MIS434 - PowerPoint PPT Presentation

Title: MIS434


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Database Management Systems Conceptual Data
Modeling Chapter 3 Charletta F. Gutierrez
The Entity-Relationship Model
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Data Modeling is the most important part of the
Database development process

• The characteristics of data captured during data
  modeling are crucial in the design of databases.
• Data rather than process play a central role in
  structuring system requirements.
• Data is more stable than the business processes
  that use that data.

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Entity-Relationship Model

• Created by Chen (1976)
• A logical representation of the data for an
  organization or for a business area
• Entity-Relationship Diagram
• a graphical representation of an
  entity-relationship model

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sends
Supplier
supplies
Customer
Shipment
includes
Item
KEY
used in
submits
Relationship
Entity
Cardinalities
Mandatory One
Mandatory Many
requests
Product
Order
Optional One
Optional Many
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E-R MODEL CONSTRUCTS

• Entity
• person, place, object, event, concept
• Entity Type
• a collection of entities that share common
  properties or characteristics
• Entity Instance
• a single occurrence of an entity type

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E-R MODEL CONSTRUCTS

• Entity Type versus System Input, Output, or User

Gives_to
TREASURER
EXPENSE REPORT
Manages
Summarizes
Has
ACCOUNT
EXPENSE
ACCOUNT
EXPENSE
Has
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E-R MODEL CONSTRUCTS - cont.

• Strong versus Weak Entity Type
• Independent versus dependent entity.
• Fig. 3-5.
• Identifying owner.
• Identifying relationship.
• Weak entity identifier is its partial identifier
  combined with that of its owner.

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E-R MODEL CONSTRUCTS - cont.

• Attribute - property or characteristic of an
  entity type
• Simple Attribute
• Composite Attribute
• Single-Valued Attribute
• Multi-valued Attribute
• Stored Attribute
• Derived Attributes

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E-R MODEL CONSTRUCTS - cont.

• Identifier or Key - An attribute (or combination
  of attributes) that uniquely identifies
  individual instances of an entity type.
• Simple Key versus Composite Key
• Candidate Key

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E-R MODEL CONSTRUCTS - cont.

• Criteria for Selecting Identifiers
• Will not change in value.
• Will not be null.
• No intelligent identifiers (containing e.g.
  locations or people that might change)
• Substitute new, simple keys for long, composite
  keys.

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RELATIONSHIPS

• Relationship - an association among the instances
  of one or more entity types that is of interest
  to the organization
• Relationship Type
• Relationship Instance

STUDENT
COURSE
Completes
STUDENT COURSE Smith COBOL Jones Databas
e Green C
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• Attributes of Relationships
• Associative Entities (Gerunds)
• All relationships involved are many
• Result has independent meaning
• Gerund has one or more non-key attributes

Date_Completed
STUDENT
COURSE
Completes
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• Rules for converting Relationship to an
  Associative entity
• All of the relationships for the participating
  entity types are many relationships.
• The resulting associative entity type has
  independent meaning to end users, and preferably
  can be identified with a single-attribute
  identifier.
• The associative entity has one or more
  attributes, in addition to the identifier.
• The associative entity participates in one or
  more relationships independent of the entities
  related in the associated relationship.

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• Degree of a Relationship - number of entity types
  that participate in it.
• Unary Relationship
• Binary Relationship
• Ternary Relationship

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Binary
Unary
Relationship degree
Ternary
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• Cardinality Constraints - the number of instances
  of one entity that can or must be associated with
  each instance of another entity.
• Minimum Cardinality
• Maximum Cardinality
• Mandatory Cardinality
• Optional Cardinality

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Mandatory One
Relationship cardinality
Mandatory Many
Optional One
Optional Many
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Basic E-R Notation
Associative entity
Strong entity
Attribute
Basic Symbols
Weak entity
Relationship
Multi-valued attribute
Identifying relationship
Derived attribute
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• Class assignment
• Page 121 Problem number 6
• Groups of 4