UNIT -1

What is Database
The database is a collection of inter-related data which is used to retrieve,
insert and delete the data efficiently. It is also used to organize the data in
the form of a table, schema, views, and reports, etc.

For example: The college Database organizes the data about the admin,
staff, students and faculty etc.

Using the database, you can easily retrieve, insert, and delete the
information.

Database Management System

o Database management system is a software which is used to manage the
database. For example: MySQL, Oracle, etc are a very popular commercial
database which is used in different applications.
o DBMS provides an interface to perform various operations like database
creation, storing data in it, updating data, creating a table in the database
and a lot more.
o It provides protection and security to the database. In the case of multiple
users, it also maintains data consistency.

DBMS allows users the following tasks:

o Data Definition: It is used for creation, modification, and removal of

definition that defines the organization of data in the database.
o Data Updation: It is used for the insertion, modification, and deletion of the
actual data in the database.
o Data Retrieval: It is used to retrieve the data from the database which can
be used by applications for various purposes.
o User Administration: It is used for registering and monitoring users,
maintain data integrity, enforcing data security, dealing with concurrency
control, monitoring performance and recovering information corrupted by
unexpected failure.
Characteristics of DBMS
o It uses a digital repository established on a server to store and manage the
information.
o It can provide a clear and logical view of the process that manipulates data.
o DBMS contains automatic backup and recovery procedures.
o It contains ACID properties which maintain data in a healthy state in case of
failure.
o It can reduce the complex relationship between data.
o It is used to support manipulation and processing of data.
o It is used to provide security of data.
o It can view the database from different viewpoints according to the
requirements of the user.

Advantages of DBMS
o Controls database redundancy: It can control data redundancy because it
stores all the data in one single database file and that recorded data is placed
in the database.
o Data sharing: In DBMS, the authorized users of an organization can share
the data among multiple users.
o Easily Maintenance: It can be easily maintainable due to the centralized
nature of the database system.
o Reduce time: It reduces development time and maintenance need.
o Backup: It provides backup and recovery subsystems which create
automatic backup of data from hardware and software failures and restores
the data if required.
o multiple user interface: It provides different types of user interfaces like
graphical user interfaces, application program interfaces

Disadvantages of DBMS
 o Cost of Hardware and Software: It requires a high speed of data
processor and large memory size to run DBMS software.
o Size: It occupies a large space of disks and large memory to run them
efficiently.
o Complexity: Database system creates additional complexity and
requirements.
o Higher impact of failure: Failure is highly impacted the database because
in most of the organization, all the data stored in a single database and if the
database is damaged due to electric failure or database corruption then the
data may be lost forever.

What is Data?
Data is a collection of a distinct small unit of information. It can be used in a
variety of forms like text, numbers, media, bytes, etc. it can be stored in
pieces of paper or electronic memory, etc.

Word 'Data' is originated from the word 'datum' that means 'single piece of
information.' It is plural of the word datum.

In computing, Data is information that can be translated into a form for

efficient movement and processing. Data is interchangeable.

What is Database?
A database is an organized collection of data, so that it can be easily
accessed and managed

You can organize data into tables, rows, columns, and index it to make it
easier to find relevant information.

Database handlers create a database in such a way that only one set of
software program provides access of data to all the users.

The main purpose of the database is to operate a large amount of

information by storing, retrieving, and managing data.

There are many dynamic websites on the World Wide Web nowadays
which are handled through databases. For example, a model that checks the
availability of rooms in a hotel. It is an example of a dynamic website that
uses a database.
There are many databases available like MySQL, Sybase, Oracle,
MongoDB, Informix, PostgreSQL, SQL Server, etc.

Modern databases are managed by the database management system

(DBMS).

SQL or Structured Query Language is used to operate on the data stored in a

database. SQL depends on relational algebra and tuple relational calculus.

A cylindrical structure is used to display the image of a database

Evolution of Databases
The database has completed more than 50 years of journey of its evolution
from flat-file system to relational and objects relational systems. It has gone
through several generations.

The Evolution
File-Based

1968 was the year when File-Based database were introduced. In file-based
databases, data was maintained in a flat file. Though files have many
advantages, there are several limitations.

One of the major advantages is that the file system has various access
methods, e.g., sequential, indexed, and random.

It requires extensive programming in a third-generation language such as

COBOL, BASIC.

