CHAPTER 2 - Database Requirements and ER...
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Copyright (c) 2016 Nenad Jukic and Prospect Press CHAPTER 2 - Database Requirements and ER Modeling Database Systems - Introduction to Databases and Data Warehouses
Transcript of CHAPTER 2 - Database Requirements and ER...
Copyright (c) 2016 Nenad Jukic and Prospect Press
CHAPTER 2 - Database Requirements and ER Modeling
Database Systems - Introduction to Databases and Data Warehouses
Copyright (c) 2016 Nenad Jukic and Prospect Press
INTRODUCTION
Entity-relationship (ER) modeling - conceptual database modeling technique • Enables the structuring and organizing of the requirements collection
process • Provides a way to graphically represent the requirements
ER diagram (ERD) - the result of ER modeling • Serves as a blueprint for the database
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 2
Copyright (c) 2016 Nenad Jukic and Prospect Press
ENTITIES
Entities - constructs that represent what the database keeps track of • The basic building blocks of an ER diagram • Represent various real world notions, such as people, places, objects,
events, items, and other concepts • Within one ERD each entity must have a different name
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 3
Presenter
Presentation Notes
An ER diagram for a retail company may contain entities, such as CUSTOMER, STORE, PRODUCT and SALES TRANSACTION.
Copyright (c) 2016 Nenad Jukic and Prospect Press
ENTITIES
Two entities
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 4
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ENTITIES
Entity instances (entity members) - occurrences of an entity • Entities themselves are depicted in the ER diagrams while entity
instances are not • Entity instances are eventually recorded in the database that is created
based on the ER diagram
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 5
Presenter
Presentation Notes
Each depicted entity contains a number of entity instances or entity members. For example entity CUSTOMER may contain entity instances such as customer Joe, customer Sue, and customer Pat.
Copyright (c) 2016 Nenad Jukic and Prospect Press
ATTRIBUTES
Attribute - depiction of a characteristic of an entity • Represents the details that will be recorded for each entity instance • Within one entity, each attribute must have a different name
Unique Attribute - attribute whose value is different for each entity instance • Every regular entity must have at least one unique attribute
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 6
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ATTRIBUTES
An entity with attributes
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 7
Presenter
Presentation Notes
This example is discussed on Page 14.
Copyright (c) 2016 Nenad Jukic and Prospect Press
RELATIONSHIPS
Relationship - ER modeling construct depicting how entities are related • Within an ER diagram, each entity must be related to at least one other
entity via a relationship
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 8
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RELATIONSHIPS
Cardinality constraints - depict how many instances of one entity can be associated with instances of another entity • Maximum cardinality
o One (represented by a straight bar: I) o Many (represented by a crow’s foot symbol)
• Minimum cardinality (participation) o Optional (represented by a circular symbol: 0) o Mandatory (represented by a straight bar: I)
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 9
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RELATIONSHIPS
A relationship between two entities
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 10
Presenter
Presentation Notes
The requirements for the relationship ReportsTo and the rules for interpreting cardinality constraints are given on Pages 14-15.
Copyright (c) 2016 Nenad Jukic and Prospect Press
RELATIONSHIPS
Four possible cardinality constraints
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 11
Copyright (c) 2016 Nenad Jukic and Prospect Press
RELATIONSHIPS
Several possible versions of the relationship ReportsTo
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 12
Presenter
Presentation Notes
The requirements for each of the versions shown are given on Pages 16-17.
Copyright (c) 2016 Nenad Jukic and Prospect Press
RELATIONSHIPS
Types of Relationships (maximum cardinality-wise) • One-to-one relationship (1:1) • One-to-many relationship (1:M) • Many-to-many relationship (M:N)
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 13
Presenter
Presentation Notes
The maximum cardinality on either side of the relationship can be either one or many.
Copyright (c) 2016 Nenad Jukic and Prospect Press
RELATIONSHIPS
Three types of relationships (maximum cardinality-wise)
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 14
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RELATIONSHIPS
A 1:M Relationship
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 15
A M:N Relationship
A 1:1 Relationship
Presenter
Presentation Notes
The requirements for the three relationships (IsLocatedIn, AssignedTo, IsAlloted) are given on Page 18.
Copyright (c) 2016 Nenad Jukic and Prospect Press
RELATIONSHIPS
Relationship instances - occurrences of a relationship • Occur when an instance of one entity is related to an instance of
another entity via a relationship • Relationship themselves are depicted in the ER diagrams while
relationship instances are not • Relationship instances are eventually recorded in the database that is
created based on the ER diagram
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 16
Copyright (c) 2016 Nenad Jukic and Prospect Press
RELATIONSHIPS
A relationship and its instances
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 17
Presenter
Presentation Notes
Illustrated relationship instances reflect the cardinality constraint symbols of the relationship.
Copyright (c) 2016 Nenad Jukic and Prospect Press
RELATIONSHIPS
Relationship attributes • In some cases M:N relationships can actually have attributes of their
own
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 18
Presenter
Presentation Notes
An attribute of a 1:M relationship or a 1:1 relationship is not necessary.
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RELATIONSHIPS
A M:N relationship with an attribute
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 19
Presenter
Presentation Notes
The requirements for the ER diagram shown are given on Page 19.
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RELATIONSHIPS
A 1:M relationship with and without an attribute
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 20
Presenter
Presentation Notes
The requirements and the discussion of the two ER diagrams shown here are given on Page 20.
Copyright (c) 2016 Nenad Jukic and Prospect Press Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 21
ER diagram example: ZAGI Retail Company Sales Department Database
Presenter
Presentation Notes
ZAGI Retail Company Sales Department Database requirements are given on Page 21. The knowledge of ER notation helps database requirements collectors gather structured and useful requirements for the future database. Knowing that an ER diagram has to be developed upon the completion of the database requirements process helps database requirements collectors focus on asking the right questions. Examples of such questions are given on Page 22.
Copyright (c) 2016 Nenad Jukic and Prospect Press
ATTRIBUTES
Composite attribute – attribute that is composed of several attributes • Not an additional attribute of an entity • Its purpose is to indicate a situation in which a collection of attributes
has an additional meaning, besides the individual meanings of each attribute
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 22
Copyright (c) 2016 Nenad Jukic and Prospect Press
ATTRIBUTES
An entity with a composite attribute
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 23
Presenter
Presentation Notes
This example is discussed on Page 22.
Copyright (c) 2016 Nenad Jukic and Prospect Press
ATTRIBUTES
Composite unique attribute – attribute that is composed of several attributes and whose value is different for each entity instance
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 24
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ATTRIBUTES
An entity with a unique composite attribute
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 25
Presenter
Presentation Notes
This example is discussed on Pages 23-24.
Copyright (c) 2016 Nenad Jukic and Prospect Press
ATTRIBUTES
Multiple unique attributes (candidate keys) - when an entity has more than one unique attribute each unique attribute is also called a candidate key
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 26
Presenter
Presentation Notes
The term “candidate key” comes from the fact that a candidate key is a “candidate” to be chosen as the primary identifier (also known as a primary key) when implementing the resulting database. One of the candidate keys is chosen later as the “primary key” for the table corresponding to the entity that contains the candidate keys. Primary keys are discussed in Chapter 3.
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ATTRIBUTES
An entity with multiple unique attributes (candidate keys)
Jukić, Vrbsky, Nestorov – Database Systems Chapter 2 – Slide 27
Presenter
Presentation Notes
This example is discussed on Page 24.
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