Download - Entity Relationship Modelling
Er. Nawaraj Bhandari
Topic 3 Entity Relationship Modelling 1
Entity Relationship(ER) Diagram
We will learn to develop ER Diagram in this topic.
Entity
An entity can be defined as anything, which can be uniquely identified and about which data is stored in a database.
A place, person, picture, concept (e.g. course, position, job) are the example of an entity.
Some other examples can be student, workers, car, departments, etc.
Entity Type
An entity type is a collection of similar entities.
An entity-type is a category.
An entity type is a group of objects, people, concepts or other items that have the same set of properties or attributes.
Entity Relationship(ER) Modelling
In 1976, Peter Chen first proposed modeling databases using a graphical technique that humans can relate easily.
An Entity–Relationship (ER) model is an abstract or conceptual way of describing a database
An ER Model is based on a perception of a real world that consists of collection of basic objects called entities and relationships among these objects.
3 Notations Type: UML, CHROWS FEET,CHEN
Entity Relationship Modelling 1 Topic 3 - 3.6
© NCC Education LimitedV1.0
Notations 1 – UML(Unified modeling language)
Student
Student on
Module
ModuleAttributes:Module Code(PK)Module Name
Attributes:Student No(PK)Student Name
Attributes:Module Code (FK)Student No (FK)
0...*
1
1
0...*
Entity Relationship Modelling 1 Topic 3 - 3.7
© NCC Education LimitedV1.0
Notations 2 - CROWS FEET
Module
Student onModule
Student
Attributes:Module Code(PK)Module Name
Attributes:Student No(PK)Student Name
Attributes:Module Code(PK)(FK)Student No (PK) (FK)
Entity Relationship Modelling 1 Topic 3 - 3.8
© NCC Education LimitedV1.0
Notations 3 -CHEN
Student
Module Module Code
Student No
M
M
Takes
Database Schema
The overall design or structure of the database is called the database schema.
Three types:1. Internal Schema2. Conceptual Schema3. External Schema
Types of Schema
1. Internal Schemadescribes the physical storage structure of database.
2. Conceptual Schemahides the details of physical storage structures and
concentrates on describing entities, data types, relationships, operations and constraints
3. External Schemadescribes a subset of the database according to the format
the user wants and hides the rest.
Data Independence
The ability to modify schema definition in one level without affecting schema definition in the next higher level is called data independence.
There are two levels of data independence.1. Physical data independence2. Logical data independence.
Physical Data Independence
Physical data independence means the capacity to change the internal schema or physical storage without affecting conceptual or external schema.
Internal schema may be changed to improve the performance.
Logical Data Independence
Logical data independence is the ability to modify the logical or conceptual schema without affecting to external schema or application program.
Logical Data independence means if we add some new columns or remove some columns from table then the user view and programs should not change.
Entity Relationship(ER) Modelling
The ER model achieves a high degree of data independence which means that the database designer do not have to worry about the physical structure of the database.
The goal of ER Modelling is to produce a model of data that is non-technical and free of ambiguities
Entity Relationship(ER) Diagram
ER diagram is a graphical modelling to standardize ER modelling.
The modeling can be carried out with the help of pictorial representation of entities, attributes, and relationships.
Attributes
An attribute is a property of an entity type.
The data that we want to keep about each entity within an entity type is contained in attributes.
Examples of Attributes :
1. Roll number, name and grade are the attributes of Student.
2. Brand, cost and weight are the attributes of Cellphone.
3. Book name, price, author, publisher, etc. are attributes of Book.
Types of Attributes
1. Single-valued attribute
2. Multi-valued attribute
3. Null Attribute
4. Simple Attribute
5. Composite Attribute
6. Key Attribute
7. Derived Attribute
8. Attribute Domain
Types of Attributes
1. Single-valued attribute : Attribute that holds a single value for an occurrence of an entity type. E.g. age, first_name, last_name, full_name, Gender etc.
2. Multi-valued attribute :Attribute where there might be more than one value for a given occurrence of an entity type, e.g. for the 'Telephone Number', where a person or company might have many of these.
3. Null attribute :Attribute that has NULL.
Types of Attributes
4. Simple AttributeIt is composed of a single component. e.g. age, first_name, last_name, Gender etc. But full_name is not simple attribute. Because it is composed of first_name and last_name.
