DBMS Notes.pdf

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.
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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.
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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.
DATABASE
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.
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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.
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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.
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Like file system, this model also had some limitations like complex implementation,
lack structural independence, can't easily handle a many-many relationship, etc.
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)
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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.
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.
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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.
NoSQL Database
A NoSQL database is an approach to design such databases that can accommodate
a wide variety of data models. NoSQL stands for "not only SQL." It is an alternative
to traditional relational databases in which data is placed in tables, and data schema
is perfectly designed before the database is built.
NoSQL databases are useful for a large set of distributed data.
Some examples of NoSQL database system with their category are:
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o MongoDB, CouchDB, Cloudant (Document-based)
o Memcached, Redis, Coherence (key-value store)
o HBase, Big Table, Accumulo (Tabular)
Advantage of NoSQL
High Scalability
NoSQL can handle an extensive amount of data because of scalability. If the data
grows, NoSQL database scale it to handle that data in an efficient manner.
High Availability
NoSQL supports auto replication. Auto replication makes it highly available
because, in case of any failure, data replicates itself to the previous consistent state.
Disadvantage of NoSQL
Open source
NoSQL is an open-source database, so there is no reliable standard for NoSQL yet.
Management challenge
Data management in NoSQL is much more complicated than relational databases. It
is very challenging to install and even more hectic to manage daily.
GUI is not available
GUI tools for NoSQL database are not easily available in the market.
Backup
Backup is a great weak point for NoSQL databases. Some databases, like MongoDB,
have no powerful approaches for data backup.
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The Object-Oriented Databases
The object-oriented databases contain data in the form of object and classes. Objects
are the real-world entity, and types are the collection of objects. An object-oriented
database is a combination of relational model features with objects oriented
principles. It is an alternative implementation to that of the relational model.
Object-oriented databases hold the rules of object-oriented programming. An object-
oriented database management system is a hybrid application.
The object-oriented database model contains the following properties.
Object-oriented programming properties
o Objects
o Classes
o Inheritance
o Polymorphism
o Encapsulation
Relational database properties
o Atomicity
o Consistency
o Integrity
o Durability
o Concurrency
o Query processing
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Graph Databases
o A graph database is a NoSQL database. It is a graphical representation of data.
It contains nodes and edges. A node represents an entity, and each edge
represents a relationship between two edges. Every node in a graph database
represents a unique identifier.
o Graph databases are beneficial for searching the relationship between data
because they highlight the relationship between relevant data.
o
o Graph databases are very useful when the database contains a complex
relationship and dynamic schema.
o It is mostly used in supply chain management, identifying the source of IP
telephony.
o
DBMS (Data Base Management System)
o Database management System is software which is used to store and retrieve
the database. For example, Oracle, MySQL, etc.; these are some popular
DBMS tools.
o DBMS provides the interface to perform the various operations like creation,
deletion, modification, etc.
o DBMS allows the user to create their databases as per their requirement.
o DBMS accepts the request from the application and provides specific data
through the operating system.
o DBMS contains the group of programs which acts according to the user
instruction.
o It provides security to the database.
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Advantage of DBMS
o Controls redundancy
o It stores all the data in a single database file, so it can control data redundancy.
o Data sharing
o An authorized user can share the data among multiple users.
o Backup
o It providesBackup and recovery subsystem. This recovery system creates
automatic data from system failure and restores data if required.
o Multiple user interfaces
o It provides a different type of user interfaces like GUI, application interfaces.
Disadvantage of DBMS
Size
It occupies large disk space and large memory to run efficiently.
Cost
DBMS requires a high-speed data processor and larger memory to run DBMS
software, so it is costly.
Complexity
DBMS creates additional complexity and requirements.
RDBMS (Relational Database Management System)
The word RDBMS is termed as 'Relational Database Management System.' It is
represented as a table that contains rows and column.
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RDBMS is based on the Relational model; it was introduced by E. F. Codd.
A relational database contains the following components:
o Table
o Record/ Tuple
o Field/Column name /Attribute
o Instance
o Schema
o Keys
An RDBMS is a tabular DBMS that maintains the security, integrity, accuracy, and
consistency of the data.
