DBMS - Interview Questions

What is Transaction in DBMS ?

webmaster — Wed, 26 Oct 2022 10:07:01 +0000

If we need one or more database access operations, we have a logical unit of processing called Database Transaction.
In simple, database transactions represent real-world events of any enterprise.
Database is inconsistent during the transaction.
Database is committed only once the state is changed from one consistent state to another.

State of Transactions

Active State

A transaction is said to be in an active state when the execution process begins.
This is the state where read write operations are performed.

Partially Committed

A Transaction is said to be in a partially committed state after the end of a transaction.

Committed State

When a transaction is executed successfully, transaction is said to be in the committed state.
Changes made are recorded in the database permanently.

Failed State

A transaction is considered to be in the failed state when any one of the checks fails, or when the transaction is aborted while it is in the active state.

Terminated State

A transaction is said to be in the terminated state when certain transactions which are leaving the system cannot be restarted.

Normal Forms in DBMS ?

webmaster — Wed, 26 Oct 2022 09:22:43 +0000

There are 4 types of form in Normalization :

First Normal Form(1NF)

If a relation does not contain any multi valued attributes, then the relation is said to be in First Normal Form.
A relation is said to be in First Normal Form if each attributes contains only single value.
Rule of First normal form states that an attribute of a table cannot have multiple values. It can have only single values.

Second Normal Form(2NF)

A table is said to be second normal form on two conditions
Table should be in First Normal Form(1NF).
Any non-prime attribute is not dependent on the proper subset of the candidate key of the table.
Non prime attribute is an attribute which is not a part of any candidate key.

Third Normal Form(3NF)

A design of the table is said to be third normal form(3NF) only if
A Table is in second normal form(2NF)
Transitive functional dependency of non-prime attribute on any super key should be removed.
Non prime attribute is an attribute which is not a part of any candidate key.

Boycee & Codd Normal Form(BCNF)

An advanced version of third normal form is known as Boycee & Codd Normal Forms(BCNF).
BCNF is stricter than third normal form.
A table is said to be in BCNF if it is already in 3NF and every functional dependency should be the super key of the table.

What are the types of databases ?

webmaster — Wed, 26 Oct 2022 09:02:24 +0000

The different types of databases are:

Hierarchical Database

In this database data is organized in a tree like structure.
Data is stored in the form of either top down or bottom up approach.
Data in this database is represented in the form of parent-child relationship.
In this database parents have many children but child can have only single parent.

Network Database

In database children can have more than one parents.
Many to many relationship complexities can be reduced in this database.
Here entities are organized in a graphical format which can be accessed through several paths.

Relational Database

One of the widely used and easy to use database is relational database.
Normalizing of data in the row and column can be acheived using relational database.
These relational models are stored in fixed structures and are manipulated using SQL.

Object Oriented Database

Data in these database are stored in the form of objects.
The structure that displays data within it is are called as classess.
Class is a component of database that displays database as a collection of objects which can store both data members values and operations.

Personal Database

Collecting and storing data on a personal system is known as Personal Database.
This database is designed only for a single user.
These type of databases are very easy to handle.
As the size of these database is very small, it needs less storage space.

Operational Database

Databases that gets created and updated in real time is called as Operational Database.
These types of databases are designed for executing and handling daily operations like per day transactions of several business.

Enterprise Database

For managing huge amount of data organization or enterprise uses these types of database and it helps them to increase and improve their efficiency.
Simultaneous access to multiple users can be done using these databases.
We can use Multiple process on these type of databases.
Parallel execution of queries on the system is allowed in these databases.

What is DBMS Queries ?

webmaster — Wed, 26 Oct 2022 08:03:59 +0000

SQL Commands

SQL commands are instruction which is used to communicate with the database.
It is also used to perform specific tasks, functions, and queries of data
It can also perform various tasks like creating a table, updating a table, deleting a table etc.

Types of SQL Commands

There are 5 types of SQL Commands

DDL (Data Definition Language)

Changing the structure of table like creating table, altering a table, and deleting a table can be done using this DDL
Commands in ddl are autocommitted which means it permanently save all changes in the database.
Some of the command in DDL are: create, alter,drop, truncate.

DML – Data Manipulation Language

These commands are used for modifing the database. Responsible for all form of changes in the database.
Commands in dml are not autocommitted which means it cannot permanently save all changes in the database. They can roll back.
Some of the commands under DML are:
Insert, Update, Delete.

