Introduction

Most Commonly Asked SQL Interview Questions and Answers in 2024

As you might have already known, SQL stands for Structured Query Language. It is used to maintain communication with a database. SQL is also considered a standard language for relational database management systems.

A lot of relatable database management systems feature SQL. The following systems are Oracle, Microsoft SQL Server, Sybase, Access, and others. MySQL is one of the most widely used databases in RDBMS.

This review was done by the American National Standards Institute (ANSI). The SQL statements can be utilized to finish the work that involves things like updating data on a database and retrieving data from a database.

SQL knowledge is essential for the majority of services. There are a lot of important questions for which the candidates need to be prepared 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.

Top SQL Interview Questions List 2024

SQL is a common language that stands for Structured Query Language, which is basically based on general English, 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 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.

2) What is meant by DBMS, and how can it be classified?

DBMS stands for Database Management System, and it is a software application that connects with the candidate as well as 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 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.

3) How are the table and field defined in SQL?

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

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.

4) In SQL, what is meant by joins?

In SQL, we generally use the JOIN clause to conjunct rows from two tables or more at times. 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.

5) How can we distinguish between CHAR and VARCHAR 2 data type in SQL?

CHAR as well as VARCHAR 2 are both used for the character's data type. But there lies a difference. We can say that VARCHAR2 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 through illustrations. For example, CHAR (10), which can be used to store only 10 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 an even number of characters lower than ten, which includes 6, 8, 4, and 2.

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

6) 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 specialty of a primary key is that it can uniquely identify a single row in a table that to in a different and unique manner.

The 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.

7) 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:

NOT NULL

DEFAULT

CHECK

PRIMARY KEY

UNIQUE

FOREIGN KEY

8) 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.

9) 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.

10) 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 tables containing data.

The foreign key constraint allows the user to avoid actions that would otherwise destroy the connection between the parent and child tables. 

Set 2 - SQL Interview Questions and Answers

11) 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 they are fed into the Database.

12) On what basis can you distinguish the clustered index and non-clustered index in SQL?

We can distinguish the clustered from the non-clustered index on the following basis:

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

Clustered indexing 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.

The non-clustered index, on the other hand, does not alter the way information is stored. 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.

13) What are the different types of joins?

Joins are basically used to recover data from 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 the most common join. The following is used to recover the rows from numerous tables that 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 that it should satisfy. That is the compatibility of the data in the right column with the data in 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 that it should satisfy. That is the compatibility of the data in the left column with the data in 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.

14) How can you define Denormalization?

Denormalization refers to approaches for accessing information in lower forms in a database, beginning with higher forms of data. The next component, which ensures consistency and dependability in a table, benefits the managers by improving the performance of the entire scenario.

The consistent data then gets added up in a table. This is accomplished by incorporating the problems obtained through a harmonious shortlisting of data from various tables into the database. 

15) How can you define entities and relationships?

Entities:

An entity can be defined as an object, or a place, or a person in the actual world that 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:

Relationships are the connections or relationships that exist between entities that correlate with one another. It is established when the entities depend upon each other.

For instance, the name of the consumer has a relationship 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 customers and accounts.

16) How can the index be defined?

An index is basically a process of performance tuning that lets the user recover the information or records from the table with great speed. The specialty 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.

17) What are the various kinds of the index, and how can you explain them?

There are three different types of indices. They are classified as either unique indices, clustered indices, or non-clustered indices.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 a 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 physical, 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.

18) 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 acceptances 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.

19) 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.

20) 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.

2) 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 issue, like an insert or update, is implemented against a particular table.

3) 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

4) 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 spaces depict numbers and characters in the following, respectively. 

5) 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.

6) 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 for the interview.

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