Types of Databases
There are various types of databases used for storing different varieties of data:
1) Centralized Database
It is the type of database that stores data at a centralized database system. It comforts
the users to access the stored data from different locations through several
applications. These applications contain the authentication process to let users access
data securely. An example of a Centralized database can be Central Library that
carries a central database of each library in a college/university.
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Advantages of Centralized Database
o It has decreased the risk of data management, i.e., manipulation of data will
not affect the core data.
o Data consistency is maintained as it manages data in a central repository.
o It provides better data quality, which enables organizations to establish data
standards.
o It is less costly because fewer vendors are required to handle the data sets.
Disadvantages of Centralized Database
o The size of the centralized database is large, which increases the response time
for fetching the data.
o It is not easy to update such an extensive database system.
o If any server failure occurs, entire data will be lost, which could be a huge loss.
o
2) Distributed Database
Unlike a centralized database system, in distributed systems, data is distributed
among different database systems of an organization. These database systems are
connected via communication links. Such links help the end-users to access the data
easily. Examples of the Distributed database are Apache Cassandra, HBase, Ignite,
etc.
We can further divide a distributed database system into:
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o Homogeneous DDB: Those database systems which execute on the same
operating system and use the same application process and carry the same
hardware devices.
o Heterogeneous DDB: Those database systems which execute on different
operating systems under different application procedures, and carries different
hardware devices.
Advantages of Distributed Database
o Modular development is possible in a distributed database, i.e., the system can
be expanded by including new computers and connecting them to the
distributed system.
o One server failure will not affect the entire data set.
3) Relational Database
This database is based on the relational data model, which stores data in the form of
rows(tuple) and columns(attributes), and together forms a table(relation). A
relational database uses SQL for storing, manipulating, as well as maintaining the
data. E.F. Codd invented the database in 1970. Each table in the database carries a
key that makes the data unique from others. Examples of Relational databases are
MySQL, Microsoft SQL Server, Oracle, etc.
Properties of Relational Database
There are following four commonly known properties of a relational model known
as ACID properties, where:
A means Atomicity: This ensures the data operation will complete either with
success or with failure. It follows the 'all or nothing' strategy. For example, a
transaction will either be committed or will abort.
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C means Consistency: If we perform any operation over the data, its value before
and after the operation should be preserved. For example, the account balance before
and after the transaction should be correct, i.e., it should remain conserved.
I means Isolation: There can be concurrent users for accessing data at the same time
from the database. Thus, isolation between the data should remain isolated. For
example, when multiple transactions occur at the same time, one transaction effects
should not be visible to the other transactions in the database.
D means Durability: It ensures that once it completes the operation and commits
the data, data changes should remain permanent.
4) NoSQL Database
Non-SQL/Not Only SQL is a type of database that is used for storing a wide range
of data sets. It is not a relational database as it stores data not only in tabular form
but in several different ways. It came into existence when the demand for building
modern applications increased. Thus, NoSQL presented a wide variety of database
technologies in response to the demands. We can further divide a NoSQL database
into the following four types:
a. Key-value storage: It is the simplest type of database storage where it stores
every single item as a key (or attribute name) holding its value, together.
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b. Document-oriented Database: A type of database used to store data as JSON-
like document. It helps developers in storing data by using the same document-
model format as used in the application code.
c. Graph Databases: It is used for storing vast amounts of data in a graph-like
structure. Most commonly, social networking websites use the graph database.
d. Wide-column stores: It is similar to the data represented in relational
databases. Here, data is stored in large columns together, instead of storing in
rows.
Advantages of NoSQL Database
o It enables good productivity in the application development as it is not required
to store data in a structured format.
o It is a better option for managing and handling large data sets.
o It provides high scalability.
o Users can quickly access data from the database through key-value.
5) Cloud Database
A type of database where data is stored in a virtual environment and executes over
the cloud computing platform. It provides users with various cloud computing
services (SaaS, PaaS, IaaS, etc.) for accessing the database. There are numerous
cloud platforms, but the best options are:
o Amazon Web Services(AWS)
o Microsoft Azure
o Kamatera
o PhonixNAP
o ScienceSoft
o Google Cloud SQL, etc.
