Structuring System Requirements: Conceptual Data Modeling

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Structuring System RequirementsConcep
tual Data Modeling

• MBA 597
• Spring 2003

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Systems Analysis

• Three Major Activities
• Requirements determination
• Requirements structuring
• Alternative generation and selection
• Concurrent and iterative

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Importance of Data Modeling

• Characteristics of data are crucial to the design
  of the database, programs, screens, and reports
• Data and their interrelationships are more
  complex than programs
• Data characteristics are more stable than
  processes
• The data structure generates the program structure

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Deliverables of Data Modeling

• Entity-Relationship Diagram (ERD) for the data in
  this application
• ERD for the application being replaced
• ERD for the whole (new) database
• ERD for the whole (old) database
• as necessary

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Components of an ERD

• Entity
• Attribute
• Relationship

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ERD vis-à-vis DFD

• Data flows on the ERD contain entities and
  attributes on the DFD
• Data stores on the ERD are composed of one or
  more entities from the DFD

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Entity
PERSON

• Person, place, object, event, or concept
• about which organization maintains data
• Entity type vs. entity instance
• Singular noun, capital letters
• Source/Sink ? Entity
• e.g., Manager is probably not an entity
• Data Flow ? Entity
• e.g., Inventory Report is not an entity

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Attribute
Gender

• Property or characteristic of entity
• of interest to the organization
• Singular noun, proper capitalization

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Identifier Attribute
PersonID

• An attribute that has a unique value for every
  instance of the entity
• a.k.a. primary key
• Ideal characteristics
• Wont change
• Never null
• Avoid intelligent keys
• Consider surrogate keys

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Multi-valued Attribute

• An attribute or group of attributes that can have
  more than one value for a specific instance of an
  entity
• a.k.a. repeating group
• e.g., phone numbers, dependents
• Important to identify them now
• Will become another entity

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Relationship

• An association between one or more entity types
• of interest to the organization
• Verb phrase
• Characteristics
• Mandatory or optional
• One or many

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Relationship Degree

• Number of entity types involved
• Binary
• Two entity types
• Most common
• Unary
• One instance of an entity related to another
  instance of the same entity
• Ternary n-ary
• More than two entity types

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Relationship Cardinality
Inner symbols indicate minimum Either 1
(required) or 0 (optional)
Outer symbols indicate maximum Either 1 or many
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Associative Entity

• Required to implement a MM unary or binary
  relationship
• Required to implement any ternary or higher
  relationship
• May have attributes of its own
• May be involved in other relationships

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