SQL Interview Questions and Answers 2021

By Gowtham Reddy

Last updated on Nov 10 2020

SQL Interview Questions and Answers 2021

Most Commonly Asked SQL Interview Questions and Answers


SQL stands for Structured Query Language. The following is used to maintain communication with a database. The following is considered as a standard language for relational database management systems. This review was done by the American National Standards Institute in ANSI. The statements of this standard language can be utilized to finish the works that involve things like updating data on a database and retrieving data from a database. A lot of relatable database management systems feature SQL. The following systems are Oracle, Microsoft SQL Server, Sybase, Access and others. Out of this, one of the most used databases in RDBMS. SQL expertise cannot be ignored in most of the services. There are a lot of important questions which the candidates need to be prepared for while going for an interview. The SQL Interview questions are very important for getting a job. One must answer the SQL Interview Questions to qualify for the interview and get a job.


How can you distinguish SQL and MySQL?


SQL is a common language that stands for Structured Query Language which is basically based on general English language whereas MySQL is a database management system.


Structured Query Language is the main part of the relational database, which can be utilized for gaining access to the Database and also plays a major role in the management of the Database. On the other hand, MySQL is a Relational Database Management System like SQL Server, Informix, and others. 


Set 1 - SQL Interview Questions and Answers


  1. State the different subsets of SQL.


There are basically three subsets of SQL:


Data Definition Language or DDL, which lets the candidate carry out different functions and perform tasks on the Database is one of the most important subsets of SQL. These operations and functions include ALTER, CREATE and DELETE.

Data Manipulating Language or DML lets the candidates manipulate and gain access to the data in the Database. The following is really helpful for the candidate in inserting, deleting and improving data from the Database. The following also helps the candidate to retrieve the data from the Database.


Data Control Language or DCL proves to be beneficial for the candidate. The following lets the candidate to gain control over the Database and also helps him to access the following. For instance, Grant and Revoke access permissions are some of the Data Control Languages.


  1. What is meant by DBMS, and how can it be classified?

DBMS is otherwise known as Database Management System, which is a software application that connects with the candidate along with the applications. The following also connects with the Database and applications to acquire and examine the data in the Database. The Database Management System lets the candidate to connect with the Database. The information that is stored in the Database can be improved, acquired and deleted and can be of any type. The types of data can be digits, strings, pictures and others.

Database Management Systems can be classified into two different categories. They are:

RDMS or the Relational Database Management System in which the necessary information is stored in the form of relations or tables is the first category. MySQL is the one of such Database Management Systems.

The other Database Management System is Non-Relational Database Management System. In the following form, there is nothing related to tables, relations and attributes. One of such Database Management System is Mongo.

  1. How are the table and field defined in SQL?

When we use the term table, it is meant that there is a cluster or cluster of data in a proper and well-settled manner which is arranged in the form of rows and columns. One of such tables is StudentInformation.

Basically, we can say that a field is the number of columns in a table. Some examples of the field are Stu Id, Stu Name, and Stu Marks.

  1. In SQL, what is meant by joins?

In SQL, we generally use JOIN clause to conjunct row from two tables or at times more than that. The following conjunction is based on a related column between the following. The following can also be utilized to amalgamate two tables. This can also be utilized for acquiring information from there. SQL comprises of 4 different kinds of joins. They are Inner Join, Right Join, Left Join and Full Join.

  1. How can we distinguish between CHAR and VARCHAR 2 data type in SQL?

CHAR as well as VARCHAR 2 are both used for the characters data type. But, there lies a difference. We can say that VARCHAR 2 is basically utilized for character strings of different lengths. On the other hand, CHAR is used for strings having the same length. We can better understand the following by illustrations. For example, CHAR (10) can be used for storing 10 characters only and cannot store more than ten characters. On the other hand, VARCHAR (10) will have the capability to store characters irrespective of their length. For example, VARCHAR 10 will be able to store even number of characters lower than ten which includes 6,8,4,2.

This is one of the most asked SQL Interview Questions in the interview.

  1. How can you define Primary Key?

A Primary Key is basically a column that verifies each row in a table. It can also be a set or cluster of columns for identifying the number of rows in a table. The speciality of a primary key is that it can identify a single row in a table that to in a different and unique manner. Primary key doesn’t allow null values in the table. For instance, Stu_Id is the primary key in the table containing information about students. 

  1. How can you define constraints?

Constraints are those elements of the table which can be used to classify the data limit type of the table. The data limit of the table can be specified while building or changing the statement of the table. We can understand the constraints of some samples. The samples of constraints are:







  1. On what basis you can distinguish the statement of DELETE and TRUNCATE?

There are certain specifications by which we can distinguish the statements of DELETE and TRUNCATE. The specifications are:

DELETE Statement:

The command of DELETE is used for deleting a particular row in the table.