DCL – Data Control Language

These command are used to grant and revoke authorities from user.
Some of the commands under DCL are:
Grant
Revoke

TCL – Transaction Control Language

These commands can only be used with DML commands like Insert, delete, update etc.
These operations are auto committed that is why they cannot be used while creating tables or dropping them.
Some of the commands under TCL are:
- Commit
- Roll Back
- Save Point

DQL – Data Query Language

These commands are used to fetch the data from a database.
Select is the only command used in this language.

What is an XML Database in DBMS

webmaster — Wed, 26 Oct 2022 07:39:04 +0000

For storing huge amount of data in xml format we use XML format.
As XML is rapidly growing in every field, we need to have a secure place to store the XML Documents.
Xquery is used to query the data stored in the database, serialized, and exported into the desired format.

Types of XML Database

XML- enabled
Native XML (NXD)

XML – Enabled Database

To convert XML documents we need XML enabled database.
In this relational database, data is stored in tables in the form of rows and columns.
The tables contain a set of records, which in turn consist of fields.

Native XML Database

In this database, the database is based on container rather than table format.
Large amount of XML document and data can be stored in this database.
This database is queried by using X-path Expressions.
This database has an advantage over the XML enabled database.
It is highly capable to store, query and maintain the XML document than XML-enabled database.

What is Relational Algebra in DBMS ?

webmaster — Wed, 26 Oct 2022 07:26:01 +0000

A procedural query language which is used widely is known as Relational Algebra.
Instances of relation are given as input data and we get output as occurrences of relations
Relational algebra uses various operations to perform this input and output action.
In a relation sql relational query operations are performed recursively.
Output for the above operations is a new relation, which is formed from one or more input relations.

Types of Relational Operation

Select Operation

Tuples that satisfy a given predicate is selected by select operation.
It is denoted by sigma(σ).
Notation: σ p(r)
- Where (σ) is used for selection prediction.
- R is used for relation,
- p is used as a propositional logic formula which may use connectors like: AND OR and NOT.

Project Operation

List of attributes that wish to appear in the result is shown in project operation.
Remaining attributes are eleminated from the table.
It is denoted by ∏.

Union Operation

In union there are two tuples, R and S.
The union operation that contains all the tuples are either in R or S or both in R & S.
It eliminates the duplicate tuples. It is denoted by ∪.

Notation: R ∪ S

Following condition should be held by Union operation.
R and S must have the attribute of the same number.
Duplicate tuples are eliminated automatically.

Set Intersection

Let us consider there are two tuples R and S. The set intersection operation contains all tuples that are in both R & S.
It is denoted by intersection ∩.
Notation: R ∩ S

Cartesian product

The Cartesian product is used to combine each row in one table with each row in the other table. It is also known as a Cross product.
It is denoted by X.
Notation: E X D

Rename Operation

The rename operation is used to rename the output relation. It is denoted by rho(ρ).

What do you understand by Query Optimization ?

webmaster — Wed, 26 Oct 2022 07:14:23 +0000

It is a phase which identifies a plan for evaluation query that has the least estimated cost.
We can use query optimizer when there is lot of algorithms and methods to execute the same task.

Advantages

Query optimizer provides faster output.
It takes less time to execute large number of queries.
It also reduces time and space complexity.

Explain levels of abstraction in DBMS ?

webmaster — Wed, 26 Oct 2022 06:48:51 +0000

There are 3 levels of abstraction in DBMS

Physical Level

It is the lowest level of data abstraction and is managed by DBMS.
This level shows how the data is stored in the database.
Data storage descriptions and details of this level are hidden from system admins, developers, and users

Logical Level

Logical level is the second level in data abstraction, which states the type of data and the relationship among the data that is stored in the database.
It is a level on which system admin and developers work.

View Level

This is the highest level of data abstraction, which shows/states only a part of the database.
View level is a level that describes a part of the database and hides the details of the table and its schema and storage from its users.
The result of a query is an example of view-level data abstraction.
A table created by selecting fields from one or more tables present in the database can be shown using this view.

What is Hierarchical model in DBMS ?

webmaster — Wed, 26 Oct 2022 06:10:29 +0000

Features of Hierarchical Model

This model is based on the tree structure.
This model contains a collection of records that are connected by each other by links.
Tree structures that are used in these models are called as routed trees.
The empty node of a tree is its root node.
The hierarchical model is a collection of routed trees and the relationship that exists in the hierarchical model is one to many and one to one.

Advantages

It is easy to understand.
More efficient than the ER model.

Disadvantages

When a parent node is deleted, child node will also be deleted because of data inconsistency occurs.
Stroage space is also wasted.
Very complex to design.
Structural Independence is not present.

