1. CS4222 Principles of Database System
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CS4222 Principles of Database System
2. Entity-Relationship model
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline Overview of Database Design Process
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Outline
Overview of Database Design Process
Entity-Relationship (ER) modeling
Entities
Entity sets
Relationship sets
Constraints on entity sets
Constraints on relationship sets
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3. Overview of Database Design Process
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4. Overview of Database Design Process
Requirement analysis
During this step, the database designers interview prospective users to understand and document their data requirements
The result of this step is a concisely written set of users’ requirements
In addition to data requirements, functional requirements are also specified
Operations (retrieval and updates) that will be applied to the database
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5. Overview of Database Design Process
Conceptual design
Use a high-level conceptual data model to create a conceptual schema
The conceptual schema is a concise description of the data requirements of the users
It includes detailed description of entities, relationships and constraints
It does not include implementation details
Can be used to communication with non-technical users
It can also be used as a reference to ensure that all users’ data requirements are met and that the requirements do not conflict.
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6. Overview of Database Design Process
Logical design or data model mappings
Translate a conceptual schema into DBMS data model
Physical design and tuning
The internal storage structures, indexes, access path, and file organizations for the database files are specified
Application programs are designed and implemented as database transactions corresponding to the high level transaction specifications
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7. Entity Relationship modeling
Problem
ER Diagram
Tables
Modeling
(Conceptual design)
Mapping
(Logical design)
Entity relationship diagram
Abstractly describe the data
Entities, relationships and attributes
Easily mapped to Tables
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8. ER Model Basics Entity Attributes
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ER Model Basics
Entity
Real-world object distinguishable from other objects
An entity may be an object with a physical existence or an object with conceptual existence
Attributes
Each entity is described (in DB) using a set of attributes
Example
For example an EMPLOYEE entity may have the attributes Name, SSN, Address, Sex, BirthDate
A particular entity has a value for each of its attributes
For example a specific employee entity may have Name='John Smith', SSN=' ', Address ='731, Fondren, Houston, TX', Sex='M', BirthDate='09-JAN-55‘
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9. Types of Attributes (1) Simple Composite Multi-valued
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Types of Attributes (1)
Simple
Each entity has a single atomic value for the attribute. For example, SSN or Sex.
Composite
The attribute may be composed of several components.
For example:
Address(Apt#, House#, Street, City, State, ZipCode, Country), or
Name(FirstName, MiddleName, LastName).
Composition may form a hierarchy where some components are themselves composite.
Multi-valued
An entity may have multiple values for that attribute.
For example, Color of a CAR or PreviousDegrees of a STUDENT.
Denoted as {Color} or {PreviousDegrees}.
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Types of Attributes (3)
In general, composite and multi-valued attributes may be nested arbitrarily to any number of levels, although this is rare.
For example
PreviousDegrees of a STUDENT is a composite multi-valued attribute denoted by {PreviousDegrees (College, Year, Degree, Field)}
Multiple PreviousDegrees values can exist
Each has four subcomponent attributes:
College, Year, Degree, Field
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11. Example of a composite attribute
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12. Types of Attributes (2) Derived attributes NULL values
In some cases, two (or more) attributes are related
Eg. Age and Birth_date of a person
The age can be derived from Birth_date
Age: derived attribute
Birth_date: stored attribute
NULL values
In some cases, a particular entity may not have an applicable value for an attribute
Eg. Aprtment_number attribute of an address
A special value called NULL is created for such situations
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13. Entity Types and Key Attributes (1)
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Entity Types and Key Attributes (1)
Entities with the same basic attributes are grouped or typed into an entity type.
For example, the entity type EMPLOYEE and PROJECT.
An attribute of an entity type for which each entity must have a unique value is called a key attribute of the entity type.
For example, SSN of EMPLOYEE.
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14. Entity Types and Key Attributes (2)
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Entity Types and Key Attributes (2)
A key attribute may be composite.
VehicleTagNumber is a key of the CAR entity type with components (Number, State).
An entity type may have more than one key.
The CAR entity type may have two keys:
VehicleIdentificationNumber (popularly called VIN)
VehicleTagNumber (Number, State), a license plate number.
Each key is underlined
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15. Displaying an Entity type
In ER diagrams, an entity type is displayed in a rectangular box
Attributes are displayed in ovals
Each attribute is connected to its entity type
Components of a composite attribute are connected to the oval representing the composite attribute
Each key attribute is underlined
Multivalued attributes displayed in double ovals
Derived attributes displayed in dashed ovals
See CAR example on next slide
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16. Entity Type CAR with two keys
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17. Entity Set
Each entity type will have a collection of entities stored in the database
Called the entity set
The Same name (CAR) is used to refer to both the entity type and the entity set
The two terms will be used interchangeably
Entity set is the current state of the entities of that type that are stored in the database
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18. Car Entity Set
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19. Example COMPANY Database
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Example COMPANY Database
We need to create a database schema design based on the following (simplified) requirements of the COMPANY Database:
The company is organized into DEPARTMENTs.
Each department has a name, number and an employee who manages the department.
We keep track of the start date of the department manager.
A department may have several locations.
Each department controls a number of PROJECTs.
Each project has a unique name, unique number and is located at a single location.
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20. Example COMPANY Database (Contd.)
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Example COMPANY Database (Contd.)
We store each EMPLOYEE’s social security number, address, salary, sex, and birthdate.
Each employee works for one department but may work on several projects.
We keep track of the number of hours per week that an employee currently works on each project.
We also keep track of the direct supervisor of each employee.
Each employee may have a number of DEPENDENTs.