The command of DELETE allows the user to revert information back to its initial state after the use of the following statement or command.

The statement of DELETE is a DML (Data Managing Language) command.

The following command works slower than the command of TRUNCATE.

TRUNCATE Statement:

The command of TRUNCATE is used for deleting or erasing all the rows of the table in one go.

The command of TRUNCATE doesn’t allow the user to revert the data back to its initial state once it is deleted.

The following is a DDL (Data Definition Language) command.

The following command is basically faster than the command of DELETE.

  1. How can you define the constraint: Unique Key?

The Unique Key classifies a particular row in the table in a different and unique way. The following key allows the user to insert multiple values that can be inserted according to the number of tables. The Unique key also allows the user to insert null values in a table.

  1. How can you define the constraint: Foreign Key?

When the constraint of Foreign Key is used in the child table, it references the parent key in the main or primary table.

The foreign key is used to maintain referential integrity. The following is done by creating a link in two table containing data.

The constraint of Foreign Key allows the user to avoid actions which would generally demolish the connection between the parent table and child.


Set 2 - SQL Interview Questions and Answers


  1. How can you explain data integrity?

We can term data integrity by saying that it maintains the consistency of the data which is stored in the Database. The following is also used to maintain the accuracy of the data in a database. Integrity constraints are often changed when entering them into the Database. These constraints are edited according to the business rules before feeding them into the Database.

  1. On what basis can you distinguish the clustered index and non clustered index in SQL?

We can distinguish the clustered and non clustered index on the following basis:

A clustered index can be utilized for recovering information from the Database in an easy way. The following works are faster than the non clustered index. Non clustered index is generally slower than the clustered index.

Clustered index basically changes the way in which the storage of records is maintained in a database in a changed manner. The clustered index does this because the following classifies the rows by the columns which are ready to be the clustered index. On the other hand, the non clustered index doesn’t change the way of storing information. It instead builds another object or element in a table which points out the original rows of the table when searched.

There can be only one clustered index in a single table. On the other hand, there can be more than one table in a non clustered index.

  1. What are the different types of joins?

Joins are basically used to recover data in a database in between the tables. Generally, there are four types of joins:

Inner Join: In MySQL, the use of an inner join is considered as the most common join. The following is used to recover the rows from numerous tables which satisfy and take care of the condition of joining.

Left join: In MySQL, the use of the left join is done to return all the rows from the tables that are on the left side. There is another criterion which it should satisfy. That is the compatibility of the data on the right column with the data on the left column.

Right, join: In MySQL, the use of the right join is done to return all the rows from the tables that are on the right side. There is another criterion which it should satisfy. That is the compatibility of the data on the left column with the data on the right column.

Full join: In MySQL, the use of full joins is done to return all the records at that time when a match is found between two or more tables. Hence, the following returns all the rows from the table on the left-hand side along with the tables on the right-hand side.

  1. How can you define Denormalization?

Denormalization can e called the techniques which can be utilized for accessing information to lower forms starting from the higher forms of data in a database. The following element proves to be beneficial to the managers by raising the performance of the whole scenario because it ensures reliability and consistency in a table. The consistent data then gets added up into a table. This is done by the incorporation of the problems in the Database obtained through a harmonious shortlisting of data from various tables.

  1. How can you define entities and relationships?


An entity can be defined as an object, or a place or a person in the actual world which has the ability to be stored in the Database. The data showing only one type of entity can be stored in tables. For instance, the Database of a bank has a customer table to store its data as a cluster of qualities which are basically the columns in the table for every single consumer.


Relationships can be defined as the connections or the relationship between the entities that correlate with each other. It is established when the entities depend upon each other. For instance, the name of the consumer has a relation with the account number of the same consumer along with its contact information. The contact information might be present on the same table. Relationships can also be established between different tables, such as the relationship between costumers to accounts.

  1. How can the index be defined?

An index is basically a process of performance tuning, which lets the user recover the information or the records from the table with a great speed. The speciality of the index is that it builds an entry for a particular value which in turn allows it to recover the data with more speed.

  1. What are the various kinds of the index, and how can you explain them?

There are three different types of index. They are unique index, clustered index, non clustered index. They can be classified in the following ways:

Unique index

The following index doesn’t let the particular field to create unreal values within it. A field with real value can be called a uniquely indexed field. The unique index can function automatically if the primary key is particularly defined.

