(ER) Entity Relationship Diagram 157a (3)

Question 1

What is an entity?

Answer 1

An Entity may be an object with a physical existence – a particular person, car,
house, or employee – or it may be an object with a conceptual existence – a
company, a job, or a university course.
Entity Set: An Entity is an object of Entity Type and a set of all entities is called an
entity set. For Example, E1 is an entity having Entity Type Student and the set of all
students is called Entity Set. In ER diagram, Entity Type is represented as:

Question 2

What is a strong entity?

Answer 2

A Strong Entity is a type of entity that has a key Attribute. Strong Entity does not depend on other Entity in the Schema. It has a primary key, that helps in identifying it uniquely, and it is represented by a rectangle. These are called Strong Entity
Types.

Question 3

What is a weak entity?

Answer 3

An Entity type has a key attribute that uniquely identifies each entity in the entity set. But some entity type exists for which key attributes can’t be defined. These are called Weak Entity types.

For Example, A company may store the information of dependents (Parents, Children, Spouse) of an Employee. But the dependents don’t have existed without the employee. So Dependent will be a Weak Entity Type and Employee will be Identifying Entity type for Dependent, which means it is Strong Entity Type.

Question 4

Why do we need ER diagrams?

Answer 4

● ER diagrams are used to represent the E-R model in a database, which makes
them easy to be converted into relations (tables).
● ER diagrams provide the purpose of real-world modeling of objects which
makes them intently useful.
● ER diagrams require no technical knowledge and no hardware support.
● These diagrams are very easy to understand and easy to create even for a
naive user.
● It gives a standard solution for visualizing the data logically.

Question 5

Database Design

Answer 5

● What it is:
● Starting from scratch, design the database schema: relation, attributes,
keys, foreign keys, constraints etc.
● Why it’s hard:
○ The database will be in operation for years.
○ Updating the schema in production is very hard
■ schema change modifications are expensive (why?)
■ making the change without introducing any bugs is hard
● this part is, by far, the most important consideration in
practice

● Consider issues such as:
○ What entities to model
○ How entities are related
○ What constraints exist in the domain
■ Several formalisms exists
○ We discuss E/R diagrams

Question 6

Database Design Process

Answer 6

Conceptual Model

Relational Model

Normalization(Conceptual schema)

Physical Storage(Physical schema)

Question 7

What are the different components of an ER diagram?

Answer 7

Entity
-Strong/Weak entity

Attribute
- key attribute
- composite attribute
- multivalued attribute
- derived attribute

Relationship
- one to one
- one to many
- many to one
- many to many

Question 8

What does a rectangle represent?

Answer 8

Entites in ER model

ER Model is used to model the logical view of the system from a data perspective
which consists of these symbols:
● Rectangles: Rectangles represent Entities in the ER Model.
● Ellipses: Ellipses represent Attributes in the ER Model.
● Diamond: Diamonds represent Relationships among Entities.
● Lines: Lines represent attributes to entities and entity sets with other
relationship types.
● Double Ellipse: Double Ellipses represent Multi-Valued Attributes.
● Double Rectangle: Double Rectangle represents a Weak Entity.

Question 9

What does an ellipse represent?

Answer 9

Attributes in an ER model

ER Model is used to model the logical view of the system from a data perspective
which consists of these symbols:
● Rectangles: Rectangles represent Entities in the ER Model.
● Ellipses: Ellipses represent Attributes in the ER Model.
● Diamond: Diamonds represent Relationships among Entities.
● Lines: Lines represent attributes to entities and entity sets with other
relationship types.
● Double Ellipse: Double Ellipses represent Multi-Valued Attributes.
● Double Rectangle: Double Rectangle represents a Weak Entity.

Question 10

What does a diamond represent?

Answer 10

Relationship among entites

ER Model is used to model the logical view of the system from a data perspective
which consists of these symbols:
● Rectangles: Rectangles represent Entities in the ER Model.
● Ellipses: Ellipses represent Attributes in the ER Model.
● Diamond: Diamonds represent Relationships among Entities.
● Lines: Lines represent attributes to entities and entity sets with other
relationship types.
● Double Ellipse: Double Ellipses represent Multi-Valued Attributes.
● Double Rectangle: Double Rectangle represents a Weak Entity.

Question 11

What does a double ellipse represent?

