken


Topics

Theory:• What is Database Design

• Basic Elements

• Representing the data model as Tables

Practice• An Example

Some Other Topics


First, a little theory


What do we mean by database design?

A process for defining a model of a subset of the “real1 “ world, then representing it as data in tables in a relational database

At least, that’s the definition we will use for the purposes of this talk.

1: Well, for small values of real, anyway.


Basic Elements

Just 3 Things:• Entities

• Attributes

• Relationships

What do we put in our model?

The “entity-relationship model” was described by Peter Chen in 1976.

See http://bit.csc.lsu.edu/~chen/chen.html


Basic Elements 1: Entities

Can be thought of as nouns• People

– author, composer, performer, seller, buyer

• Places– home, IP address, URL, destination, factory,

store

• Things– song, recording, instrument, car, invoice

Is “telephone number” a place or a thing?


Basic Elements 2: Attributes

Can be thought of as adjectives (but only loosely):• Length• Color• Horsepower• Part number• Song Title• Publication Date• Size• Fabric• Owner

Is “telephone number” a attribute or an entity?

Entities have attributes


Basic Elements 3: Relationships

Can be thought of as verbs:• has a• owns• contains• supervises• performs• called• sold• purchased• proved

Entities are connected by relationships

Is “telephone number” a relationship?


In May 1995,

Andrew Wiles

published

a proof

of Fermat’s Last Theorem

Relationships have attributes too


In May 1995,

Andrew Wiles

published

a proof

of Fermat’s Last Theorem

Relationships have attributes too

entity

entity

relationship

attribute


What goes in an entity ?

Identifying attributes• Must be able to uniquely identify the entity

• Can have more than one way to id

• Id can be composite

Descriptive attributes• the values you need to keep track of

• generally should be simple, not complex


What to include in your model

The things your application has to keep track of• Telephones, wires, switches

The actions your application or its users perform• Make calls, send telephone bills, collect payments

Some attributes of the things and actions• Originating number, date and time of call, duration, called

number

Keep it simple Be accurate Keep it up to date


What to include in your model

Consider the goals of the system Everything you include should be there for a

reason you can state• in no more than two sentences

Everything should have a clear name• if you can’t name it, it doesn’t belong

Talk to the stakeholders !!!


What to leave out of your model

The real world has properties that don’t matter (to your application)

The real world has relationships that don’t matter

Things happen in the real world that don’t matter

Keep it simple• If you can’t say why you need it, leave it out


Logical vs Physical Data Models

Logical entities often require multiple tables to represent them• Tables can be thought of as logical or physical• It depends on your point of view

There is also the physical storage database layout• storage areas• data extents• disks• etc.

We aren’t going to talk about the physical database layout

We will talk about tables


Mapping Your Model to a Database

Entities become tables• Identifiers become indexes

Attributes become columns• Data types: pick appropriate

Relationships become tables or foreign keys

Simply put,


“In theory, there is no difference betweentheory and practice, but in practice there is.”

Jan van de Snepscheut


Now for some practice.


An example

A Music store (perhaps the pTunes store)• Buys compact disc recordings from

distributors

• Has inventory

• Allows customers to search for what they want– Maybe in an in-store kiosk or on the web

• Sells compact discs to customers


What should we do first?


Activities

We buy discs from a distributor Orders are sent to a distributor Orders are delivered to the store Orders may be cancelled We sell discs to customers in sales transactions Customers buy discs in sales transactions Customers search for what they want to buy

Which of these must be remembered by the system?


What do we need to keep track of

Discs we have Discs we sold Discs we know about and can get Discs we have ordered Information needed to do our income tax

• what we paid for stock• when we bought it• what we sold it for• when we sold it


Disc entities

UPC Code: 8697-07416-2 Manufacturer: Sony BMG Cost to us: $ 2.00 Price charged: $ 17.95 Tax charged: $ 0.80 Date purchased: March 19, 2007 Date sold: June 9, 2007


Disc table might look like this

upc manufacturer cost price tax datePurch dateSold

8697-07416-2 Sony BMG 2.00 17.95 0.90 2007-03-19 2007-06-09

8697-07416-2 Sony BMG 2.00 ? ? 2007-06-09 ?