Hierarchical Data Model

1968-1980 was the era of the Hierarchical Database. Prominent hierarchical

database model was IBM's first DBMS. It was called IMS (Information
Management System).

In this model, files are related in a parent/child manner.

Below diagram represents Hierarchical Data Model. Small circle represents objects.

Network data model

Charles Bachman developed the first DBMS at Honeywell called Integrated Data Store
(IDS). It was developed in the early 1960s, but it was standardized in 1971 by the CODASYL
group (Conference on Data Systems Languages).

In this model, files are related as owners and members, like to the common network model.

Network data model identified the following components:

o Network schema (Database organization)

o Sub-schema (views of database per user)
o Data management language (procedural)

This model also had some limitations like system complexity and difficult to design and
maintain.

Relational Database

1970 - Present: It is the era of Relational Database and Database Management. In 1970,
the relational model was proposed by E.F. Codd.

Relational database model has two main terminologies called instance and schema.

The instance is a table with rows or columns

Schema specifies the structure like name of the relation, type of each column and name.

This model uses some mathematical concept like set theory and predicate
logic.
Like file system, this model also had some limitations like complex
implementation, lack structural independence, can't easily handle a many-
many relationship, etc.

The first internet database application had been created in 1995.

During the era of the relational database, many more models had introduced
like object-oriented model, object-relational model, etc.

Cloud database
Cloud database facilitates you to store, manage, and retrieve their
structured, unstructured data via a cloud platform. This data is accessible
over the Internet. Cloud databases are also called a database as service
(DBaaS) because they are offered as a managed service.

Some best cloud options are:

o AWS (Amazon Web Services)

o Snowflake Computing
o Oracle Database Cloud Services
o Microsoft SQL server
o Google cloud spanner
Advantages of cloud database

Lower costs

Generally, company provider does not have to invest in databases. It can

maintain and support one or more data centers.

Automated

Cloud databases are enriched with a variety of automated processes such as

recovery, failover, and auto-scaling.

Increased accessibility

You can access your cloud-based database from any location, anytime. All
you need is just an internet connection.

Data Independence

o Data independence can be explained using the three-schema

architecture.
o Data independence refers characteristic of being able to modify the
schema at one level of the database system without altering the
schema at the next higher level.

There are two types of data independence:

1. Logical Data Independence

o Logical data independence refers characteristic of being able to

change the conceptual schema without having to change the external
schema.
o Logical data independence is used to separate the external level from
the conceptual view.
o If we do any changes in the conceptual view of the data, then the user
view of the data would not be affected.
o Logical data independence occurs at the user interface level.

2. Physical Data Independence

 o Physical data independence can be defined as the capacity to change
the internal schema without having to change the conceptual schema.
o If we do any changes in the storage size of the database system
server, then the Conceptual structure of the database will not be
affected.
o Physical data independence is used to separate conceptual levels from
the internal levels.
o Physical data independence occurs at the logical interface level.

Fig: Data Independence

Database Languages in DBMS

o A DBMS has appropriate languages and interfaces to express database

queries and updates.
o Database languages can be used to read, store and update the data in
the database.

Types of Database Languages

1. Data Definition Language (DDL)

o DDL stands for Data Definition Language. It is used to define database

structure or pattern.
o It is used to create schema, tables, indexes, constraints, etc. in the
database.
o Using the DDL statements, you can create the skeleton of the
database.
o Data definition language is used to store the information of metadata
like the number of tables and schemas, their names, indexes, columns
in each table, constraints, etc.

Here are some tasks that come under DDL:

o Create: It is used to create objects in the database.

o Alter: It is used to alter the structure of the database.
o Drop: It is used to delete objects from the database.
o Truncate: It is used to remove all records from a table.
o Rename: It is used to rename an object.
o Comment: It is used to comment on the data dictionary.

These commands are used to update the database schema that's why they
come under Data definition language.
2. Data Manipulation Language (DML)
DML stands for Data Manipulation Language. It is used for accessing and
manipulating data in a database. It handles user requests.

Here are some tasks that come under DML:

o Select: It is used to retrieve data from a database.

o Insert: It is used to insert data into a table.
o Update: It is used to update existing data within a table.
o Delete: It is used to delete all records from a table.
o Merge: It performs UPSERT operation, i.e., insert or update operations.
o Call: It is used to call a structured query language or a Java
subprogram.
o Explain Plan: It has the parameter of explaining data.
o Lock Table: It controls concurrency.

