E-r Model
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Chapter 2: Entity-Relationship Model
Overview of Database Design
What are the entities and relationships in the enterprise? _ What information about these entities and relationships should we store in the database? _ _ A database `schema’ in the ER Model can be represented pictorially (ER diagrams). _
Entity Sets A database can be modeled as: a collection of entities, relationship among entities. An entity is an object that exists and is
distinguishable from other objects.
Example: specific person, company, event, plant
Entities have attributes Example: people have names and addresses An entity set is a set of entities of the same type that
share the same properties.
Example: set of all persons, companies, trees, holidays
Entity Sets customer and loan
customerid customer customer customer loan amount name street city number
Attributes An entity is represented by a set of attributes, that is
descriptive properties possessed by all members of an entity set. Example:
customer = (customerid, customername, customerstreet, customercity) loan = (loannumber, amount)
Domain – the set of permitted values for each attribute Attribute types:
Simple and composite attributes. Single-valued and multi-valued attributes
E.g. multivalued attribute: phone-numbers
Derived attributes
Can be computed from other attributes E.g. age, given date of birth
Composite Attributes
Relationship Sets A relationship is an association among several
entities Example: Hayes depositor A-102 customer entityrelationship setaccount entity A relationship set is a mathematical relation among n ≥ 2 entities, each taken from entity sets {(e1, e2, … en) | e1 ∈ E1, e2 ∈ E2, …, en ∈ En} where (e1, e2, …, en) is a relationship
Example: (Hayes, A-102) ∈ depositor
Degree of a Relationship Set Refers to number of entity sets that participate in a
relationship set. Relationship sets that involve two entity sets are binary (or degree two). Generally, most relationship sets in a database system are binary. Relationship sets may involve more than two entity sets. ★E.g. Suppose employees of a bank may have jobs (responsibilities) at multiple branches, with different jobs at different branches. Then there is a ternary relationship set between entity sets employee, job and branch
Relationships between more than two entity sets are rare.
Most relationships are binary. (More on this later.)
Mapping Cardinalities Express the number of entities to which another
entity can be associated via a relationship set. Most useful in describing binary relationship sets. For a binary relationship set the mapping cardinality must be one of the following types:
One to one One to many Many to one Many to many
Mapping Cardinalities
One to one
One to many
Note: Some elements in A and B may not be mapped to any elements in the other set
Mapping Cardinalities
Many to one
Many to many
Note: Some elements in A and B may not be mapped to any elements in the other set
Mapping Cardinalities affect ER Design ■ Can make accessdate an attribute of account, instead of a relationship
attribute, if each account can have only one customer
■ I.e., the relationship from account to customer is many to one, or
equivalently, customer to account is one to many
E-R Diagrams
■ Rectangles represent entity sets. ■ Diamonds represent relationship sets. ■ Lines link attributes to entity sets and entity sets to relationship sets. ■ Ellipses represent attributes ■ Double ellipses represent multivalued attributes. ■ Dashed ellipses denote derived attributes. ■ Underline indicates primary key attributes (will study later)
Introduction to Entity-Relationship (E-R) Modeling Notation uses three main constructs
Data entities Relationships Attributes
Entity-Relationship (E-R) Diagram
10.14
A detailed, logical representation of the entities, associations and data elements for an organization or business
Entity-Relationship (E-R) Modeling Key Terms
Entity
A person, place, object, event or concept in the user environment about which the organization wishes to maintain data Represented by a rectangle in E-R diagrams
Entity Type
A collection of entities that share common properties or characteristics
Attribute
10.15
A named property or characteristic of an entity that is of interest to an organization
Entity-Relationship (E-R) Modeling Key Terms
Candidate keys and identifiers
Each entity type must have an attribute or set of attributes that distinguishes one instance from other instances of the same type Candidate key Attribute (or combination of attributes) that uniquely identifies each instance of an entity type
Depicting Entities and Attributes
Draw a portion of the ERD for each of these systems: DePaul Campus Connect Registration System Illinois Bureau of Motor Vehicles System Amazon.com Product Information System
Entity-Relationship (E-R) Modeling Key Terms
Identifier A candidate key that has been selected as the unique identifying characteristic for an entity type Selection rules for an identifier 1. Choose a candidate key that will not change its value 2. Choose a candidate key that will never be null 3. Avoid using intelligent keys 4. Consider substituting single value surrogate keys for large composite keys
Entity-Relationship (E-R) Modeling Key Terms
Relationship
An association between the instances of one or more entity types that is of interest to the organization Association indicates that an event has occurred or that there is a natural link between entity types Relationships are always labeled with verb phrases
Naming and Defining Relationships Relationship name is a verb phrase Avoid vague names Guidelines for defining relationships
Definition explains what action is being taken and why it is important Give examples to clarify the action Optional participation should be explained Explain reasons for any explicit maximum cardinality
Overview of Database Design
What are the entities and relationships in the enterprise? _ What information about these entities and relationships should we store in the database? _ _ A database `schema’ in the ER Model can be represented pictorially (ER diagrams). _
Entity Sets A database can be modeled as: a collection of entities, relationship among entities. An entity is an object that exists and is
distinguishable from other objects.