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6) Object-oriented Databases
The type of database that uses the object-based data model approach for storing data
in the database system. The data is represented and stored as objects which are
similar to the objects used in the object-oriented programming language.
7) Hierarchical Databases
It is the type of database that stores data in the form of parent-children relationship
nodes. Here, it organizes data in a tree-like structure.
Data get stored in the form of records that are connected via links. Each child record
in the tree will contain only one parent. On the other hand, each parent record can
have multiple child records.
8) Network Databases
It is the database that typically follows the network data model. Here, the
representation of data is in the form of nodes connected via links between them.
Unlike the hierarchical database, it allows each record to have multiple children and
parent nodes to form a generalized graph structure.
9) Personal Database
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Collecting and storing data on the user's system defines a Personal Database. This
database is basically designed for a single user.
Advantage of Personal Database
o It is simple and easy to handle.
o It occupies less storage space as it is small in size.
10) Operational Database
The type of database which creates and updates the database in real-time. It is
basically designed for executing and handling the daily data operations in several
businesses. For example, An organization uses operational databases for managing
per day transactions.
11) Enterprise Database
Large organizations or enterprises use this database for managing a massive amount
of data. It helps organizations to increase and improve their efficiency. Such a
database allows simultaneous access to users.
Advantages of Enterprise Database:
o Multi processes are supportable over the Enterprise database.
o It allows executing parallel queries on the system.
What is RDBMS
RDBMS stands for Relational Database Management Systems..
All modern database management systems like SQL, MS SQL Server, IBM DB2,
ORACLE, My-SQL and Microsoft Access are based on RDBMS.
It is called Relational Data Base Management System (RDBMS) because it is based
on relational model introduced by E.F. Codd.
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How it works
Data is represented in terms of tuples (rows) in RDBMS.
Relational database is most commonly used database. It contains number of tables
and each table has its own primary key.
Due to a collection of organized set of tables, data can be accessed easily in RDBMS.
Brief History of RDBMS
During 1970 to 1972, E.F. Codd published a paper to propose the use of relational
database model.
RDBMS is originally based on that E.F. Codd's relational model invention.
What is table
The RDBMS database uses tables to store data. A table is a collection of related data
entries and contains rows and columns to store data.
A table is the simplest example of data storage in RDBMS.
Let's see the example of student table.
ID Name AGE COURSE
1 Ajeet 24 B.Tech
2 aryan 20 C.A
3 Mahesh 21 BCA
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4 Ratan 22 MCA
5 Vimal 26 BSC
What is field
Field is a smaller entity of the table which contains specific information about every
record in the table. In the above example, the field in the student table consist of id,
name, age, course.
What is row or record
A row of a table is also called record. It contains the specific information of each
individual entry in the table. It is a horizontal entity in the table. For example: The
above table contains 5 records.
Let's see one record/row in the table.
1 Ajeet 24 B.Tech
What is column
A column is a vertical entity in the table which contains all information associated
with a specific field in a table. For example: "name" is a column in the above table
which contains all information about student's name.
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Ajeet
Aryan
Mahesh
Ratan
Vimal
NULL Values
The NULL value of the table specifies that the field has been left blank during record
creation. It is totally different from the value filled with zero or a field that contains
space.
Data Integrity
There are the following categories of data integrity exist with each RDBMS:
Entity integrity: It specifies that there should be no duplicate rows in a table.
Domain integrity: It enforces valid entries for a given column by restricting the
type, the format, or the range of values.
Referential integrity: It specifies that rows cannot be deleted, which are used by
other records.
User-defined integrity: It enforces some specific business rules that are defined by
users. These rules are different from entity, domain or referential integrity.
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Difference between DBMS and RDBMS
Although DBMS and RDBMS both are used to store information in physical
database but there are some remarkable differences between them.
The main differences between DBMS and RDBMS are given below:
No. DBMS RDBMS
1) DBMS applications store
data as file.
RDBMS applications store
data in a tabular form.
2) In DBMS, data is generally stored in
either a hierarchical form or a
navigational form.
In RDBMS, the tables have an identifier calle
primary key and the data values are stored in th
form of tables.
