Entity-Relationship Model

• Entity, Relationship, and E-R Diagram
• Entity
• Attributes
• Weak/Regular Entity
• Relationship
• Degree
• Mapping Cardinality
• Relationship with Attributes
• Participation
• Keys
• Extended E-R Features
• Entity Specialization/Generalization
• Relationship Aggregation
• Design of an E-R Database Schema

2.1

Entity, Relationship, and E-R Diagram

• A database can be modeled as:
• a collection of entities,
• relationship among entities.
• A database can be illustrated by an E-R diagram

2.2

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. (will study later)
• Dashed ellipses denote derived attributes. (will study later)
• Underline indicates primary key attributes (will study later)

2.3

Entity Sets

• 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

2.4

Entity Sets customer and loan

customer-id customer- customer- customer- loan- amount� name street city number

2.5

Attributes

• An entity is represented by a set of attributes, that is descriptive properties possessed by all members of an entity set.

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• 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

Example:

customer = (customer-id, customer-name, customer-street, customer-city)� loan = (loan-number, amount)

2.6

Composite Attributes

2.7

E-R Diagram With Composite, Multivalued, and Derived Attributes

2.8

Weak Entity and Regular/Strong Entity

• A weak entity is an entity that is existence-dependent on some other entity. By contrast, a regular entity (or “a strong entity”) is an entity which is not weak.
• The existence of a weak entity set depends on the existence of a identifying entity set
• it must relate to the identifying entity set via a total, one-to-many relationship set from the identifying to the weak entity set
• E.g. An employee’s dependents might be weak entities --- they can’t exist (so far as the database is concerned) if the relevant employee does not exist.
• A weak entity type can be related to more than one regular entity type.

2.9

Weak Entity and Regular/Strong Entity

• We depict a weak entity by double rectangles.
• The identifying relationship is depicted using a double diamond.

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2.10

2.11

Relationship Sets

• A relationship is an association among several entities

Example:� Hayes depositor A-102� customer entity relationship set account entity

• A relationship set is a mathematical relation among n ≥ 2 entities, each taken from entity sets

{(e₁, e₂, … e_n) | e₁ ∈ E₁, e₂ ∈ E₂, …, e_n ∈ E_n}��where (e₁, e₂, …, e_n) is a relationship

• Example:

(Hayes, A-102) ∈ depositor

2.12

Relationship Set borrower

2.13

Relationship Sets (Cont.)

• An attribute can also be property of a relationship set.
• For instance, the depositor relationship set between entity sets customer and account may have the attribute access-date

2.14

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.

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• Relationships between more than two entity sets are rare. Most relationships are binary. (More on this later.)

• 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

2.15

E-R Diagram with a Ternary Relationship

2.16

Binary Vs. Non-Binary Relationships

• Some relationships that appear to be non-binary may be better represented using binary relationships
• E.g. A ternary relationship parents, relating a child to his/her father and mother, is best replaced by two binary relationships, father and mother
• Using two binary relationships allows partial information (e.g. only mother being know)
• But there are some relationships that are naturally non-binary
• E.g. works-on

2.17

Roles

• Entity sets of a relationship need not be distinct
• The labels “manager” and “worker” are called roles; they specify how employee entities interact via the works-for relationship set.
• Roles are indicated in E-R diagrams by labeling the lines that connect diamonds to rectangles.
• Role labels are optional, and are used to clarify semantics of the relationship

2.18

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

2.19

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

2.20

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

2.21

Mapping Cardinality

• We express cardinality constraints by drawing either a directed line (→), signifying “one,” or an undirected line (—), signifying “many,” between the relationship set and the entity set.
• E.g.: One-to-one relationship:
• A customer is associated with at most one loan via the relationship borrower
• A loan is associated with at most one customer via borrower

2.22

One-To-Many Relationship

• In the one-to-many relationship a loan is associated with at most one customer via borrower, a customer is associated with several (including 0) loans via borrower

2.23

Many-To-One Relationships

• In a many-to-one relationship a loan is associated with several (including 0) customers via borrower, a customer is associated with at most one loan via borrower

2.24

Many-To-Many Relationship

• A customer is associated with several (possibly 0) loans via borrower
• A loan is associated with several (possibly 0) customers via borrower

2.25

Mapping Cardinalities affect ER Design

• Can make access-date 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

2.26

Relationship Sets with Attributes

2.27

Participation of an Entity Set in a Relationship Set

• Total participation (indicated by double line): every entity in the entity set participates in at least one relationship in the relationship set
• E.g. participation of loan in borrower is total
• every loan must have a customer associated to it via borrower
• Partial participation: some entities may not participate in any relationship in the relationship set
• E.g. participation of customer in borrower is partial

2.28

Keys

• A super key of an entity set is a set of one or more attributes whose values uniquely determine each entity.
• A candidate key of an entity set is a minimal super key
• Customer-id is candidate key of customer
• account-number is candidate key of account
• Although several candidate keys may exist, one of the candidate keys is selected to be the primary key.