For each dependent, we keep track of their name, sex, birthdate, and relationship to the employee.
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21. Initial Design of Entity Types for the COMPANY Database Schema
Based on the requirements, we can identify four initial entity types in the COMPANY database:
DEPARTMENT
PROJECT
EMPLOYEE
DEPENDENT
Their initial design is shown on the following slide
The initial attributes shown are derived from the requirements description
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22. Initial Design of Entity Types
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23. Refining the initial design by introducing relationships
The initial design is typically not complete
Some aspects in the requirements will be represented as relationships
ER model has three main concepts:
Entities (and their entity types and entity sets)
Attributes (simple, composite, multivalued)
Relationships (and their relationship types and relationship sets)
We introduce relationship concepts next
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24. Relationships and Relationship Types (1)
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Relationships and Relationship Types (1)
A relationship relates two or more distinct entities with a specific meaning.
For example, EMPLOYEE John Smith works on the ProductX PROJECT, or EMPLOYEE Franklin Wong manages the Research DEPARTMENT.
Relationships of the same type are grouped or typed into a relationship type.
For example, the WORKS_ON relationship type in which EMPLOYEEs and PROJECTs participate, or the MANAGES relationship type in which EMPLOYEEs and DEPARTMENTs participate.
The degree of a relationship type is the number of participating entity types.
Both MANAGES and WORKS_ON are binary relationships.
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25. Relationship type vs. relationship set
Is the schema description of a relationship
Identifies the relationship name and the participating entity types
Also identifies certain relationship constraints
Relationship Set:
The current set of relationship instances represented in the database
The current state of a relationship type
The two terms will be used interchangeably
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Relationship instances of the M:N WORKS_ON relationship between EMPLOYEE and PROJECT
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27. Relationship type vs. relationship set
Previous figures displayed the relationship sets
Each instance in the set relates individual participating entities – one from each participating entity type
In ER diagrams, we represent the relationship type as follows:
Diamond-shaped box is used to display a relationship type
Connected to the participating entity types via straight lines
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28. Example
Works_For
Employee
Department
Works_On
Employee
Project
The same entity type may be involved in different relationship sets
Eg. Employee is involved in 2 relationship types: Works_For and Works_On
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29. Attributes of Relationship types
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Attributes of Relationship types
A relationship type can have attributes:
For example, HoursPerWeek of WORKS_ON
Its value for each relationship instance describes the number of hours per week that an EMPLOYEE works on a PROJECT.
A value of HoursPerWeek depends on a particular (employee, project) combination
Hours
Works_On
Employee
Project
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30. Recursive Relationship Type
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Recursive Relationship Type
An relationship type with the same participating entity type in distinct roles
Example: the SUPERVISION relationship
EMPLOYEE participates twice in two distinct roles:
supervisor (or boss) role
supervisee (or subordinate) role
Each relationship instance relates two distinct EMPLOYEE entities:
One employee in supervisor role
One employee in supervisee role
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31. Displaying a recursive relationship
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Displaying a recursive relationship
In a recursive relationship type.
Both participations are same entity type in different roles.
For example, SUPERVISION relationships between EMPLOYEE (in role of supervisor or boss) and (another) EMPLOYEE (in role of subordinate or worker).
In ER diagram, need to display role names to distinguish participations.
Supervision
supervisor
subordinate
Employees
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32. Constraints on Entity Sets
Domain Constraints on Entity Sets
Cardinality constrains on relationship sets
Participation Constraints
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33. Domain Constraints on Entity Sets
An attribute is associated with a domain.
The value of the attribute for each entity is constrained to be in the domain only.
Example:
Gender: “F” or “M”
Age: “> 20”
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34. Cardinality constrains on relationship sets
Many-to-Many Relationship (M:N)
By default
One-to-Many Relationship(1:N)
Many-to-One Relationship (N:1)
One-to-One Relationship (1:1)
In ER diagram, cardinality ratio(1:1, 1:N, N:1, or M:N) is shown by placing appropriate numbers on the relationship edges
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35. Many-to-Many Relationship in ER diagram
An entity in A is associated to multiple entities in B
An entity in B is associated to multiple entities in A A B
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36. Many-to-Many Relationship
Example:
Multiple students can take the same course
One student can take multiple courses M N
Students
Take
Courses
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37. One-to-Many Relationship
An entity in A is associated to many entities in B
Or an entity in B is associated to at most one entity in A A B
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38. One-to-Many Relationship
Example
A supervisor can supervise multiple students
A student has at most one supervisor 1 N
Supervisor
Supervise
Students
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39. One-to-One Relationship
An entity in A is associated to at most one entity in B
And an entity in B is associated to at most one entity in A
Example
A woman is married to at most one man
A man is married to at most one woman
Is_Married_to 1 1
Women
Men
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40. Participation Constraints
Two kinds of participation constraints
Total participation
Each department must have a manager
The participation of the Departments in the relationship set Manages is said to be total
Indicated in ER diagram by double lines
Partial participation
A participation that is not total
Indicated in ER diagram by single lines
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41. Participation Constraints
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Participation Constraints
Each department must have a manager
Manages
Employee
Department
Employee
Department
Manages
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42. Exercise Each customer can have multiple accounts
customers
custacct
accounts
opendate
Each customer can have multiple accounts
Several customers can share the same accounts
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43. Exercise (cont.) Each customer has at most one account1
customers
custacct
accounts
opendate
Each customer has at most one account
Each account is owned by at least one customer
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44. Exercise (cont.) Each customer has at least one account1 N
customers
custacct
accounts
opendate
Each customer has at least one account
Each accounts is owned by exactly one customer
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