3) Normalization is not present in
DBMS.
Normalization is present in RDBMS.
4) DBMS does not apply any
security with regards to data
manipulation.
RDBMS defines the integrity constraint
for the purpose of ACID
(Atomocity, Consistency, Isolation and Durability
property.
5) DBMS uses file system to store
data, so there will be no relation
between the tables.
in RDBMS, data values are stored in the form o
tables, so a relationship between these data value
will be stored in the form of a table as well.
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6) DBMS has to provide some uniform
methods to access the stored
information.
RDBMS system supports a tabular structure of th
data and a relationship between them to access th
stored information.
7) DBMS does not support
distributed database.
RDBMS supports distributed database.
8) DBMS is meant to be for small
organization and deal with small
data. it supports single user.
RDBMS is designed to handle large
amount of data. it supports multiple users.
9) Examples of DBMS are file
systems, xml etc.
Example of RDBMS are
mysql, postgre, sql server, oracle etc.
After observing the differences between DBMS and RDBMS, you can say that
RDBMS is an extension of DBMS. There are many software products in the market
today who are compatible for both DBMS and RDBMS. Means today a RDBMS
application is DBMS application and vice-versa.
DBMS vs. File System
File System Approach
File based systems were an early attempt to computerize the manual system. It is
also called a traditional based approach in which a decentralized approach was taken
where each department stored and controlled its own data with the help of a data
processing specialist. The main role of a data processing specialist was to create the
necessary computer file structures, and also manage the data within structures and
design some application programs that create reports based on file data.
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In the above figure:
Consider an example of a student's file system. The student file will contain
information regarding the student (i.e. roll no, student name, course etc.). Similarly,
we have a subject file that contains information about the subject and the result file
which contains the information regarding the result.
Some fields are duplicated in more than one file, which leads to data redundancy. So
to overcome this problem, we need to create a centralized system, i.e. DBMS
approach.
DBMS:
A database approach is a well-organized collection of data that are related in a
meaningful way which can be accessed by different users but stored only once in a
system. The various operations performed by the DBMS system are: Insertion,
deletion, selection, sorting etc.
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In the above figure,
In the above figure, duplication of data is reduced due to centralization of data.
There are the following differences between DBMS and File systems:
Basis DBMS Approach File System Approach
Meaning DBMS is a collection of
data. In DBMS, the user is not
required to write the
procedures.
The file system is a collection of
data. In this system, the user
has to write the procedures for
managing the database.
Sharing of data Due to the centralized
approach, data sharing is
easy.
Data is distributed in many files,
and it may be of different
formats, so it isn't easy to share
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data.
Data Abstraction DBMS gives an abstract
view of data that hides the
details.
The file system provides the
detail of the data
representation and storage of data.
Security and Protection
DBMS provides a good
protection mechanism.
It isn't easy to protect a file under
the file system.
Recovery Mechanism
DBMS provides a crash
recovery mechanism, i.e.,
DBMS protects the user from
system failure.
The file system doesn't have a
crash mechanism, i.e., if the
system crashes while entering som
data, then the content of the file
will be lost.
Manipulation
Techniques
DBMS contains a wide
variety of sophisticated
techniques to store and
retrieve the data.
The file system can't efficiently stor
and retrieve the data.
Concurrency
Problems
DBMS takes care of
Concurrent access of data using
some form of locking.
In the File system, concurrent
access has many problems lik
redirecting the file while
deleting some information or
updating some information.
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Where to use Database approach used in large
systems which interrelate many
files.
File system approach used
in large systems which
interrelate many files.
Cost The database system is
expensive to design.
The file system approach is
cheaper to design.
DataRedundancy and
Inconsistency
Due to the centralization
of the database, the
problems of data
redundancy and
inconsistency is controlled.
In this, the files and
application programs are
created by different
programmers so that there
exists a lot of duplication
of data which
may lead to inconsistency.
Structure The database structure is
complex to design.
The file system approach has a
simple structure.
Data
Independence
In this system, Data
Independence exists, and it
can be of two types.
Logical Data Independence
Physical Data Independence
In the File system approach, ther
exists no Data Independence.
Integrity
Constraints
Integrity Constraints are
easy to apply.
