Representing Information

Data Models

A data model is a precise description of information contentTypes of data models

Conceptual (high-level): in terms that users will understandLogical: in terms that a relational database system will understand (close to the way data is stored)Physical (low-level, how data is actually stored): in terms of the underlying computer hardware and operating system (memory/file system)

Data Models

A Schema is a data model that is intended to be used with a specific database system

External schemas are defined for the users of a database

Logical schema defines the representation as a collection of tables that are stored in a database server

Internal schema defines the representation used by the database server to store the tables in memory or files

External level

Internal to logical mapping

Logical to external mappings

disk

Internal schema Internal level

External schema 1 External schema 2 External schema 3

Logical schema Logical level

Databasetables

Data Models

Distinction between a schema and actual database: intension vs. extension

Populating a database

Requirements

In this phase database designers gather information about the database system in need

Database designers model the database according to user requirements

Database attributes and characteristics conform to users’ actual “ vocabulary”

Organizing Information

Entity An abstraction of the object that contains those characteristics important to the information system

Entity classThe common characteristics that represent a set of entities

AttributeA characteristic/property that helps to describe an object

Attribute valueThe value of a particular characteristic

Organizing Information

Examples

Entity: A customer, or the characteristics of a customer that are of interest

Entity class/type: All of the potential customers, or the information about them

Attribute: hair color or last name

Attribute value: hair color is brown or last name is ‘Breaux’

Examples of Entity Classes

Entity Class Description

Customer A customer of the business

Video An item in the rental inventory

Employee A person who works in one or more stores

Store One of the retail outlets of BigHit Video

Rental The rental of a video by a customer for a specific period and cost

PurchaseOrder

A request to purchase an item

Attributes

An attribute has: Name, Type, Restriction, Description

Domain of an attribute: set of all possible values

Attribute Single valued /simple/atomicMulti-valued (each element is a possible value): college degree, car colorComposite (the whole is one value): consists of several atomic attributes (date, address)

Examples

Attribute Type Restrictions on Values

Description Sample Value

title String Unbounded The title of an item

Annie Hall

ssn String 9 digits (0–9)

A Social Security number

211009008

accountId Number 4 byte integer

The identifier of a customer account

101

Address Composite 2 strings Street, city, st., Zip

….

otherUsers Set Set of strings of 30 characters

Names of other people authorised to use the account

Jeremiah Block

Kenneth Block

Key Attributes

A key attribute is an attribute that uniquely identifies a single entity within the class

Examples Employee : SSN Book : ISBN Customer : AccountId

A key may be a composite attribute: Full Name

Attribute Constraints

Key Constraint: The system respects this constraint by rejecting an entity with an existing key

Null Constraint: the value can (not) be NULL(unknown, not applicable)

Derived: value not entered but computed based on other attributes’ values

Relationships

A relationship type between 2 entity classes represents the possibility that two entities may be related (x-product)

A customer may rent a video

A relationship (or relationship instance) is a connection between 2 entities (actual tuple)

Customer Jane Block rents the video with videoId 90987

To maintain flexibility in relationships, relationship types are not defined as attributes

The customer accountId is not an attribute of the Video entity class

Example (Customer and Video)

Account ID: 101 video ID: 90987

Date: January 9, 2002

Cost: $2.99

Jane Block1010 Main St.Apopka, FL 30458

Elizabeth Date due: January 11, 2002

Constraints on RelationshipsA relationship type may be limited in how many relationships an object may haveThe cardinality of a relationship type may be

One-to-one: Each object of a class may have no more than one object of the other classOne-to-many: One of the classes allows and entity to have any number of related entities, but the other class restricts its entities to be related to no more than one entity.Many-to-many: Entities of both classes may have any number of related entities of the other class.

Other Constraints

Min/Max constraints

Mandatory Participation Constraint and Optional Participation Constraint

Higher-order relationships

A relationship can involve more than two entities

Example: Store-Video-Supplier relationship

Example(Case in Point)

Determining entity classes, attributes and relationship types for BigHit Online video salesProcess

Evaluate statement of goals for information systemDefine entity classesDefine attributes for entity classesDefine relationship typesDetermine cardinalities of relationship rolesAdd attributes to relationship types as necessaryEvaluate entity classes, attributes, and relationship types for clarity, accuracy, and completeness

Example

Statement of goals is highlighted to identify different kinds of information

Example

Entities:

Customer (key: account , last name, first name, address, email, credit card, password)

Movie (key: id, title, genre, length)

Sale (key: id, cost, date, credit card)

Shopping Cart (key: id, date)

Example

Relationship Type

Entity Class Entity Class Cardinality Ratio

Attibutes

Purchases Customer Sale one-to-many

Includes Sale Movie many-to-many

quantity

Selects Customer ShoppingCart

one-to-many

Includes ShoppingCart

Movie many-to-many

quantity