Historical Perspectives
Chapter 2
Data Modeling
BLCN-534: Fundamentals of Database Systems
Chapter Objectives
- Explain the concept and practical use of data modeling.
- Recognize which relationships in the business environment are unary, binary, and ternary relationships.
- Describe one-to-one, one-to-many, and many-to-many unary, binary, and ternary relationships.
- Recognize and describe intersection data.
- Model data in business environments by drawing entity-relationship diagrams that involve unary, binary, and ternary relationships.
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Data Modeling Basics
- Exploring the different ways that entities can relate to each other as they always do in the real world
- Devising a way of recording, of diagramming, the entities and the ways in which they interrelate in the business environment
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Entity-Relationship (E-R) Model
- A diagramming technique
- Diagrams entities (with attributes) and the relationship between the entities.
- There are many variations of E-R diagrams in use.
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E-R Model Entity (and its attributes)
Rectangular shape
Salesperson = a type of entity
Name of entity is in caps above the separator line.
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E-R Model Entity (and its attributes)
- Entity type’s attributes are shown below the separator line.
- PK and boldface denote the attribute(s) that constitute the entity type’s unique identifier.
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Binary Relationships
- Simplest kind of relationship
- Relationship between two entity types
- A salesperson “sells” products or products are “sold” by salespersons
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Cardinality
- Represents the maximum number of entities that can be involved in a particular relationship.
- One-to-One Binary Relationship
- One-to-Many Binary Relationship
- Many-to-Many Binary Relationship
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One-to-One Binary Relationship
- 1-1
- A single occurrence of one entity type can be associated with a single occurrence of the other entity type and vice versa.
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One-to-Many Binary Relationship
- 1-M
- Use “crow’s foot” to represent the multiple association.
- “many” = the maximum number of occurrences that can be involved, means a number that can be 1, 2, 3, ... n.
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Many-to-Many Binary Relationship
- M-M
- “many” can be either an exact number or have a known maximum.
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Cardinality
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Modality
- The minimum number of entity occurrences that can be involved in a relationship.
- “inner” symbol on E-R diagram (“outer” symbol is cardinality)
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Cardinality & Modality
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Intersection Data
- Describes the relationship between two entities.
- Used with many-to-many relationships.
- Represented on E-R diagram as an “associative entity”
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Many-to-Many Binary Relationship with Intersection Data
- For example, we know not only that salesperson 137 sold some of product 24013 but also how many units of that product that salesperson sold.
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Associative Entity
- Entities can have attributes; many-to-many relationships can have attributes.
- Many-to-many relationship may be treated similarly to entities in an E-R diagram.
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Associative Entity
- The unique identifier of the associative entity is usually the combination of the unique identifiers of the two entities in the many-to-many relationship.
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Unary Relationships
- Associate occurrences of an entity type with other occurrences of the same entity type.
- Cardinality:
- One-to-One Unary Relationship
- One-to-Many Unary Relationship
- Many-to-Many Unary Relationship
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Unary Relationships
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Ternary Relationship
- Involves three different entity types.
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Example Use Cases
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- The General Hardware Company E-R Diagram
- Customer Employee is a dependent entity.
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Good Reading Bookstores
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World Music Association
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Lucky Rent-A-Car
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