Integrity Constraints are
difficult to implement in file system.
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Data Models In the database approach,
3 types of data models exist:
o Hierarchal data models
o Network data models
o Relational data models
In the file system approach,
there is no concept of data
models exists.
Flexibility Changes are often a
necessity to the content of
the data stored in any
system, and these
changes are more easily
with a database approach.
The flexibility of the system is
less as compared to the
DBMS approach.
Examples Oracle, SQL Server, Sybase etc.
Cobol, C++ etc.
DBMS Architecture
o The DBMS design depends upon its architecture. The basic client/server
architecture is used to deal with a large number of PCs, web servers, database
servers and other components that are connected with networks.
o The client/server architecture consists of many PCs and a workstation which
are connected via the network.
o DBMS architecture depends upon how users are connected to the database to
get their request done.
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Types of DBMS Architecture
Database architecture can be seen as a single tier or multi-tier. But logically, database
architecture is of two types like: 2-tier architecture and 3-tier architecture.
1-Tier Architecture
o In this architecture, the database is directly available to the user. It means the
user can directly sit on the DBMS and uses it.
o Any changes done here will directly be done on the database itself. It doesn't
provide a handy tool for end users.
o The 1-Tier architecture is used for development of the local application, where
programmers can directly communicate with the database for the quick
response.
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2-Tier Architecture
o The 2-Tier architecture is same as basic client-server. In the two-tier
architecture, applications on the client end can directly communicate with the
database at the server side. For this interaction, API's like: ODBC, JDBC are
used.
o The user interfaces and application programs are run on the client-side.
o The server side is responsible to provide the functionalities like: query
processing and transaction management.
o To communicate with the DBMS, client-side application establishes a
connection with the server side.
Fig: 2-tier Architecture
3-Tier Architecture
o The 3-Tier architecture contains another layer between the client and server. In
this architecture, client can't directly communicate with the server.
o The application on the client-end interacts with an application server which
further communicates with the database system.
o End user has no idea about the existence of the database beyond the application
server. The database also has no idea about any other user beyond the
application.
o The 3-Tier architecture is used in case of large web application.
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Three schema Architecture
o The three schema architecture is also called ANSI/SPARC architecture or
three-level architecture.
o This framework is used to describe the structure of a specific database system.
o The three schema architecture is also used to separate the user applications and
physical database.
o The three schema architecture contains three-levels. It breaks the database
down into three different categories.
The three-schema architecture is as follows:
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In the above diagram:
o It shows the DBMS architecture.
o Mapping is used to transform the request and response between various
database levels of architecture.
o Mapping is not good for small DBMS because it takes more time.
o In External / Conceptual mapping, it is necessary to transform the request from
external level to conceptual schema.
o In Conceptual / Internal mapping, DBMS transform the request from the
conceptual to internal level.
Objectives of Three schema Architecture
The main objective of three level architecture is to enable multiple users to access
the same data with a personalized view while storing the underlying data only once.
Thus it separates the user's view from the physical structure of the database. This
separation is desirable for the following reasons:
o Different users need different views of the same data.
o The approach in which a particular user needs to see the data may change over
time.
o The users of the database should not worry about the physical implementation
and internal workings of the database such as data compression and encryption
techniques, hashing, optimization of the internal structures etc.
o All users should be able to access the same data according to their
requirements.
o DBA should be able to change the conceptual structure of the database without
affecting the user's
o Internal structure of the database should be unaffected by changes to physical
aspects of the storage.
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1. Internal Level
o The internal level has an internal schema which describes the physical storage
structure of the database.
o The internal schema is also known as a physical schema.
o It uses the physical data model. It is used to define that how the data will be
stored in a block.
o The physical level is used to describe complex low-level data structures in
detail.
The internal level is generally is concerned with the following activities:
o Storage space allocations.
For Example: B-Trees, Hashing etc.
o Access paths.
For Example: Specification of primary and secondary keys, indexes, pointers
and sequencing.
o Data compression and encryption techniques.
o Optimization of internal structures.
o Representation of stored fields.
