 |
Session One
Introduction to the Course
and Entities |
Topics
Introduction to the course
Overview of Data Modeling and Database Design
Power
Point: Overview
The Goal
The Process
CONCEPTUAL
(Business View) |
LOGICAL
(Systems View) |
| Analysis |
Design |
| Entity |
Table |
| Relationship |
Foreign Key |
| Attribute |
Column |
| Unique Identifier |
Primary Key
Unique Key |
Database Development Process
| Phase |
Objectives |
| Conceptual data modeling |
Define and model the things of significance about which the business
needs to know or hold information and the relationships between them. |
| Logical database design |
Map the information requirements reflected in an ER model into a relational
database design |
| Physical database design |
Create physical relational database tables to implement the design. |
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First Cut - "Space is FREE and processing is INSTANTANEOUS"
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DANGER = ANALYSIS PARALYSIS
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Entity Relationship Modeling
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Create a model of the information requirements of a business, using entities,
attributes and relationships.
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Hardware and Software Independence
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The conceptual model should be built independently of any future implementation
issues.
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Will be using ERWIN - It can do both forward and reverse engineering.
Entity Definitions
Power
Point: Entities
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An object of interest to the business
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A class or category of thing
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A named thing
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A noun
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A thing of significance about which the business needs information
Attribute Definitions (We will cover attributes more in a later
session.)
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Nouns used to describe entities
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Specific pieces of information which need to be known
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An entity should have attributes
All entities must have attributes or they are not entities. Remember the
definition of an entity ‘. . .about which the business needs to hold information’.
Attributes are the way information about entities are stored.
Instance VS Entity
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Instances should not be mistaken for entities
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An entity is a class or category of thing, for example, EMPLOYEE
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An instance is a specific thing, for example, Jane Smith
If an entity does not have multiple instances then it is probably not an
entity but rather an attribute of another entity. This is not a hard and
fast rule but is very unusual.
Identifying a Unique Instance
Each instance of an entity must be uniquely identifiable from other
instances of the same entity. An attribute or set of attributes that do
this is called a Unique Identifier (UID).
Identifying and Modeling Entities
Follow the steps below to help you identify and model entities from
a set of interview notes or other documentation.
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Examine each noun. Is each one significant to the business?
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Is there information of interest about it that the business needs to hold?
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Name the entity using a singular form.
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Is it possible to identify one entity instance distinctly from another?
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Describe the entity to make sure that everyone in the team has the same
understanding of its meaning. This is of the utmost importance as it helps
to define the entity and sets the rule for what it takes to be a member
of the set.
Summary
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Entities
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An entity is a thing of significance about which the business needs to
hold information.
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Diagram Conventions
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Entities are drawn as soft boxes with the name in the singular and in uppercase.
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Entity Instances
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When you identify an entity, you identify a group of things. Within the
group there are individual instances.
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Attributes
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Attributes are the way to store the information that is held about an entity.
Each attribute is a descriptor for the entity.
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Identifying Entities and Attributes
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Examine each noun.
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Name each entity.
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Does the business need to hold information about the entity?
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Is each instance of the entity uniquely identifiable?
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Write a description of the entity.
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Diagram each entity and a few of its attributes.
Two other problems:
SCOPE CREEP
A projects information scope increases as work in developing the model
proceeds. Maybe adding new or additional entities
AMBIGUITY
"I have a dog and it is black and white"
Assignment for next class.
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Worksheet One
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Read chapters 2 and 3 from text.
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To memorize for next class "Crows fly East or South"