Clustered index

The following index rearranges the basic or the physic arrangement of the table along with the research which depends upon the key values of the table. There can be only one clustered index in one table.

Non-clustered index:

The following index doesn’t contribute to changing the general or the physical arrangement of the tables. The following index is also used to maintain the data in a logical manner. Each table can have more than one non-clustered index.

  1. How can normalization be defined, and what benefits can it provide?

Normalization can be called as the method of arranging data in a proper way to prevent the use of unreal information in a table along with the redundancy. There are a lot of benefits that the user can enjoy with normalization. Some benefits of the following are:

  • Proper arrangements of the Database
  • More tables consisting of smaller rows
  • Making data access efficient
  • More acceptance towards the queries
  • Accessing information fast
  • Easy implementation of security measures
  • Ensures flexibility in modifications
  • Eradicating unreal data and decreasing the redundancy
  • Compact database storage
  • Maintaining data consistency

This is one of the most crucial SQL Interview questions which can be essential for the candidate.

  1. What can be the basic difference between DROP and TRUNCATE commands?

The command of DROP is used to remove a table completely, and the following can be reverted from the Database.

On the other hand, the command of TRUNCATE removes all the rows in the table.

This is one of the basic SQL Interview questions that can be asked.

  1. What are the different kinds of normalization and explain the following?

There are a lot of successive levels of normalization. The following successive levels of normalization are known as normal forms. Each consecutive normal form is dependent on its preceding one. The first three normal forms are mostly sufficient for SQL practices. The types of normalization are:

1NF or the first normal form: This normalization doesn’t repeat groups within the rows.

2NF or the second normal form: This normalization states that each and every non-key value of the column has to function according to the whole primary key.

3NF or the third normal form: This normalization depends on the main or the primary key without depending on the non-key supporting value.


Set 3 - SQL Interview Questions and Answers


There are more SQL Interview questions which the candidates are asked.


  1. What is the use of ACID in a database?

In a database, ACID stands for Atomicity, Consistency, Isolation and Durability. The following element is used to make sure that the transactions made on the basis of information are reliably transmitted to the Database.

Atomicity: The following term refers to the complete or incomplete transactions referring to a single logical operation of data. This means, when any part of the transaction fails or remains incomplete, it negatively affects the entire transaction. This also doesn’t allow the state of the Database to change in any way.

Consistency: The following term makes sure that the information must fit into and satisfy all the rules and conditions of the validation. In other words, we can say that the transaction never moves out of the Database or never gets erased without finishing its state.

Isolation: The main objective of the following term is that it can control concurrency. 

Durability: The following term ensures that after a transaction is completed, there is no way that it can be reverted back. Power loss and other errors of this sort cannot stop it.

This is one of the most important SQL interview questions that are likely to be asked during an interview.

  1. How can Trigger in SQL be defined?

The following are specific procedures that are conserved and are used to implement automatically in place or else implement the following after the improvement and upgrading of data. The following also allows the user to deploy a batch containing code when any kind of issues like insert or update is implemented against a particular table.

  1. A lot of operators are available in SQL. What are the various operators in SQL?

These kinds of SQL interview questions confuse the candidate at times. So, coming to the question, there are three different kinds of operators available in SQL. They are:

  • Arithmetical operators
  • Logical operators
  • Comparison operators
  1. Are the terms zero or blank space the same as that of NULL values?

No, absolutely not. We can never say that the terms zero or blank space as same as that of NULL values. The element of NULL value depicts a particular value which is not known or not present in a particular data. On the other hand, terms like zero value or blank space depict numbers and characters in the following, respectively. 

  1. How can you distinguish between natural join and cross join?

The natural join basically focuses on all the columns of the table that have the same data types as well as the same names in both the tables. The cross join builds the cross product or the Cartesian product of two different tables.

  1. How can you define subquery in SQL?

The term subquery is an issue within another issue where the same problem or issue is used to recover necessary data again from the Database. There are two parts of a subquery. The two parts involve the outer query and the inner query. The inner query is basically called as a subquery, and the outer query is known as the main query in the Database. The implementation of subqueries is always done first, and the outcome of the following is always passed on to the main query. The use of comparison operators like <,> or = can be done in a subquery. These kinds of SQL Interview Questions are a bit difficult to understand and answer accordingly.


There are a lot of SQL Interview Questions apart from this, which the candidates must practice qualifying the interview they


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About the Author

Sprintzeal   Gowtham Reddy

Gowtham is a digital marketer and a content writer skilled in creating high quality, latest, and informative content in the education domain. His works majorly focus on concepts beneficial for professionals aspiring to enhance their careers.

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