What is Join in DBMS and what are its types ?

webmaster — Wed, 26 Oct 2022 05:57:29 +0000

DBMS join is a binary operation that allows combining join products and selecting in one single statement.
If data needs to be retrieved from two or more tables, we can use joins to retrieve data.
Tables in DBMS are associated with the primary key and foreign keys.

Types of Joins

Inner Joins: Theta, Natural, EQUI

Outer Join: Left, Right, Full

Inner Joins

We can use an inner join to return rows from both tables based on a satisfying condition.
It is the default join type and is the most widely used join operation.
Inner join can classify into three subtypes

Theta Join

It is represented by the symbol θ which allows us to join two tables based on condition.
This join works for all comparison operators
This join is also called Theta Join.

Syntax

[pastacode lang=”sql” manual=”A%20%E2%8B%88%CE%B8%20B” message=”” highlight=”” provider=”manual”/]

Theta join can use any conditions in the selection criteria.

Natural Join

This join does not use any comparison operators.
Attributes should have the same name and domain in this join.
In this join, two relations should have at least one common attribute between relations.
It performs selection forming equality on those attributes which appear in both relations by eliminating the duplicate attributes.

EQUI join

When Theta join uses an equivalence condition, the EQUI join can be used.
This join is the most difficult join to be done on an RDBMS because RDBMS have fundamental performance problems.

Outer Join

This join returns all the attributes of both tables depending on certain conditions.
If there are no attributes present for any one of the tables, it returns NULL irrespective of a row of the table attribute.
Outer Join is further classified as:
- Left Outer Join
- Right Outer Join
- Full Outer Join

Left Outer Join

If a left outer join is performed on two tables, it returns all the rows of the left table, even if there is no matching row for it in the right table.

Syntax

[pastacode lang=”sql” manual=”A%C2%A0Left%20Outer%20Join%C2%A0B” message=”” highlight=”” provider=”manual”/]

Right Outer Join

It returns all the rows of the second table even if there is no matching row in the first table, performing Right Outer Join.

Syntax

[pastacode lang=”sql” manual=”A%20Right%20Outer%20Join%C2%A0B” message=”” highlight=”” provider=”manual”/]

Full Outer Join

It returns all the rows of the first and second Table.

Syntax

[pastacode lang=”sql” manual=”A%C2%A0Full%20Outer%20Join%C2%A0B” message=”” highlight=”” provider=”manual”/]

What is ER Model in DBMS ?

webmaster — Sat, 04 Jun 2022 07:46:51 +0000

ER stands for Entity-Relationship model. It is a high level data model.
If any data elements or relationship of any specified system can defined using this model.
Conceptual design for database is developed in this model.
Design view of data is very simple and easy to develop.
Structure of the database is shown as a diagram called the Entity-Relationship diagram.

Component of ER Diagram:

Entity

In an ER Diagram, Entity is represented as rectangles.
An entity may be any object, class, person or place.

Attribute

In an ER diagram attributes are represented using eclipse.
Attribute describes the property of an entity.

Relationship

Relationship are represented in an er diagram by diamond or rhombus.
It is used to represent the relationship between entities.

Types of Relationship

One to one relationship

When only one instance of an entity is associated with an relationship is known as one to one relationship.

One to many Relationship

When only one instance of entity in the left is associated with more than one instance on the right is called one to many relationship.

Many-to-one relationship

When more than one instance of entity in the left is associated with only one instance on the right associates with the relationship is called many to one relationship.

Many-to-many relationship

When more than one instance of entity in the left is associated with more than one instance on the right associates with the relationship is called many to many relationship

What are the disadvantages of DBMS ?

webmaster — Sat, 04 Jun 2022 07:27:21 +0000

High Cost

Disadvantage of DBMS is its high cost of software and hardware.
For users to use a DBMS they require a high speed processor and huge memory size and also need costly machines to maintain database.
Highly trained database administrator is needed for using and maintaining the large database systems.

Huge Size

In the initial stage the size of the database is not big, but when user stores a large amount of data it causes many problems.
Due to the heavy data, dbms donot provide good results and donot run efficiently.

Database Failure

In a DBMS system all the data or information is stored in one centralized data base. If this database fails, the data is lost.

Complexity

For a non technical user, DBMS is so complex. So it is not easy to manage and maintain the database.
Training is need for users, designers and administrators to efficiently run the database systems.

Increased Staff Cost

To maintain and manage the database skilled staffs are required. So lot of money needs to be spent for the skilled staffs to be trained.

