Representing Information

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Representing Information
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Data Models

• A data model is a precise description of
  information content
• Types of data models
• Conceptual (high-level) in terms that users will
  understand
• Logical 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)

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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

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Data Models

• Distinction between a schema and actual database
  intension vs. extension
• Populating a database

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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

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Organizing Information

• Entity
• An abstraction of the object that contains those
  characteristics important to the information
  system
• Entity class
• The common characteristics that represent a set
  of entities
• Attribute
• A characteristic/property that helps to describe
  an object
• Attribute value
• The value of a particular characteristic

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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

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Examples of Entity Classes
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Attributes

• An attribute has Name, Type, Restriction,
  Description
• Domain of an attribute set of all possible
  values
• Attribute
• Single valued /simple/atomic
• Multi-valued (each element is a possible value)
  college degree, car color
• Composite (the whole is one value) consists of
  several atomic attributes (date, address)

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Examples
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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

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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

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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

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Example (Customer and Video)
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Constraints on Relationships

• A relationship type may be limited in how many
  relationships an object may have
• The 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 class
• One-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.

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Other Constraints

• Min/Max constraints
• Mandatory Participation Constraint and Optional
  Participation Constraint

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Higher-order relationships

• A relationship can involve more than two
  entities
• Example Store-Video-Supplier relationship

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Example(Case in Point)

• Determining entity classes, attributes and
  relationship types for BigHit Online video sales
• Process
• Evaluate statement of goals for information
  system
• Define entity classes
• Define attributes for entity classes
• Define relationship types
• Determine cardinalities of relationship roles
• Add attributes to relationship types as necessary
• Evaluate entity classes, attributes, and
  relationship types for clarity, accuracy, and
  completeness

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Example

• Statement of goals is highlighted to identify
  different kinds of information

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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)

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Example