Post on 25-Dec-2015
3.3.11
CSIS 3310CSIS 3310
Chapter 3The Entity-Relationship
Model
Chapter 3The Entity-Relationship
Model
Conceptual Data Modeling
3.3.22
CSIS 3310CSIS 3310
SDLC Revisited – Data Modeling is an Analysis Activity
SDLC Revisited – Data Modeling is an Analysis Activity
Project Identification and Selection
Project Initiation and Planning
Analysis
Physical Design
Implementation
Maintenance
Logical Design
Purpose –thorough analysisDeliverable – functional system specifications
Database activity – conceptual data modelingcommunication between DB designer & user
3.3.33
CSIS 3310CSIS 3310 Business RulesBusiness Rules
• Statements that define or constrain some aspect of the business
• Assert business structure• Control/influence business behavior• Expressed in terms familiar to end
users• Automated through DBMS software
3.3.44
CSIS 3310CSIS 3310 A Good Business Rule is:A Good Business Rule is:
• Declarative – what, not how• Precise – clear, agreed-upon meaning• Atomic – one statement• Consistent – internally and externally• Expressible – structured, natural language• Distinct – non-redundant• Business-oriented – understood by
business people
3.3.66
CSIS 3310CSIS 3310 E-R Model ConstructsE-R Model Constructs
• Entity– Person: customer, vendor, supplier– Place: location, city, region, territory– Object: vehicle, monitor, part– Event: order, registration, renewal– Concept: expense, flight, project
• Entity Type versus Entity Instance• Entity Type versus System Input,
Output, or User
3.3.77
CSIS 3310CSIS 3310
Entity symbols
Relationship symbols
Attribute symbols
A special entity that is also a relationship
Basic E-R NotationBasic E-R Notation
3.3.88
CSIS 3310CSIS 3310
What Should an Entity Be?
What Should an Entity Be?
• SHOULD BE:– An object that will have many instances in the
database– An object that will be composed of multiple
attributes– An object that we are trying to model
• SHOULD NOT BE:– A user of the database system – An output of the database system (e.g. a
report)
3.3.99
CSIS 3310CSIS 3310
System userSystem user System outputSystem output
Appropriate entities
Inappropriate entitiesInappropriate entities
3.3.1010
CSIS 3310CSIS 3310 AttributesAttributes
• Attribute - property or characteristic of an entity type
• Classifications of attributes:–Simple versus Composite Attribute–Single-Valued versus Multivalued
Attribute–Derived Attributes– Identifier Attributes
3.3.1212
CSIS 3310CSIS 3310
Entity with a multivalued attribute (Skill) and derived attribute
(Years_Employed)
Entity with a multivalued attribute (Skill) and derived attribute
(Years_Employed)
Multivalued: an employee can have more than one skill
Derived from date employed and current date
3.3.1313
CSIS 3310CSIS 3310 E-R Model ConstructsE-R Model Constructs
• 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– an attribute that could be a key…satisfies
the requirements for being a key
3.3.1515
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Composite key attribute
Composite key attribute
The key is composed of two subparts
3.3.1616
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Criteria for Selecting Identifiers (Keys)
Criteria for Selecting Identifiers (Keys)
• Will not change in value.
• Will not be null.
• No intelligent identifiers–e.g. containing locations or people
that might change
• Substitute new, simple keys for long, composite keys.
3.3.1717
CSIS 3310CSIS 3310
Strong vs. Weak Entities, and
Identifying Relationships
Strong vs. Weak Entities, and
Identifying Relationships
• Strong entities – exist independently of other types of entities– has its own unique identifier– represented with single-line rectangle
• Weak entity– dependent on a strong entity…cannot exist on its
own– Does not have a unique identifier– represented with double-line rectangle
• Identifying relationship– links strong entities to weak entities– represented with double line diamond
3.3.2020
CSIS 3310CSIS 3310 RelationshipsRelationships
• Relationship Types vs. Relationship Instances– The relationship type is modeled as the
diamond and lines between entity types
• Relationships can have attributes– These describe features pertaining to the
association between the entities in the relationship
• Two entities can have more than one type of relationship between them (multiple relationships)
3.3.2424
CSIS 3310CSIS 3310Associative EntityAssociative Entity
• Its an Entity – It’s a Relationship
• When should a relationship be an Associative Entity?
–All relationships involved are “many”
–Associative entity has independent meaning
–Has one or more non-key attributes
– Independent relationships
3.3.2626
CSIS 3310CSIS 3310
Degree of Relationships
Degree of Relationships
• Degree of a Relationship - number of entity types that participate in it.–Unary (or Recursive) Relationship
–Binary–Ternary
3.3.2727
CSIS 3310CSIS 3310
One entity related to another of the same entity type
Entities of two different types related to each other
Entities of three different types related to each other
Degree of relationships
Degree of relationships
3.3.3030
CSIS 3310CSIS 3310
Represents a bill-of -materials structure
This has a many-to-many relationship
A unary relationship with an attribute
A unary relationship with an attribute
3.3.3131
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An Associative EntityBill of Materials Structure
An Associative EntityBill of Materials Structure
3.3.3333
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A ternary relationship as an associative entity
A ternary relationship as an associative entity
3.3.3434
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Multivalued attribute vs. relationship. Alternative
approaches
Multivalued attribute vs. relationship. Alternative
approaches
3.3.3535
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Cardinality of RelationshipsCardinality of Relationships
• One – to – One– Each entity in the relationship will have
exactly one related entity
• One – to – Many– An entity on one side of the relationship can
have many related entities, but an entity on the other side will have a maximum of one related entity
• Many – to – Many– Entities on both sides of the relationship can
have many related entities on the other side
3.3.3636
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Cardinality ConstraintsCardinality Constraints
• The number of instances of one entity that can or must be associated with each instance of another entity–Minimum Cardinality
• If zero, then optionalIf zero, then optional• If one or more, then mandatoryIf one or more, then mandatory
–Maximum Cardinality–Mandatory One - when min & max both =
1
3.3.3939
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One optional, one mandatory cardinality
Cardinality constraintsCardinality constraints
3.3.4040
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Optional cardinalities with unary degree, one-to-one
relationship
Optional cardinalities with unary degree, one-to-one
relationship
3.3.4141
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Cardinality constraints in a ternary relationship
Cardinality constraints in a ternary relationship
3.3.4343
CSIS 3310CSIS 3310 Multiple RelationshipsMultiple Relationships
• More than one relationship between the same entity types
3.3.4444
CSIS 3310CSIS 3310
Multiple relationship with fixed cardinality constraintMultiple relationship with
fixed cardinality constraint
3.3.4646
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Pine Valley Furniture User View 1: Orders for
customers
Pine Valley Furniture User View 1: Orders for
customers
3.3.4747
CSIS 3310CSIS 3310
Pine Valley FurnitureUser View 2: Orders for products
Pine Valley FurnitureUser View 2: Orders for products