3. Data Control Language (DCL)

o DCL stands for Data Control Language. It is used to retrieve the stored
or saved data.
o The DCL execution is transactional. It also has rollback parameters.

(But in Oracle database, the execution of data control language does

not have the feature of rolling back.)

Here are some tasks that come under DCL:

o Grant: It is used to give user access privileges to a database.

o Revoke: It is used to take back permissions from the user.

There are the following operations which have the authorization of Revoke:

CONNECT, INSERT, USAGE, EXECUTE, DELETE, UPDATE and SELECT.

4. Transaction Control Language (TCL)

TCL is used to run the changes made by the DML statement. TCL can be
grouped into a logical transaction.

Here are some tasks that come under TCL:

o Commit: It is used to save the transaction on the database.

o Rollback: It is used to restore the database to original since the last
Commit.

ACID Properties in DBMS

DBMS is the management of data that should remain integrated when any
changes are done in it. It is because if the integrity of the data is affected,
whole data will get disturbed and corrupted. Therefore, to maintain the
integrity of the data, there are four properties described in the database
management system, which are known as the ACID properties. The ACID
properties are meant for the transaction that goes through a different group
of tasks, and there we come to see the role of the ACID properties.

In this section, we will learn and understand about the ACID properties. We
will learn what these properties stand for and what does each property is
used for. We will also understand the ACID properties with the help of some
examples.

ACID Properties
The expansion of the term ACID defines for:

1) Atomicity
The term atomicity defines that the data remains atomic. It means if any
operation is performed on the data, either it should be performed or
executed completely or should not be executed at all. It further means that
the operation should not break in between or execute partially. In the case of
executing operations on the transaction, the operation should be completely
executed and not partially.

Example: If Remo has account A having $30 in his account from which he
wishes to send $10 to Sheero's account, which is B. In account B, a sum of $
100 is already present. When $10 will be transferred to account B, the sum
will become $110. Now, there will be two operations that will take place. One
is the amount of $10 that Remo wants to transfer will be debited from his
account A, and the same amount will get credited to account B, i.e., into
Sheero's account. Now, what happens - the first operation of debit executes
successfully, but the credit operation, however, fails. Thus, in Remo's
account A, the value becomes $20, and to that of Sheero's account, it
remains $100 as it was previously present.

In the above diagram, it can be seen that after crediting $10, the amount is
still $100 in account B. So, it is not an atomic transaction.

The below image shows that both debit and credit operations are done
successfully. Thus the transaction is atomic.
2) Consistency
The word consistency means that the value should remain preserved always. In DBMS, the
integrity of the data should be maintained, which means if a change in the database is made, it
should remain preserved always. In the case of transactions, the integrity of the data is very
essential so that the database remains consistent before and after the transaction. The data should
always be correct.

Example:

In the above figure, there are three accounts, A, B, and C, where A is making a transaction T one
by one to both B & C. There are two operations that take place, i.e., Debit and Credit. Account A
firstly debits $50 to account B, and the amount in account A is read $300 by B before the
transaction. After the successful transaction T, the available amount in B becomes $150. Now, A
debits $20 to account C, and that time, the value read by C is $250 (that is correct as a debit of
$50 has been successfully done to B). The debit and credit operation from account A to C has
been done successfully. We can see that the transaction is done successfully, and the value is also
read correctly. Thus, the data is consistent. In case the value read by B and C is $300, which
means that data is inconsistent because when the debit operation executes, it will not be
consistent.
3) Isolation
The term 'isolation' means separation. In DBMS, Isolation is the property of a
database where no data should affect the other one and may occur
concurrently. In short, the operation on one database should begin when the
operation on the first database gets complete. It means if two operations are
being performed on two different databases, they may not affect the value of
one another. In the case of transactions, when two or more transactions
occur simultaneously, the consistency should remain maintained. Any
changes that occur in any particular transaction will not be seen by other
transactions until the change is not committed in the memory.

Example: If two operations are concurrently running on two different

accounts, then the value of both accounts should not get affected. The value
should remain persistent. As you can see in the below diagram, account A is
making T1 and T2 transactions to account B and C, but both are executing
independently without affecting each other. It is known as Isolation.

4) Durability
Durability ensures the permanency of something. In DBMS, the term
durability ensures that the data after the successful execution of the
operation becomes permanent in the database. The durability of the data
should be so perfect that even if the system fails or leads to a crash, the
database still survives. However, if gets lost, it becomes the responsibility of
the recovery manager for ensuring the durability of the database. For
committing the values, the COMMIT command must be used every time we
make changes.