Requirement of Technical Staff

A non technical person does not understand the complexity of the database.
Skilled person with good skills is needed for managing and maintaining the database.

Cost of Data Conversion

The biggest disadvantage of DBMS is cost of conversion is very high.
There is a need for skilled, trained and experienced database administrators for conversion of data smoothly.

Performance

Another biggest disadvantage of database systems is speed of the database for small firms and organizations is very slow.
Hence the performance of database in these organizations is poor.

Different types of Keys in DBMS

webmaster — Sat, 04 Jun 2022 07:10:54 +0000

There are 7 keys in DBMS

Primary Key

Key which is used to identify one and only instance of entity uniquely. An Enity can contains multiple keys, and keys which is most suitable becomes the primary key.
For each entity the primary key selection is based on requirements and developers.

Candidate Key

A key or an attribute that uniquely identifies a tuple is called Candidate key.
Apart from primary key, other keys are considered as candiate key.
The candidate keys are as strong as the primary key.

Super Key

An attribute set that can uniquely identify a tuple is called as Super Key. It is a superset of candidate key.

Foreign Key

Column of table that is used to point the primary key of another table is called as Foreign key.

Alternate Key

One of more attributes or a combination of attributes that uniquely identifies a tuples in a relation is called Alternate Key.
These attributes or combinations of the attributes are called the candidate keys.
One key is chosen as primary key and the other keys remain as candidate keys if they exist and is called as alternate keys.
Total number of alternate keys is the total number of candidate keys minus the primary key.
In a relation if there is only one candidate key alternate keys may or may not exist.

Composite Key

Whenever there is more than one attribute in a primary key, it is known as Composite key or concatenated key.

Artificial Key

Keys that are created using arbitarily assigned data is known as artificial keys.
When primary key is large and complex and has no relationship with other relations, then the artificial key is created.
Data values of an artificial key are usually numbered in serial order.

What are the data models in DBMS ?

webmaster — Sat, 04 Jun 2022 06:46:19 +0000

There are 4 data models in DBMS

Relational Data Model

Edgar F.Codd described the model in 1969.
Data are designed using rows and column within a table. So data and in between relationships are represented using these models.
In this model tables are called as relations.
It is a widely used models for commercial data processing applications.

Entity-Relationship Data Model

Data in this model is logically represented asobjects and relationships among them.
Objects in this models are known as entities and relationship among these entities.
Peter Chen designed this relationship model. This relationship model is used in database desigining.
Set of attributes describe entities and set of same type of entities is known as Entity Set and set of the same type of relationship is known as relationship set.

Object-based Data Model

Object based model is an extension of ER model with the notions of functions, encapsulations and object identity.
Structure and collection types are supported in this model.
Database systems following the object oriented approach were developed during 1980.
In this model objects are nothing but data carrying its properties.

Semistructured Data Model

This model is different from other 3 models.
Data specification is allowed in this model where the individual data items of the same type have different attribute sets.
Semi Structured data is widely used and represented by XML(Extensible Markup Language).
When XML was developed for desiging the mark up information to the text document, it gains importance because of its applcation in the exchange of data.

Difference between Trigger and Stored procedure in DBMS

webmaster — Sat, 04 Jun 2022 05:41:20 +0000

Triggers	Stored Procedures
A Trigger is created in such a way that it fires when a specific event occurs.	A group of sql commands that can be reused again and again are called stored procedures. They are created and stored in a database.
A trigger cannot be called or execute directly by a user. Only when the corresponding events are fired, triggers are created.	Stored procedures can be executed whenever wanted by using exec command.
Trigger cannot be scheduled.	You can schedule a job to execute the stored procedure at a pre-defined time.
Calling a trigger within a trigger is not possible	Calling a stored procedure within a stored procedure is possible
You cannot pass parameters as input.	Inputs can be passed as parameters
It does return a value	Can return zero or n values.
In a trigger, we cannot perform any transactions	You can use transactions within a stored procedure.