Example: specific person, company, event, plant
Entities have attributes Example: people have names and addresses An entity set is a set of entities of the same type that
share the same properties.
Example: set of all persons, companies, trees, holidays
Entity Sets customer and loan
customerid customer customer customer loan amount name street city number
Attributes An entity is represented by a set of attributes, that is
descriptive properties possessed by all members of an entity set. Example:
customer = (customerid, customername, customerstreet, customercity) loan = (loannumber, amount)
Domain – the set of permitted values for each attribute Attribute types:
Simple and composite attributes. Single-valued and multi-valued attributes
E.g. multivalued attribute: phone-numbers
Derived attributes
Can be computed from other attributes E.g. age, given date of birth
Composite Attributes
Relationship Sets A relationship is an association among several
entities Example: Hayes depositor A-102 customer entityrelationship setaccount entity A relationship set is a mathematical relation among n ≥ 2 entities, each taken from entity sets {(e1, e2, … en) | e1 ∈ E1, e2 ∈ E2, …, en ∈ En} where (e1, e2, …, en) is a relationship
Example: (Hayes, A-102) ∈ depositor
Degree of a Relationship Set Refers to number of entity sets that participate in a
relationship set. Relationship sets that involve two entity sets are binary (or degree two). Generally, most relationship sets in a database system are binary. Relationship sets may involve more than two entity sets. ★E.g. Suppose employees of a bank may have jobs (responsibilities) at multiple branches, with different jobs at different branches. Then there is a ternary relationship set between entity sets employee, job and branch
Relationships between more than two entity sets are rare.
Most relationships are binary. (More on this later.)
Mapping Cardinalities Express the number of entities to which another
entity can be associated via a relationship set. Most useful in describing binary relationship sets. For a binary relationship set the mapping cardinality must be one of the following types:
One to one One to many Many to one Many to many
Mapping Cardinalities
One to one
One to many
Note: Some elements in A and B may not be mapped to any elements in the other set
Mapping Cardinalities
Many to one
Many to many
Note: Some elements in A and B may not be mapped to any elements in the other set
Mapping Cardinalities affect ER Design ■ Can make accessdate an attribute of account, instead of a relationship
attribute, if each account can have only one customer
■ I.e., the relationship from account to customer is many to one, or
equivalently, customer to account is one to many
E-R Diagrams
■ Rectangles represent entity sets. ■ Diamonds represent relationship sets. ■ Lines link attributes to entity sets and entity sets to relationship sets. ■ Ellipses represent attributes ■ Double ellipses represent multivalued attributes. ■ Dashed ellipses denote derived attributes. ■ Underline indicates primary key attributes (will study later)
Introduction to Entity-Relationship (E-R) Modeling Notation uses three main constructs
Data entities Relationships Attributes
Entity-Relationship (E-R) Diagram
10.14
A detailed, logical representation of the entities, associations and data elements for an organization or business
Entity-Relationship (E-R) Modeling Key Terms
Entity
A person, place, object, event or concept in the user environment about which the organization wishes to maintain data Represented by a rectangle in E-R diagrams
Entity Type
A collection of entities that share common properties or characteristics
Attribute
10.15
A named property or characteristic of an entity that is of interest to an organization
Entity-Relationship (E-R) Modeling Key Terms
Candidate keys and identifiers
Each entity type must have an attribute or set of attributes that distinguishes one instance from other instances of the same type Candidate key Attribute (or combination of attributes) that uniquely identifies each instance of an entity type
Depicting Entities and Attributes
Draw a portion of the ERD for each of these systems: DePaul Campus Connect Registration System Illinois Bureau of Motor Vehicles System Amazon.com Product Information System
Entity-Relationship (E-R) Modeling Key Terms
Identifier A candidate key that has been selected as the unique identifying characteristic for an entity type Selection rules for an identifier 1. Choose a candidate key that will not change its value 2. Choose a candidate key that will never be null 3. Avoid using intelligent keys 4. Consider substituting single value surrogate keys for large composite keys
Entity-Relationship (E-R) Modeling Key Terms
Relationship
An association between the instances of one or more entity types that is of interest to the organization Association indicates that an event has occurred or that there is a natural link between entity types Relationships are always labeled with verb phrases
Naming and Defining Relationships Relationship name is a verb phrase Avoid vague names Guidelines for defining relationships
Definition explains what action is being taken and why it is important Give examples to clarify the action Optional participation should be explained Explain reasons for any explicit maximum cardinality
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