Answer 11

Multi valued attributes

ER Model is used to model the logical view of the system from a data perspective
which consists of these symbols:
● Rectangles: Rectangles represent Entities in the ER Model.
● Ellipses: Ellipses represent Attributes in the ER Model.
● Diamond: Diamonds represent Relationships among Entities.
● Lines: Lines represent attributes to entities and entity sets with other
relationship types.
● Double Ellipse: Double Ellipses represent Multi-Valued Attributes.
● Double Rectangle: Double Rectangle represents a Weak Entity.

Question 12

What does a double rectangle represent?

Answer 12

Weak entity

ER Model is used to model the logical view of the system from a data perspective
which consists of these symbols:
● Rectangles: Rectangles represent Entities in the ER Model.
● Ellipses: Ellipses represent Attributes in the ER Model.
● Diamond: Diamonds represent Relationships among Entities.
● Lines: Lines represent attributes to entities and entity sets with other
relationship types.
● Double Ellipse: Double Ellipses represent Multi-Valued Attributes.
● Double Rectangle: Double Rectangle represents a Weak Entity.

Question 13

What are attributes?

Answer 13

Attributes are the properties that define the entity type. For example, Roll_No, Name, DOB, Age, Address, and Mobile_No are the attributes that define entity type Student. In ER diagram, the attribute is represented by an oval.

1. Key Attribute
   The attribute which uniquely identifies each entity in the entity set is called the key attribute. For example, Roll_No will be unique for each student. In ER diagram, the key attribute is represented by an oval with underlying lines.
2. Composite Attribute
   An attribute composed of many other attributes is called a composite attribute. For example, the Address attribute of the student Entity type consists of Street, City, State, and Country. In ER diagram, the composite attribute is represented by an oval comprising of ovals.
3. Multivalued Attribute
   An attribute consisting of more than one value for a given entity. For example, Phone_No (can be more than one for a given student). In ER diagram, a multivalued attribute is represented by a double oval.
4. Derived Attribute
   An attribute that can be derived from other attributes of the entity type is known as a derived attribute. e.g.; Age (can be derived from DOB). In ER diagram, the derived attribute is represented by a dashed oval.

Question 14

What is a relationship type?

Answer 14

A Relationship Type represents the association between entity types. For example,
‘Enrolled in’ is a relationship type that exists between entity type Student and Course. In ER diagram, the relationship type is represented by a diamond and connecting the entities with lines.

Question 15

Relationship set?

Answer 15

its just multiple relationship types

A Relationship Type represents the association between entity types.

Question 16

What is degree of relationship set?

Answer 16

The number of different entity sets participating in a relationship set is called the degree of a relationship set.

1. Unary Relationship: When there is only ONE entity set participating in a relation, the relationship is called a unary relationship. For example, one person is married to only one person.

Binary Relationship: When there are TWO entities set participating in a relationship, the relationship is called a binary relationship. For example, a Student is enrolled in a Course.

Question 17

Multiplicity of ER Relations

Answer 17

• One to One
  When each entity in each entity set can take part only once in the relationship, the cardinality is one-to-one. Let us assume that a male can marry one female and a female can marry one male. So the relationship will be one-to-one.

*One to many
In one-to-many mapping as well where each entity can be related to more than one relationship and the total number of tables that can be used in this is 2. Let us assume that one surgeon deparment can accomodate many doctors. So the Cardinality will be 1 to M. It means one deparment has many Doctors.

• Many to One
  When entities in one entity set can take part only once in the relationship set and entities in other entity sets can take part more than once in the relationship set, cardinality is many to one. Let us assume that a student can take only one course but one course can be taken by many students. So the cardinality will be n to 1. It means that for one course there can be n students but for one student, there will be only one course.
• many to many

When entities in all entity sets can take part more than once in the relationship cardinality is many to many. Let us assume that a student can take more than one course and one course can be taken by many students. So the relationship will be many to many.

Question 18

What are participation constraints?

Answer 18

Participation Constraint is applied to the entity participating in the relationship set.

1. Total Participation – Each entity in the entity set must participate in the relationship. If each student must enroll in a course, the participation of students will be total. Total participation is shown by a double line in the ER diagram.
2. Partial Participation – The entity in the entity set may or may NOT participate in the relationship. If some courses are not enrolled by any of the students, the participation in the course will be partial.

Question 19

H

How to draw a ER Diagram?

Answer 19

● The very first step is Identifying all the Entities, and place them in a
Rectangle, and labeling them accordingly.
● The next step is to identify the relationship between them and pace them
accordingly using the Diamond, and make sure that, Relationships are not
connected to each other.
● Attach attributes to the entities properly.
● Remove redundant entities and relationships.
● Add proper colors to highlight the data present in the database.