Therefore, the ACID property of DBMS plays a vital role in maintaining the
consistency and availability of data in the database.
Thus, it was a precise introduction of ACID properties in DBMS. We have
discussed these properties in the transaction section also.

ER (Entity Relationship) Diagram in DBMS

o ER model stands for an Entity-Relationship model. It is a high-level data

model. This model is used to define the data elements and relationship for a
specified system.
o It develops a conceptual design for the database. It also develops a very
simple and easy to design view of data.
o In ER modeling, the database structure is portrayed as a diagram called an
entity-relationship diagram.

For example, Suppose we design a school database. In this database, the

student will be an entity with attributes like address, name, id, age, etc. The
address can be another entity with attributes like city, street name, pin code,
etc and there will be a relationship between them.

Component of ER Diagram
1. Entity:
An entity may be any object, class, person or place. In the ER diagram, an
entity can be represented as rectangles.

Consider an organization as an example- manager, product, employee,

department etc. can be taken as an entity.

a. Weak Entity

An entity that depends on another entity called a weak entity. The weak
entity doesn't contain any key attribute of its own. The weak entity is
represented by a double rectangle.

2. Attribute
The attribute is used to describe the property of an entity. Eclipse is used to
represent an attribute.

For example, id, age, contact number, name, etc. can be attributes of a
student.

a. Key Attribute

The key attribute is used to represent the main characteristics of an entity. It represents a primary
key. The key attribute is represented by an ellipse with the text underlined.

b. Composite Attribute

An attribute that composed of many other attributes is known as a composite attribute. The
composite attribute is represented by an ellipse, and those ellipses are connected with an ellipse.

c. Multivalued Attribute
An attribute can have more than one value. These attributes are known as a multivalued attribute.
The double oval is used to represent multivalued attribute.

For example, a student can have more than one phone number.

d. Derived Attribute

An attribute that can be derived from other attribute is known as a derived attribute. It can be
represented by a dashed ellipse.

For example, A person's age changes over time and can be derived from
another attribute like Date of birth.
3. Relationship
A relationship is used to describe the relation between entities. Diamond or
rhombus is used to represent the relationship.

Types of relationship are as follows:

a. One-to-One Relationship

When only one instance of an entity is associated with the relationship, then
it is known as one to one relationship.

For example, A female can marry to one male, and a male can marry to
one female.

b. One-to-many relationship

When only one instance of the entity on the left, and more than one instance
of an entity on the right associates with the relationship then this is known
as a one-to-many relationship.

For example, Scientist can invent many inventions, but the invention is
done by the only specific scientist.

Notation of ER diagram
Database can be represented using the notations. In ER diagram, many
notations are used to express the cardinality. These notations are as follows:
Fig: Notations of ER diagram

c. Many-to-one relationship

When more than one instance of the entity on the left, and only one instance
of an entity on the right associates with the relationship then it is known as a
many-to-one relationship.

For example, Student enrolls for only one course, but a course can have
many students.
d. Many-to-many relationship

When more than one instance of the entity on the left, and more than one
instance of an entity on the right associates with the relationship then it is
known as a many-to-many relationship.

For example, Employee can assign by many projects and project can have
many employees.

Extended Entity-Relationship (EE-R) Model

EER is a high-level data model that incorporates the extensions to the
original ER model. Enhanced ERD are high level models that represent the
requirements and complexities of complex database.
In addition to ER model concepts EE-R includes −

 Subclasses and Super classes.

 Specialization and Generalization.
 Category or union type.
 Aggregation.
These concepts are used to create EE-R diagrams.

Subclasses and Super class

Super class is an entity that can be divided into further subtype.
For example − consider Shape super class.
Super class shape has sub groups: Triangle, Square and Circle.
Sub classes are the group of entities with some unique attributes.Sub class
inherits the properties and attributes from super class.

Specialization and Generalization

Generalization is a process of generalizing an entity which contains
generalized attributes or properties of generalized entities.

It is a Bottom up process i.e. consider we have 3 sub entities Car, Truck and
Motorcycle. Now these three entities can be generalized into one super class
named as Vehicle.
Specialization is a process of identifying subsets of an entity that share
some different characteristic. It is a top down approach in which one entity
is broken down into low level entity.
In above example Vehicle entity can be a Car, Truck or Motorcycle.