314-510347-2 Island Records 2.21 15.95 0.80 2006-01-12 2007-02-14

314-510347-2 Island Records 2.21 ? ? 2006-01-12


What’s wrong?

Is upc a unique identifier? Might have bought from a distributor Have no information about what is on the disc

• How do customers search? Don’t know when disc was made Could be more than one tax jurisdiction

• provincial tax, city tax Don’t know if disc is on order Don’t know who bought it Duplicated data Etc., etc.


Disc entities take 2

UPC Code: 8697-07416-2 Manufacturer: Sony BMG Distributor: Bob’s Wholesale CD’s Cost to us: $ 2.00 Price charged: $ 17.95 Tax charged: $ 0.80 Date ordered: March 19, 2007 Date received: March 20, 2007 Date sold: June 9, 2007 Disc Title: “The Essential Joshua Bell” Artist: Joshua Bell Track 1: “Danse Russe” Track 2: “Violin Concerto in E Minor” Track 3: “Nocturne in C-sharp Minor” etc.


Example: Now What’s wrong?

This is getting messy Activities combined with disc’s attributes Have duplicated information How many tracks can there be? What if there is more than one artist? Don’t have all the information a customer

might want to use to search


Discs revisited

Discs have titles Discs have pictures on the cover Discs contain tracks Discs are made by manufacturers Discs are purchased from distributors Discs are ordered from distributors Discs are delivered to the store Discs are sold to customers


“Discs contain tracks …”

Tracks contain songs Tracks occur in order Tracks have a duration Songs are performed in performances Songs have performers (usually) Songs have composers Songs have names (titles) Songs have a key (but not always) Performances are done by performers Performers can be groups (bands, orchestras, etc.) Performances are performed in a location or venue


We seem to need these entities

Discs Manufacturers Distributors Orders Customers Inventory

Tracks Songs Performers Groups ?


Songs have names (titles).

Are names properties of songs?

Or are they entities related to songs?

Or are they something else?


Song data (track 1)

Title “Danse Russe” from Swan Lake, Op.20

Time 4:30

Composer Peter Tchaikovsky

Category Classical, violin, orchestra

Performers Joshua Bell, Michael Tilson Thomas, Berlin Philharminic Orchestra

Track number 1

Disc upc 8697-07416-2


Song data (track 2)

Title Violin Concerto in E Minor, Op. 64

Time 6:27

Composer Felix Mendelssohn


Performers Joshua Bell, Sir Roger Norrington, Camerata Salzburg

Track number 2

Disc upc 8697-07416-2


Performance data


Time 6:27





Performance data take 2


Time 6:27




Performance Date

?

Performance Location

?


Performer data

id name

1 Joshua Bell

2 Sir Roger Norrington

3 Camerata Salzburg

4 Michael Tilson Thomas

5 Berlin Philharmonic

6 Bono

7 The Edge

8 Adam Clayton

9 Larry Mullen


Performance to Performer Relationship

performance id performer id

1 1

1 2

1 3

1 …

2 1

2 4

2 5

2 …

325 6

325 7

325 8

325 9


Performance data take 3

Performance id 2


Time 6:27




Track to Performance Relationship

Disc upc Track Num Performance id

8697-07416-2 1 1

8697-07416-2 2 2

… … …

314-510347-2 1 325


Relationships (so far):

disctrack

track

track

trackperformance

performance

performance

performance

performer

performerone to one

one to many

many to many


What happened to Songs?


Relationships (take 2):

disctrack

track

track

tracksong

performance

performance

performance

performer

performer

one to one

one to many

many to many

performance

performance

performance

song

one to many


Relationships (take 3):

disc

track

track

track

performance

performance

performance

performer

performer

song

song

song


What about“business entities”

?

Where are they?


Business entities

disc

track

track

track

performance

performance

performance

performer

performer

song

song

song

Here is one kind of “business entity”


Business entities

disc

track

track

track

performance

performance

performance

performer

performer

song

song

song

Here is a different kind of “business entity”


Business entities

disc

track

track

track

performance

performance

performance

performer

performer

song

song

song

Here is still another kind of “business entity”


Should you use arrays?