2.29

Keys for Relationship Sets

• The combination of primary keys of the participating entity sets forms a super key of a relationship set.
• (customer-id, account-number) is the super key of depositor
• NOTE: this means a pair of entity sets can have at most one relationship in a particular relationship set.
• E.g. if we wish to track all access-dates to each account by each customer, we cannot assume a relationship for each access. We can use a multivalued attribute though
• Must consider the mapping cardinality of the relationship set when deciding the what are the candidate keys
• Need to consider semantics of relationship set in selecting the primary key in case of more than one candidate key

2.30

 Keys in This Table

• Super Keys:
• {StudentID}
• {Email}
• {Phone}
• {StudentID, Name}
• {Email, Name}
• {StudentID, Email, Phone}
• Candidate Keys:
• {StudentID}
• {Email}
• {Phone}
• Primary Key:
• {StudentID} (chosen among candidate keys)

�

Example Table: Student

StudentID Name Email Phone

2.31

Recursive Relationship Type is: SUPERVISION�(participation role names are shown)

2.32

Attribute of a Relationship Type is: �Hours of WORKS_ON

2.33

COMPANY ER Schema Diagram� using (min, max) notation

2.34

ER DIAGRAM FOR A BANK �DATABASE

© The Benjamin/Cummings Publishing Company, Inc. 1994, Elmasri/Navathe, Fundamentals of Database Systems, Second Edition

2.35

Specialization

• Top-down design process; we designate subgroupings within an entity set that are distinctive from other entities in the set.
• These subgroupings become lower-level entity sets that have attributes or participate in relationships that do not apply to the higher-level entity set.
• Depicted by a triangle component labeled ISA (E.g. customer “is a” person).
• Attribute inheritance – a lower-level entity set inherits all the attributes and relationship participation of the higher-level entity set to which it is linked.

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2.36

Specialization Example

2.37

Generalization

• A bottom-up design process – combine a number of entity sets that share the same features into a higher-level entity set.
• Specialization and generalization are simple inversions of each other; they are represented in an E-R diagram in the same way.
• The terms specialization and generalization are used interchangeably.

2.38

Specialization and Generalization (Contd.)

• Can have multiple specializations of an entity set based on different features.
• E.g. permanent-employee vs. temporary-employee, in addition to officer vs. secretary vs. teller
• Each particular employee would be
• a member of one of permanent-employee or temporary-employee,
• and also a member of one of officer, secretary, or teller
• The ISA relationship also referred to as superclass - subclass relationship

2.39

Design Constraints on a Specialization/Generalization

• Constraint on which entities can be members of a given lower-level entity set.
• condition-defined
• E.g. all customers over 65 years are members of senior-citizen entity set; senior-citizen ISA person.
• user-defined
• Constraint on whether or not entities may belong to more than one lower-level entity set within a single generalization.
• Disjoint
• an entity can belong to only one lower-level entity set
• Noted in E-R diagram by writing disjoint next to the ISA triangle
• Overlapping
• an entity can belong to more than one lower-level entity set

2.40

Design Constraints on a Specialization/Generalization (Contd.)

• Completeness constraint -- specifies whether or not an entity in the higher-level entity set must belong to at least one of the lower-level entity sets within a generalization.
• total : an entity must belong to one of the lower-level entity sets
• partial: an entity need not belong to one of the lower-level entity sets

2.41

1. Disjoint Constraint

• Definition: An entity can belong to only one subclass.
• Example:�Consider a superclass Employee with subclasses Engineer and Manager.�If the constraint is disjoint, then an employee cannot be both an Engineer and a Manager.
• An employee must be either an Engineer or a Manager.
• An employee cannot be both at the same time.

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2. Overlapping Constraint

• Definition: An entity can belong to multiple subclasses.
• Example:�Consider a superclass Person with subclasses Student and Employee.�If the constraint is overlapping, a person can be both a Student and an Employee.

3. Completeness Constraint

• Definition: Specifies whether all entities in the superclass must be members of at least one subclass.
• Types:
• Total: Every entity in the superclass must belong to at least one subclass.
• Partial: Some entities in the superclass may not belong to any subclass.
• Example:
• Total: Every Vehicle is either a Car or a Truck.
• Partial: Some Person entities may be neither Student nor Employee.

�

2.42

Aggregation

• Consider the ternary relationship works-on, which we saw earlier
• Suppose we want to record managers for tasks performed by an � employee at a branch

2.43

Aggregation (Cont.)