Category or Union
Relationship of one super or sub class with more than one super class.
Owner is the subset of two super class: Vehicle and House.

Aggregation
Represents relationship between a whole object and its component.

Consider a ternary relationship Works_On between Employee, Branch and

Manager. Now the best way to model this situation is to use aggregation, So,
the relationship-set, Works_On is a higher level entity-set. Such an entity-set
is treated in the same manner as any other entity-set. We can create a
binary relationship, Manager, between Works_On and Manager to represent
who manages what tasks.

Mapping Constraints

o A mapping constraint is a data constraint that expresses the number of

entities to which another entity can be related via a relationship set.
o It is most useful in describing the relationship sets that involve more than two
entity sets.
o For binary relationship set R on an entity set A and B, there are four possible
mapping cardinalities. These are as follows:
1. One to one (1:1)
2. One to many (1:M)
 3. Many to one (M:1)
4. Many to many (M:M)

One-to-one
In one-to-one mapping, an entity in E1 is associated with at most one entity
in E2, and an entity in E2 is associated with at most one entity in E1.

Many-to-many
In many-to-many mapping, an entity in E1 is associated with any number of
entities in E2, and an entity in E2 is associated with any number of entities in
E1.

One-to-many
In one-to-many mapping, an entity in E1 is associated with any number of
entities in E2, and an entity in E2 is associated with at most one entity in E1.
Many-to-one
In one-to-many mapping, an entity in E1 is associated with at most one
entity in E2, and an entity in E2 is associated with any number of entities in
E1.

Keys
o Keys play an important role in the relational database.
o It is used to uniquely identify any record or row of data from the table. It is
also used to establish and identify relationships between tables.

For example, ID is used as a key in the Student table because it is unique

for each student. In the PERSON table, passport_number, license_number,
SSN are keys since they are unique for each person.
Types of keys:

1. Primary key
o It is the first key used to identify one and only one instance of an entity
uniquely. An entity can contain multiple keys, as we saw in the PERSON table.
The key which is most suitable from those lists becomes a primary key.
o In the EMPLOYEE table, ID can be the primary key since it is unique for each
employee. In the EMPLOYEE table, we can even select License_Number and
Passport_Number as primary keys since they are also unique.
o For each entity, the primary key selection is based on requirements and
developers.
2. Candidate key
o A candidate key is an attribute or set of attributes that can uniquely identify a
tuple.
o Except for the primary key, the remaining attributes are considered a
candidate key. The candidate keys are as strong as the primary key.

For example: In the EMPLOYEE table, id is best suited for the primary key.
The rest of the attributes, like SSN, Passport_Number, License_Number, etc.,
are considered a candidate key.

3. Super Key
Super key is an attribute set that can uniquely identify a tuple. A super key is
a superset of a candidate key.
For example: In the above EMPLOYEE table, for(EMPLOEE_ID,
EMPLOYEE_NAME), the name of two employees can be the same, but their
EMPLYEE_ID can't be the same. Hence, this combination can also be a key.

The super key would be EMPLOYEE-ID (EMPLOYEE_ID, EMPLOYEE-NAME), etc.

4. Foreign key
o Foreign keys are the column of the table used to point to the primary key of
another table.
o Every employee works in a specific department in a company, and employee
and department are two different entities. So we can't store the department's
information in the employee table. That's why we link these two tables
through the primary key of one table.
o We add the primary key of the DEPARTMENT table, Department_Id, as a new
attribute in the EMPLOYEE table.
o In the EMPLOYEE table, Department_Id is the foreign key, and both the tables
are related.
5. Alternate key
There may be one or more attributes or a combination of attributes that
uniquely identify each tuple in a relation. These attributes or combinations of
the attributes are called the candidate keys. One key is chosen as the
primary key from these candidate keys, and the remaining candidate key, if
it exists, is termed the alternate key. In other words, the total number of
the alternate keys is the total number of candidate keys minus the primary
key. The alternate key may or may not exist. If there is only one candidate
key in a relation, it does not have an alternate key.

For example, employee relation has two attributes, Employee_Id and

PAN_No, that act as candidate keys. In this relation, Employee_Id is chosen
as the primary key, so the other candidate key, PAN_No, acts as the
Alternate key.