Difference between DBMS and File System

webmaster — Sat, 04 Jun 2022 05:28:14 +0000

File System	DBMS
A software that manages and organizes the file in storage medium is called as file system.	DBMS is software application that is used for creating, accessing and managing databases.
Data Storage and Data representation details is provided by the file system	Data that hides the details is given an abstraction in DBMS.
Storing and retrieving data cannot be done efficiently in this system.	A wide variety of methods to store data and retrieve data can be done efficiently through DBMS.
Data Recovery process is not possbile	In DBMS Data Backup Recovery is possible.
File system does not have a crash recovery mechanism	It provides crash recovery mechanism.
Protecting a file system is very difficult	Offers good protection mechanism
Redundancy of data is greater in file system	Redundancy of data is lower in DBMS
File system has a higher data inconsistency	DBMS has a lower data inconsistency
File system offer less security	DBMS offer higher security
File System allows you to stores the data as isolated data files and entities.	Database Management System stores data as well as defined constraints and interrelation.
Support for complicated transactions is not supported	Easy to implement complicated transactions.
The centralization process is hard in File Management System.	Centralization is easy to achieve in the DBMS system.
Back up and recovery of data is not possible if the data is lost.	DBMS system provides backup and recovery of data even if it is lost.
No efficient query processing in file system	You can easily query data in a database using the SQL language.
This system doesn’t offer concurrency.	DBMS system provides a concurrency facility.

What are Database Management Tools ?

webmaster — Fri, 03 Jun 2022 12:42:43 +0000

phpMyAdmin:

PhpMyAdmin is the most popular open-source web-based database management tool by MySQL.
Using phpMyAdmin we can create or manage databases, tables, DB users, optimize and check errors in your database.
PhpMyAdmin also allows to import and export data in different file formats all with very easy procedures.

Navicat for MySQL:

This relational database was created by Premium Soft.
It is a cross-platform database of windows, mac, Linux relational database.
For creating complex database queries it has a visual query builder which helps creating database faster.
This builder also helps to import/export/backup tools for your databases.
It has report builder for printing out db queries, task scheduling assistant for scheduled execution of jobs, and much more.
Navicat also produces front-ends for Oracle and PostgreSQL.
It has both a lite version and a premium version.

SQLyog:

SQLyog is an GUI administration tool for MySQL which was developed by an Indian company Webyog.
It has a paid Enterprise version($99 – $999) and a free community version.
This tool has an outstanding list of features, and it is very useful for professional database admins

Sequel Pro:

It is a open source MySQL database management tool for Mac users.
Its crafty and instinctive barebones GUI helps us to easily navigate to an app.

SQl Server Management Studio:

Best Microsoft tool for database admins is SQL Server Management studio.
GUI interface of this tool is straightforward for users using windows OS.
It has an innovative feature called Object Explorer which allows user to browse, select and perform operations on database objects.
This single tool can be used for creating and administering SQL server databases..

What is DBMS Utilities ?

webmaster — Fri, 03 Jun 2022 12:19:30 +0000

Data Loading Utility:

Data loading utility helps to load data from an external format without writing programs.

Backup utility:

In case of crashes and disasters this utility helps to take copies of the database periodically.

Recovery Utility:

From the database backup and history of transactions this utility helps in reconstructing the correct state of database.

Monitoring Tools:

This tool monitors the performance of internal scheme that can be changed and optimize the data access.

File Organization:

This utility helps in restructuring of data from one type to another.

Difference between RDBMS and DBMS ?

webmaster — Fri, 03 Jun 2022 11:55:11 +0000

DBMS	RDBMS
Data is stored in file format	Data is stored in table format.
Data is stored either in a hierarchical form or navigational form	Tables have an identifier called primary key, and data values are stored in the form of tables.
No Normalization is present.	Normalization is present in RDBMS.
Do not provide security for data manipulation	RDBMS defines the integrity constraint for the purpose of ACID (Atomocity, Consistency, Isolation and Durability) property.
Uses file system to store data, so there will be no relation between the tables	In RDBMS since the data values are stored in the form of tables, relationship between these data values will also be stored in the form of table.
Provides some methods to access the stored information.	For accessing the stored information RDBMS supports a tabular structure of the data and relationship between them
Does not support distributed data base.	RDBMS supports distributed database.
It is meant for small organization and deal with small data. Supports single user.	RDBMS is designed to handle large amount of data. it supports multiple users.
Examples of DBMS are file systems, xml	Example of RDBMS are mysql, postgre, sql server, oracle etc.

Difference between Delete and Truncate

webmaster — Fri, 03 Jun 2022 11:42:11 +0000

Delete

Truncate

Used to delete rows from a table based on the condition provided on the where clause.

Used to delete the entire data from rows from the table. Similar to delete command with no where clause

Delete commands deletes only the rows that are mentioned in where clause.

Truncate command deletes complete row from a table.

We can roll back the delete commands.

We cannot roll back the truncate commands.

Delete command is very slow because it maintains a log

That’s why it is fast. This command is very fast as it does not maintain any log

It uses row lock when performing delete function

Truncate command uses table log while performing the truncate function