• Relationship sets works-on and manages represent overlapping information
• Every manages relationship corresponds to a works-on relationship
• However, some works-on relationships may not correspond to any manages relationships
• So we can’t discard the works-on relationship
• Eliminate this redundancy via aggregation
• Treat relationship as an abstract entity
• Allows relationships between relationships
• Abstraction of relationship into new entity
• Without introducing redundancy, the following diagram represents:
• An employee works on a particular job at a particular branch
• An employee, branch, job combination may have an associated manager

2.44

E-R Diagram With Aggregation

2.45

E-R Design Decisions

• The use of an attribute or entity set to represent an object.
• Whether a real-world concept is best expressed by an entity set or a relationship set.
• The use of a ternary relationship versus a pair of binary relationships.
• The use of specialization/generalization – contributes to modularity in the design.
• The use of aggregation – can treat the aggregate entity set as a single unit without concern for the details of its internal structure.

2.46

E-R Diagram for a Banking Enterprise

2.47

Summary of Symbols Used in E-R Notation

2.48

Summary of Symbols (Cont.)

2.49

Alternative E-R Notations

2.50

Scenario 1: University Course Registration System

Activity Description:

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Design an ER diagram for a university system that manages students, courses, professors, and registrations.

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Entities and Attributes:

• Student: Student_ID (PK), Name, Email, Department
• Course: Course_ID (PK), Title, Credits, Department
• Professor: Prof_ID (PK), Name, Email, Specialization
• Registration: Reg_ID (PK), Semester, Grade
• Department: Dept_ID (PK), Name, Location

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Relationships:

• A student can register for multiple courses.
• A course can be taught by multiple professors, and a professor can teach multiple courses.
• Each course belongs to one department, and each student is enrolled in one department.
• The Registration entity connects Student and Course with additional attributes like semester and grade.

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Special Features:

• Use of weak entity (Registration).
• Many-to-Many relationships with attributes.
• Generalization: TeachingStaff ⟶ Professor, AssistantProfessor

2.51

1. Student — Registers — Course

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Type: Many-to-Many

Explanation: A student can register for multiple courses, and each course can have multiple students.

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Resolution: This is resolved using the Registration entity, which includes attributes like Semester and Grade.

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Attributes Involved:

Student: Student_ID, Name, Email, Department

Course: Course_ID, Title, Credits

Registration: Reg_ID, Semester, Grade

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🔹 2. Course — Taught by — Professor

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Type: Many-to-Many

Explanation: A course can be taught by multiple professors (e.g., co-teaching), and a professor can teach multiple courses.

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Attributes Involved:

Professor: Prof_ID, Name, Specialization

2.52

🔹 3. Course — Belongs to — Department

Type: Many-to-One

Explanation: Each course is offered by one department, but a department can offer many courses.

Attributes Involved:

Department: Dept_ID, Name, Location

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🔹 4. Student — Enrolled in — Department

Type: Many-to-One

Explanation: Each student is enrolled in one department, but a department can have many students.

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🔹 Optional Enhancements:

Generalization: TeachingStaff ⟶ Professor, AssistantProfessor

Derived Attributes: GPA can be derived from grades in the Registration entity.

2.53

Scenario 2: E-Commerce Order Management System

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Activity Description:

Design an ER diagram for an e-commerce platform that handles customers, products, orders, payments, and shipping.

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Entities and Attributes:

• Customer: Cust_ID (PK), Name, Email, Address
• Product: Prod_ID (PK), Name, Price, Category
• Order: Order_ID (PK), Order_Date, Status
• Payment: Payment_ID (PK), Method, Amount, Date
• Shipping: Ship_ID (PK), Carrier, Tracking_No, Ship_Date

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Relationships:

• A customer can place multiple orders.
• An order can contain multiple products, and a product can appear in multiple orders.
• Each order has one payment and one shipping record.
• Product belongs to a category (can be modeled as a separate entity or attribute).

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Special Features:

• Aggregation: Order includes Product, Payment, and Shipping.
• Multivalued attributes: Customer may have multiple addresses.
• Derived attributes: Total amount from products in an order.

2.54

1. Customer — Places — Order

Type: One-to-Many

Explanation: A single customer can place multiple orders, but each order is placed by one customer.

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Attributes Involved:

Customer: Cust_ID (Primary Key)

Order: Order_ID, Order_Date, Status

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2. Order — Contains — Product

Type: Many-to-Many

Explanation: An order can contain multiple products, and a product can be part of multiple orders.

Resolution: This relationship is typically resolved using a junction table or Order Details entity with attributes like quantity and price at the time of order.

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Attributes Involved:

Product: Prod_ID, Name, Price

Order: Order_ID

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2.55

3. Order — Has — Payment

Type: One-to-One

Explanation: Each order is associated with one payment transaction.

Attributes Involved:

Payment: Payment_ID, Method, Amount, Date.

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🔹 4. Order — Has — Shipping

Type: One-to-One

Explanation: Each order has one shipping record detailing how and when it was shipped.

Attributes Involved:

Shipping: Ship_ID, Carrier, Tracking_No, Ship_Date

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🔹 Optional Enhancements:

Product — Belongs to — Category (One-to-Many)

Customer — Has — Multiple Addresses

(Multivalued Attribute or separate Address entity)

2.56