6. Composite key
Whenever a primary key consists of more than one attribute, it is known as a
composite key. This key is also known as Concatenated Key.
For example, in employee relations, we assume that an employee may be
assigned multiple roles, and an employee may work on multiple projects
simultaneously. So the primary key will be composed of all three attributes,
namely Emp_ID, Emp_role, and Proj_ID in combination. So these attributes
act as a composite key since the primary key comprises more than one
attribute.

Generalization
o Generalization is like a bottom-up approach in which two or more entities of
lower level combine to form a higher level entity if they have some attributes
in common.
o In generalization, an entity of a higher level can also combine with the
entities of the lower level to form a further higher level entity.
o Generalization is more like subclass and superclass system, but the only
difference is the approach. Generalization uses the bottom-up approach.
o In generalization, entities are combined to form a more generalized entity,
i.e., subclasses are combined to make a superclass.

For example, Faculty and Student entities can be generalized and create a
higher level entity Person.

Specialization
o Specialization is a top-down approach, and it is opposite to Generalization. In
specialization, one higher level entity can be broken down into two lower
level entities.
 o Specialization is used to identify the subset of an entity set that shares some
distinguishing characteristics.
o Normally, the superclass is defined first, the subclass and its related
attributes are defined next, and relationship set are then added.

For example: In an Employee management system, EMPLOYEE entity can

be specialized as TESTER or DEVELOPER based on what role they play in the
company.

Aggregation
In aggregation, the relation between two entities is treated as a single entity.
In aggregation, relationship with its corresponding entities is aggregated into
a higher level entity.

For example: Center entity offers the Course entity act as a single entity in
the relationship which is in a relationship with another entity visitor. In the
real world, if a visitor visits a coaching center then he will never enquiry
about the Course only or just about the Center instead he will ask the
enquiry about both.
Reduction of ER diagram to Table
The database can be represented using the notations, and these notations
can be reduced to a collection of tables.

In the database, every entity set or relationship set can be represented in

tabular form.

The ER diagram is given below:

There are some points for converting the ER diagram to the table:

o Entity type becomes a table.

In the given ER diagram, LECTURE, STUDENT, SUBJECT and COURSE forms
individual tables.

o All single-valued attribute becomes a column for the table.

In the STUDENT entity, STUDENT_NAME and STUDENT_ID form the column of

STUDENT table. Similarly, COURSE_NAME and COURSE_ID form the column of
COURSE table and so on.

o A key attribute of the entity type represented by the primary

key.

In the given ER diagram, COURSE_ID, STUDENT_ID, SUBJECT_ID, and

LECTURE_ID are the key attribute of the entity.

o The multivalued attribute is represented by a separate table.

In the student table, a hobby is a multivalued attribute. So it is not possible

to represent multiple values in a single column of STUDENT table. Hence we
create a table STUD_HOBBY with column name STUDENT_ID and HOBBY.
Using both the column, we create a composite key.

o Composite attribute represented by components.

In the given ER diagram, student address is a composite attribute. It contains

CITY, PIN, DOOR#, STREET, and STATE. In the STUDENT table, these
attributes can merge as an individual column.

o Derived attributes are not considered in the table.

In the STUDENT table, Age is the derived attribute. It can be calculated at

any point of time by calculating the difference between current date and
Date of Birth.

Using these rules, you can convert the ER diagram to tables and columns
and assign the mapping between the tables. Table structure for the given ER
diagram is as below:
Figure: Table structure

Relationship of higher degree

The degree of relationship can be defined as the number of occurrences in
one entity that is associated with the number of occurrences in another
entity.

There is the three degree of relationship:

1. One-to-one (1:1)
2. One-to-many (1:M)
3. Many-to-many (M:N)

1. One-to-one
o In a one-to-one relationship, one occurrence of an entity relates to only one
occurrence in another entity.
o A one-to-one relationship rarely exists in practice.
o For example: if an employee is allocated a company car then that car can
only be driven by that employee.
o Therefore, employee and company car have a one-to-one relationship.
2. One-to-many
o In a one-to-many relationship, one occurrence in an entity relates to many
occurrences in another entity.
o For example: An employee works in one department, but a department has
many employees.
o Therefore, department and employee have a one-to-many relationship.

3. Many-to-many
o In a many-to-many relationship, many occurrences in an entity relate to
many occurrences in another entity.
o Same as a one-to-one relationship, the many-to-many relationship rarely
exists in practice.
o For example: At the same time, an employee can work on several projects,
and a project has a team of many employees.
o Therefore, employee and project have a many-to-many relationship.