5. Composite AttributeIt is composed of more than one component. For example, 'address' might have city, state, country, zip code, etc.
Types of Attributes
6. Key AttributePrimary Key is a key attribute.Not Null and Unique attribute of an entityAt most one attribute in each entity
7. Derived AttributeThe value of the derived attribute can be derived from the
values of other attributesAge of a person can be derived from the date of birth and
current date i.e. age = current date – date of birth.
Types of Attributes
8. Attribute DomainA domain is the set of allowable values for an attribute or
number of attributes. A domain therefore limits the values that an attribute can
have. For example, the domain of 'gender' would include the
values 'Male' ,'Female’, ’Other’. Similarly, the domain of fruit would include the values
'Apple', 'Orange' etc.
Relationship
A relationship is an association of entities showing how the entities are related with each other.
The examples of relationship are: Teaching is the relationship between Lecturer and Student. Buying is the relationship between Vendor and Customer. Treatment is the relationship between Doctor and Patient.
Types of Relationship or Multiplicity
Number or range of possible occurrences of an entity type in relation to another entity type (CARDINALITY)
1. One-to-One (1 : 1)
2. One-to-Many (1 : N)
3. Many-to-One (N : 1)
4. Many-to-Many (M : N)
Types of Relationship or Multiplicity
1. One-to -One (1 : 1)Each department can have only one department head.Each worker works in one department.
2. One-to-Many (1 : N)One course can have many students registered.A lecturer may be teaching to many students.
Types of Relationship or Multiplicity
3. Many-to-One (N : 1)Many cities come under one country.Many books can be written by one writer.Many authors can write one book.
4. Many-to-Many (M : N)An Author can write several Books, and a Book can be written by several AuthorsA teacher teaches many class and a class studies from many teachers.
ER Diagram Notation (CHEN)
Represents an entity set, But in CROWS FEET entity set are represents by rounded rectangle.
Represents an attribute
Represent linking of attributes to entity sets and of entity sets to relationship sets.
Represents a relationship
ER Diagram
ER Diagram
Draw an ER Diagram for ClassRoom which has two attributes : room_no and building_name.
ER Diagram(Chen)
N 1
ER Diagram(CROWS FEET )
ER Diagram (Chen)
N
1
N
M
ER Diagram(CROWS FEET )
Linking Table(Entity) or Associative or Composite entity
Attribute Notation (CHEN)
Single Valued Attribute
Multi-valued Attribute
Key Attribute
Derived Attribute
Composite Attribute
An employee has first name, last name and can have more than one qualifications.
Multi-valued Attribute
Key Attribute
Primary Key
A department has Primary Key dept_no, department_name and location
foreign key is expressed by dotted underline.
Derived Attribute
The value of the derived attribute can be derived from the values of other related attributes.
Age of a person can be derived from the date of birth and current date i.e. age = current date – date of birth. In this example, age is the derived attribute.
Composite Attribute
Composite attribute is one which can be further subdivided into simple attributes.
Strong entityStrong entity is one whose existence does not depend on other entity.
Weak entity
Weak entity is an entity that depends on another entity. Weak entity doesn't have key attribute of their own.
Associative or composite entity
Associative entity is used to create many-to-many relationship between different entities
Classification of Entity Sets (CHEN)
Entity sets can be broadly classified into:
Classification of Entity Sets (CHEN)
Consider the example, student takes course. Here student is a strong entity.
Course is considered as weak entity because, if there are no students to take a particular course, then that course cannot be offered. The Course entity depends on the student entity.
Associative or Composite entity
The database relational model does not offer direct support to many-to-many relationships. So, we require associative or composite entity.
Consider the example, customer purchases book.
A customer can purchase many books. And a book can be purchased by many customers. So, here purchases is an associative entity. The entity purchases is mediator for customer and book.
Task : ER Diagram
1. Draw an ER diagram for student.
2. Draw an ER diagram for customers and loan relation.
3. Draw an ER diagram for hospital with a set of patients and medical doctors.
4. Draw an ER diagram for banking system.
5. Draw an ER diagram for car insurance company.
Relationship Degree
Relationship degree refers to the number of associated entities in a relationship.
The relationship degree can be broadly classified into
1. Unary(Recursive) relationship
2. Binary relationship
3. Ternary relationship
Unary(Recursive) relationship
The number of associated entity is one.
An entity related to itself is known as recursive relationship.