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2. Conceptual Level
o The conceptual schema describes the design of a database at the conceptual
level. Conceptual level is also known as logical level.
o The conceptual schema describes the structure of the whole database.
o The conceptual level describes what data are to be stored in the database and
also describes what relationship exists among those data.
o In the conceptual level, internal details such as an implementation of the data
structure are hidden.
o Programmers and database administrators work at this level.
3. External Level
o At the external level, a database contains several schemas that sometimes
called as subschema. The subschema is used to describe the different view of
the database.
o An external schema is also known as view schema.
o Each view schema describes the database part that a particular user group is
interested and hides the remaining database from that user group.
o The view schema describes the end user interaction with database systems.
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Mapping between Views
o The three levels of DBMS architecture don't exist independently of each other.
There must be correspondence between the three levels i.e. how they actually
correspond with each other. DBMS is responsible for correspondence between
the three types of schema. This correspondence is called Mapping.
o There are basically two types of mapping in the database architecture:
o Conceptual/ Internal Mapping
o External / Conceptual Mapping
o Conceptual/ Internal Mapping
o The Conceptual/ Internal Mapping lies between the conceptual level and the
internal level. Its role is to define the correspondence between the records and
fields of the conceptual level and files and data structures of the internal level.
o External/ Conceptual Mapping
o The external/Conceptual Mapping lies between the external level and the
Conceptual level. Its role is to define the correspondence between a particular
external and the conceptual view.
Data Models
Data Model is the modeling of the data description, data semantics, and consistency
constraints of the data. It provides the conceptual tools for describing the design of
a database at each level of data abstraction. Therefore, there are following four data
models used for understanding the structure of the database:
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1) Relational Data Model: This type of model designs the data in the form of rows
and columns within a table. Thus, a relational model uses tables for representing data
and in-between relationships. Tables are also called relations. This model was
initially described by Edgar F. Codd, in 1969. The relational data model is the widely
used model which is primarily used by commercial data processing applications.
2) Entity-Relationship Data Model: An ER model is the logical representation of
data as objects and relationships among them. These objects are known as entities,
and relationship is an association among these entities. This model was designed by
Peter Chen and published in 1976 papers. It was widely used in database designing.
A set of attributes describe the entities. For example, student_name, student_id
describes the 'student' entity. A set of the same type of entities is known as an 'Entity
set', and the set of the same type of relationships is known as 'relationship set'.
3) Object-based Data Model: An extension of the ER model with notions of
functions, encapsulation, and object identity, as well. This model supports a rich type
system that includes structured and collection types. Thus, in 1980s, various
database systems following the object-oriented approach were developed. Here, the
objects are nothing but the data carrying its properties.
4) Semistructured Data Model: This type of data model is different from the other
three data models (explained above). The semistructured data model allows the data
specifications at places where the individual data items of the same type may have
different attributes sets. The Extensible Markup Language, also known as XML, is
widely used for representing the semistructured data. Although XML was initially
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designed for including the markup information to the text document, it gains
importance because of its application in the exchange of data.
Data model Schema and Instance
o The data which is stored in the database at a particular moment of time is called
an instance of the database.
o The overall design of a database is called schema.
o A database schema is the skeleton structure of the database. It represents the
logical view of the entire database.
o A schema contains schema objects like table, foreign key, primary key, views,
columns, data types, stored procedure, etc.
o A database schema can be represented by using the visual diagram. That
diagram shows the database objects and relationship with each other.
o A database schema is designed by the database designers to help programmers
whose software will interact with the database. The process of database
creation is called data modeling.
A schema diagram can display only some aspects of a schema like the name of record
type, data type, and constraints. Other aspects can't be specified through the schema
diagram. For example, the given figure neither show the data type of each data item
nor the relationship among various files.
In the database, actual data changes quite frequently. For example, in the given
figure, the database changes whenever we add a new grade or add a student. The
data at a particular moment of time is called the instance of the database.
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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.
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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 Language
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.
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Types of Database Language
1. Data Definition Language
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.
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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 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
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:
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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 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.
o
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.
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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.
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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.
Thus, when the amount loses atomicity, then in the bank systems, this becomes a
huge issue, and so the atomicity is the main focus in the bank systems.
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:
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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.
4) 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.
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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.
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