Indexes

Enforce uniqueness Make searches faster Enable fast retrieval of entities by their

identities Enable finding entities with certain attributes


What indexes do we needfor the music store database?


Tables

0) Discs1) Tracks2) Songs3) Performers4) Performances5) Tracks of discs6) Performances of songs7) Performers of performances


What indexes do we need

0) Indexes for identifying attributes1) A unique row identifier2) Indexes for the queries you will do


What should we do next ?


Other Topics

Normalization Unique keys Word indexes Naming Customisation


Normalization

Oversimplified, it means:• Don’t duplicate data

Attributes should be simple• have only one value• be necessary• not derived data• don’t repeat

Complicated attributes are often entities in their own right• For example, addresses might be


Unique keys

EVERY table must have a unique key EVERY row needs a unique identifier

• that never changes even if moved to another database (i.e. if you replicate)

Often, users don’t need to see it Use a UUID or sequence or maybe datetime Unique key is the ONLY way to identify rows

unambiguously ROWID’s are temporary and can change Use the same method throughout

• You’ll be glad you did


Word indexes

Can be used to hold multiple status or attribute values• Conflicts with normalisation• Flexible

Easy to add new ones Queries are fast

Example:• Category: classical, violin, orchestral, concerto


Naming

• What is in the column “GL01262” ?

Good names are crucial to understanding


Naming

Table and column names should have clear meanings everyone can understand• “GL01262” vs “dateEntered”

Names with dashes cause inconvenience with SQL• “order-date”

Booleans should be named for truth value• “backOrdered”

No double negations• “notOutOfStock”

Good names are crucial to understanding


Making tables customizable

Spare columns Separate table with spare columns Separate table with name/value pairs Name/value pairs in word-indexed column

We will look at 4 ways:


Table and columns

custnum name city

001 Bob Phoenix

002 Alice Boston

003 Eve Denver


Spare columns in table

custnum name city

001 Bob Phoenix

002 Alice Boston

003 Eve Denver

extra1 extra2 extra3

frozen ? 0.0

? 125.46 0.12

? ? ?


Spare columns in table

custnum name city

001 Bob Phoenix

002 Alice Boston

003 Eve Denver

extra1 extra2 extra3

frozen ? 0.0

? 125.46 0.12

? ? ?

What data types should you use?How many spare columns?Wasted columns when not usedHow do you know what each spare got used for?How do you know how many unused spares you have?


Separate table for spare columns

custnum name city

001 Bob Phoenix

002 Alice Boston

003 Eve Denver

custnum extra1 extra2 extra3

001 frozen ? 0.0

002 ? 125.46 0.12


Separate table for spare columns

custnum name city

001 Bob Phoenix

002 Alice Boston

003 Eve Denver

custnum status owed discount

001 frozen ? 0.0

002 ? 125.46 0.12


Separate table with name/value pairs

custnum name city

001 Bob Phoenix

002 Alice Boston

003 Eve Denver

custnum name value

001 status frozen

002 owed 125.46

002 discount 0.12


Name/value pairs in word-indexed column

custnum name city

001 Bob Phoenix

002 Alice Boston

003 Eve Denver

extra1

status: frozen

owed: 125.46, discount: 0.12

?


Modeling Tools

PCase Enterprise Architect Power Designer ConceptDraw Erwin Rational

Pencil and paper !

Blackboard !


Summary

Understand the requirements Leave out what is not needed Review the design with stakeholders Evolve the design as changes come up Test to make sure it works

• Can it do everything that is needed?

• Does it perform adequately?

Expect changes to come


Homework

Papers• Wiles, A.: "Modular elliptic curves and Fermat's Last

Theorem”, Annals of Mathematics 141 (3): 443-551• Chen, P.: “The Entity-Relationship Model -- Toward a

Unified View of Data”, ACM TODS Vol 1, No 1, 1976 Wikipedia articles to start from:

• entity-relationship model• data model

Books:• Teorey, Lightstone, Nadeau: “Database Modeling and

Design”, Morgan Kaufmann.


Questions

database design, a practical guide

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