Binary relationship
In a binary relationship, two entities are involved.
Consider the example: each department will have many workers.
Ternary relationship
In a ternary relationship, three entities are simultaneously involved.
Example : Consider a project is assigned to many employee. And the project is assigned to one location.
ER Diagram Notation (CROW'S FEET)
Entity Name
attribute namePK
attribute name
attribute name
Entity Name
Entity RelationshipIllustrates an association between two entities. It has a name which is a verb. It also has cardinality.
is assigned to
ER Diagram Notation (CROW'S FEET)
zero or more
one or more
one and only one
zero or 1
Cardinality Cardinality refers to the maximum number of times an instance in one entity can be associated with instances in the related entity. It can be zero or one or more.
Some examples (CROW'S FEET)
courseteacherteaches
coursestudenttakes
ER Diagram Notation (UML)
A Book is written by one or more authors. An author writes one or more Books. A Category may have zero or many books.
Nouns: Book, Author, Category
[Note : Many to Many Relationship cannot be shown in relational model]
1..NAuthorBook AuthorBook
11..N1
Category
1
0..N
ER Diagram Notation (CROW'S FEET)
A Gardening Company
The company hires out workers as they need to.
Workers in the town stay in lodgings. A lodging may have many
workers in it. We are only concerned with the workers’ current
lodging. Workers possess certain skills such as ‘Tree Surgery’
and ‘Garden Design’. They may have more than one skill.
[Note : Try to identify nouns that could possibly be entities.]
A Gardening Company - Nouns
The company hires out workers as they need to.
Workers in the town stay in lodgings. A lodging may have many
workers in it. We are only concerned with the workers’ current
lodging. Workers possess certain skills such as ‘Tree Surgery’ and
‘Garden Design’. They may have more than one skill.
[Note : We are only concerned with one town. So, town is not entity here. But 'Town' is an attribute of the Lodging.]
Possible Solution(UML)
0..*SkillWorker SkillWorker
10..*1
Lodging
0..*
1
Possible Solution(CROW'S FEET)
Equipment & Students
A college department of photography allocates equipment to its students.
A student might be allocated up to 3 types of equipment at a time.
Equipment types can be allocated to many students.
Sports club
A junior sports club keeps track of what sports its members do.
Each member might be involved in more than one sport.
Any sport will have many members involved
Possible Solution(CROW'S FEET)
Many-to-Many Relationship
Module having many Students and Students taking many Modules
Taken ByModule Student0..N 0..N
Attributes
ModuleID(PK)
ModuleName
Attributes
StudentID(PK)
StudentName
Many-to-Many Relationship
Module Student0..N 0..N
ModuleStudent1 1
Attributes
ModuleID(PK)
ModuleName
Attributes
StudentID(PK)
StudentName
Attributes
ModuleID(FK)
StudentID(FK)
Draw the ER : Car Hire Company
Company has a number of models of car available for hire.
Customers hire cars and this is recorded against a particular car rather than a model, as the company might have several cars of the same model.
When a customer wants to reserve a particular model they can do so. This is recorded against a model rather than the car.
When a car of that model becomes available, it will be held for the customer.
When they come in for it, a record is made on the reservation to say which car satisfied the reservation.
Draw the ER : Car Hire Company
Company has a number of models of car available for hire.
Customers hire cars and this is recorded against a particular car rather than a model, as the company might have several cars of the same model.
When a customer wants to reserve a particular model they can do so. This is recorded against a model rather than the car.
When a car of that model becomes available, it will be held for the customer.
When they come in for it, a record is made on the reservation to say which car satisfied the reservation.
Car Hire Company
Model
HireCar
Reservation
Customer0..N
1
0..N 11 0..N
0..N 1
1
0..N
1 0..N
Each supplier has a unique name.
More than one supplier can be located in the same city.
Each part has a unique part number.
Each part has a color.
A supplier can supply more than one part.
A part can be supplied by more than one supplier.
A supplier can supply a fixed quantity of each part.
Manufacturing Industry
Manufacturing Industry(Chen)
Manufacturing Industry(UML)
Manufacturing Industry (CROWS FEET)
Primary Key
The PRIMARY KEY constraint uniquely identifies each record in a database table.
Primary keys must contain unique values.
A primary key column cannot contain NULL values.
