Relational Databases

COMP 416

Fall 2010

Lecture 21

What’s a database?

• A collection of data

• Examples of collections of data?– Library– Web– Stacks of papers on your desk– Set of baseball cards.

• Are all of these things databases?

Web vs. Library

• What’s the big difference?– Organization– In what ways is the library organized?

• Databases have organization.

Library vs Baseball Cards• What’s the primary object in these two

collections?– Libraries: books– Baseball card collections: baseball cards

• How alike are books?– Somewhat, but large variatations

• How alike are baseball cards?– Very alike.

• Things in databases are highly structured.

One library vs another

• What’s the difference between the Brauer Library and the House Library?

• Different purposes leads to different priorities (in organization and content).

• Databases are built for a purpose.– The more specific the purpose, the more

specific its structure and organization.

So, what’s a database

• A database is an collection of structured information organized for a specific purpose.

Relational Databases

• Relational databases are the most prevalent type of database used.

• Information is organized into related tables.

• Each table captures information about a different entity.– Columns are different fields of information

(attributes of the entity).– Each row represents one instance (a specific

example of the entity).

Design Goals• What kinds of information do we want to

keep track of?

• What do we want to do with that information?

Entities

• First step in database design is to identify entities.– Think of entities as “things” that you want to

know information about.– What do we care about for our bookstore?

• Books (duh?)

Attributes

• Next step is to identify attributes of those entities.

• An attribute is labeled piece of information (i.e., a name/value pair)

• In general, we expect every instance of a particular entity to have specific values for a set of common attributes.

Book Entity

BookAuthor(s)

Title

Publisher

Genre

Price

Normalization

• Not all database designs are equal.

• Experience and research has shown that certain structures and relationships are easier to maintain and process than others.

• Normalization: a process through which a database design is “cleaned up”

• Well-defined set of “normal forms” which are the incremental result of this process.

1NF

• First Normal Form– All attributes are single-valued.– All instances have a unique identifier.

Book Entity Revisited

• Is our book entity in 1NF?

BookAuthor(s)

Title

Publisher

Genre

Price

Bookstore Entities (1NF)

• Multi-valued attributes generally indicate the need for a new entity.

BookTitle

Publisher

Genre

Price

AuthorFirst

Last

Birthday

Unique Identifiers

• What in our book and author entities can act as a unique identifier?

• Often (almost always) the best way to create a unique identifier is to create an artificial one.– Book ID, Author ID.– Assigned by the database itself.– No inherent semantics.

Book Entities (1NF) v2

Book

Title

Publisher

Genre

Price

ID

Author

First

Last

Birthday

ID

Modeling Relationships

• Two relationship types.– One-to-Many– Many-to-Many

• For now, we’ll just model this pictorially like this:

Book

Title

Publisher

Genre

Price

ID

Author

First

Last

Birthday

ID

2NF

• Second Normal Form– Already in 1NF– Non-identifying attributes are dependent on the

entity’s unique identifier.• Rule of thumb: if the same value appears multiple

times for a particular attribute, think hard if what you really need is another entity.

Bookstore Entities

• What might we pull out into an entity? Book

Title

Publisher

Genre

Price

ID

Author

First

Last

Birthday

ID

Bookstore Entities (2NF)

Book

Title

Price

ID

Author

First

Last

Birthday

ID

Publisher

Name

Address

State

State Abbrev.

ID

Genre

Genre Name

ID

3NF

• Third Normal Form– In 2NF– No attributes dependent on each other.

• What part of our data model violates this?

• To fix, generally want to pull the dependent attributes out into their own entity.

Bookstore Entities (3NF)

Book

Title

Price

ID

Author

First

Last

Birthday

ID

Publisher

Name

Address

ID

Genre

Genre Name

ID

Long NameAbbrev.

StateID

Logical vs Physical Design• Result so far is “logical” database design.• Still need to implement this design as a specific

database.• Relational databases:

– Each entity associated with a table.– Attributes are columns of the table.– Each attribute is given a data type.– Unique identifiers are “primary keys”– Relationships are embodied as “foreign keys”

• An attribute whose value is the unique identifier in another table.

Implementing 1-to-many

• To implement a 1-to-many relationship, add an attribute on the “many” side which is the unique identifier of the “one” side.

Implementing 1-to-many

Author

First

Last

Birthday

ID

Genre

Genre Name

ID

Long NameAbbrev.

StateID

Publisher

NameAddress

ID

StateID

Book

TitlePrice

ID

PubIDGenreID

Resolving M-to-M

• Many-to-many relationships are hard to implement in a database.

• Why is this?– Foreign key attribute which is supposed to

implement the relationship requires multiple values.

– This breaks 1NF structure.

• How might we fix it?

Junction Entities

• A junction entity is an abstract entity provides a level of indirection for a many-to-many relationship.

Adding BookAuthor Junction

Author

First

Last

Birthday

ID

Genre

Genre Name

ID

Long NameAbbrev.

StateID

Publisher

NameAddress

ID

StateID

Book

TitlePrice

ID

PubIDGenreID

BookAuthor

BookIDID

AuthorID

SQL

• Structured Query Language (SQL)– The language in which we express actions to be

performed on a relational database.– Standardized to allow portability across

different products.• SQL92 (aka SQL2) is the latest standard.

– Product specific differences and extensions still exist, but much better than before.

MySQL• MySQL

– Open-source– Great for small to mid-sized organizations.– Fast, efficient, cheap– Doesn’t support full SQL but a good portion of it.

Web App Model

JavaScriptProgrammable,

dynamic interface to the

document

Browser Web ServerHTTP Requests

HTTP Responses

DBOn Disk

SQLPHPProgrammable,

dynamic, document

construction

DatabaseStructured, table-based, information

storage