Each table should have a primary key, and each table can have only ONE primary key.
departments------------------------dept_no (PK)department_namelocation
Foreign Key
A FOREIGN KEY in one table points to a PRIMARY KEY in another table
A FOREIGN KEY is a column or a combination of columns that is used to establish and enforce a link between two tables or entities.departments------------------------dept_no (PK)department_namelocation
workers------------------emp_no (PK)first_namelast_nameagejob_titledept_no (FK)
Creating Foreign Key
Problems(pit·fall) with ER Models
There are several problems that may arise when designing a ER model. These are known as connection traps.
There are two main types of connection traps:
1. Fan traps
2. Chasm traps
Fan Traps
Fan traps occur in a situation when a model represents relationship between entity types however a path between certain entity occurrences is ambiguous.
Example: (Staff)-1:N-works in-1-(Campus)-1-has-1:N-(Department).
In this model, it may be impossible to determine the department a staff belongs to.
DepartmentStaff Campus1 1 1..*1..*
Entity Relationship Modelling 2 Topic 4 - 4.72
© NCC Education LimitedV1.0
Fan Traps- 1
Campus ID Campus Name
C1 London
C2 Lagos
C3 Cairo
Staff ID Name Campus ID (FK)
S1 Eric Hudson
C2
S2 Sandra Smith
C1
S3 Malia Shah
C1
Department ID Name Campus ID (FK)
D1 Humanities C1
D2 Computing C2
D3 Maths C2
Campus
Staff Department
In which Department does Eric work?
Fan Traps – The Solution
Restructuring the model resolves trap (Campus)-1-has-1:N-(Department)-1-has-1:N-(Staff)
CampusStaff Department1 1..* 11..*
Entity Relationship Modelling 2 Topic 4 - 4.74
© NCC Education LimitedV1.0
Fan Traps- 2
Campus ID
Campus Name
C1 London
C2 Lagos
C3 Cairo
Staff ID Name Department ID (FK)
S1 Eric Hudson
D3
S2 Sandra Smith
D1
S3 Malia Shah
D1
Department ID Name Campus ID (FK)
D1 Humanities C1
D2 Computing C2
D3 Maths C2
Campus
StaffDepartment
We now know in which department Eric works
Chasm Traps
Chasm Traps occur where there are relationships between entities, but one of the relationships is non-mandatory.
A model suggests the existence of a relationship between entity types, but the pathway does not exist between certain entity occurrences. This connection traps is known as Chasm Traps.
http://db.grussell.org/section005.html
Chasm Traps – The Problem
A branch has many staff members who manage properties, but not all properties are managed by a member of staff, and not all staff manage properties.
Staff PropertyBranch1...* 0...*0...11
Entity Relationship Modelling 2 Topic 4 - 4.77
© NCC Education LimitedV1.0
Chasm Trap - 1Branch ID
Branch Name
1 Ilford
2 Redbridge
Staff ID Name Branch ID (FK)
S1 Davinda 1
S2 Roberta 1
S3 Eddie 2
Property ID Property Name
Staff ID (FK)
P1 Yap Mansions
S1
P2 Hill House
P2 Usher House
S2
Branch
Staff
Property
Which Branch manages‘Hill House’?
Chasm Traps – The Solution
The solution is to change the structure and represent both relationships.
PropertyStaff
Branch
1...* 0...*0...11
1 0...*
Chasm Traps – The Solution
Branch ID
Branch Name
1 Ilford
2 Redbridge
Staff ID Name Branch ID (FK)
S1 Davinda
1
S2 Roberta 1
S3 Eddie 2
Property ID
Property Name
Staff ID (FK) Branch ID (FK)
P1 Yap Mansions S1 1
P2 Hill House 2
P3 Usher House S2 1
Branch Staff
Property
We now know which Branch manages ‘Hill House’
References
http://www.studytonight.com/dbms/er-diagram.php
http://www.blackwasp.co.uk/SQLDerivedTables.aspx
http://www2.cs.uregina.ca/~bernatja/crowsfoot.html
http://revj.sourceforge.net/traps.html
http://db.grussell.org/section005.html
http://www.tutorialspoint.com/dbms/dbms_data_independence.htm
http://en.wikipedia.org/wiki/Three_schema_approach
http://en.wikipedia.org/wiki/Data_independence
http://www.tutorialspoint.com/dbms/er_model_basic_concepts.htm
END OF TOPIC 3
Any Questions