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Top Asked SQL Interview Questions

Q: Which are joins in SQL? Name the most commonly used SQL joins.

There are four types of SQL joins, which are as follows: Inner Join: It fetches the records from both of the joined tables with values that match. Queries are often joined via an inner join. SELECT * FROM Table_A JOIN Table_B; SELECT * FROM Table_A INNER JOIN Table_B; Left (Outer) Join: With a left outer join, each and every single row or record from the left are retrieved. Furthermore, the records that match those from the right are also retrieved. SELECT * FROM Table_A A LEFT JOIN Table_B B ON A.col = B.col; Right (Outer) Join: The purpose of a right outer join is to get each and every row or record from the right and even the matching ones from the left. SELECT * FROM Table_A A RIGHT JOIN Table_B B ON A.col = B.col; Full Join: This join method finds records that match either in the right or the left table. SELECT * FROM Table_A A FULL JOIN Table_B B ON A.col = B.col; Let’s go to the next SQL query interview question.

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The most common and helpful SQL interview questions and answers for both new and seasoned candidates are included below. These inquiries were made to expressly acquire you with the kinds of queries you would encounter throughout your SQL interview.

HeroVired’s observations show that effective interviewers infrequently prepare questions on a predetermined subject before the interview. As an alternative, questions typically start with a fundamental understanding of the issue, and subsequent discussion follows depending on what you say.

What is SQL?

SQL is abbreviated for Structured Query Language. It is considered the default language for RDBMS and helps manage structured data that comes with variables or entities with relationships between them. SQL is typically used for interacting with databases.

According to ANSI, SQL is designed to manage RDBMS and carry on various data manipulation activities on various sorts of data. To be precise, it’s a database language leveraged to create or delete databases.

sql interview questions

Why is SQL so Popular?

Even after 50 years of its existence, SQL is still widely used due to its effectiveness. Relational databases were created to address a concern or need. Since SQL was designed to interact with such databases, it swiftly took over as the standard language.

SQL and Relational databases are commonly used interchangeably. It is one of the reasons why RDBMS are frequently referred to as SQL databases.

Making an attempt to remake the wheel would be pointless, right? SQL and relational databases are similar in that both aren’t going anywhere since they function flawlessly.

sql interview questions

Together, these two techniques guarantee the continued reliability of a large number of technologies, including structured web architecture, back-end technologies that enable enterprises to operate smoothly, and more.

25 Most Important SQL Query Interview Questions and Answers

This blog includes SQL interview questions and answers for topics like MS SQL Server, MySQL databases, etc., from novices to seasoned pros. It is a one-stop shop where you may get the most advantages and prepare quickly for job interviews.

Take a look at the top SQL query interview questions being used by employers right now:

What is the difference between SQL and MySQL?

SQL is the industry standard language for retrieving and modifying structured databases. In contrast, MySQL is a popular RDBMS (relational database management system) that is leveraged to handle or deal with SQL databases.

What are the different subsets of SQL?

SQL comes with four different subsets. These include the following:

DML or Data Manipulation Language: It is utilized to alter the database’s already-existing data. The group of commands includes INSERT, UPDATE, SELECT, and others.
DDL or Data Definition Language: This language describes the data structure, which includes operations like DROP, ALTER, CREATE, etc.
DCL or Data Control Language: It regulates access to the information kept in databases. This category of commands includes the verbs REVOKE and GRANT.
TCL or Transaction Control Language: It deals with the database’s transaction actions. This group of commands includes SAVEPOINT, SET TRANSACTION, ROLLBACK, COMMIT, and others.

Let’s go to the next SQL interview question.

What do you mean by DBMS? What are its different types?

To collect and process data, a DBMS (Database Management System) interfaces with the user, other programs, and the database itself.

A DBMS enables seamless communication between the databases and the users. Any sort of data, including photos, numbers, strings, etc., can be saved within the database and can be retrieved, edited, or deleted.

Typically, there are two types of DBMS:

Relational Database Management System: Here, tables are used to store data. An example is MySQL.
Non-Relational Database Management System: It is characterized by the absence of the concepts of attributes, tuples, and relations. An example is MongoDB.

Let’s go to the next SQL interview question.

What is RDBMS? How is it different from DBMS?

RDBMS stands for Relational Database Management System. The primary distinction between DBMS and RDBMS is that relations can be established between the shared attributes of these tables.

And that’s how RDBMS performs data storage in contrast to DBMS. RDBMS is the foundation of the majority of contemporary database management systems, including Amazon Redshift, IBM DB2, Oracle, Microsoft SQL Server, and MySQL.

Let’s go to the next SQL query interview question.

Which are joins in SQL? Name the most commonly used SQL joins.

There are four types of SQL joins, which are as follows:

Inner Join: It fetches the records from both of the joined tables with values that match. Queries are often joined via an inner join.

SELECT *
FROM Table_A
JOIN Table_B;
SELECT *
FROM Table_A
INNER JOIN Table_B;

Left (Outer) Join: With a left outer join, each and every single row or record from the left are retrieved. Furthermore, the records that match those from the right are also retrieved.

SELECT *
FROM Table_A A
LEFT JOIN Table_B B
ON A.col = B.col;

Right (Outer) Join: The purpose of a right outer join is to get each and every row or record from the right and even the matching ones from the left.

SELECT *
FROM Table_A A
RIGHT JOIN Table_B B
ON A.col = B.col;

Full Join: This join method finds records that match either in the right or the left table.

SELECT *
FROM Table_A A
FULL JOIN Table_B B
ON A.col = B.col;

Let’s go to the next SQL query interview question.

What is the SELECT statement?

A SELECT statement retrieves 0–n rows from n–tables or n–views of a database. Most programs use the SELECT command, which is the most common DML or data manipulation language command.

Due to the declarative nature of SQL as a programming language, SELECT queries only declare the result set rather than how to compute it.

Let’s go to the next SQL query interview question.

What are some common clauses used with SELECT queries in SQL?

Here are some common clauses used with SELECT queries in SQL:

SQL WHERE: This clause is utilized to filter records in accordance with certain criteria, and it only keeps those that are required.
SQL ORDER BY: This clause is utilized to either sort the entries in DESC (descending) or ASC (ascending) order depending on one or more fields.
SQL GROUP BY: When combined with some aggregation procedures, this clause can summarize the database’s findings by grouping records with the same data.
SQL HAVING: This clause is used in conjunction with the GROUP BY clause to sort and filter records.

Let’s go to the next SQL interview question.

What is the difference between a primary key and a unique key?

While a unique key can have a null value, a primary key cannot. Both unique and primary keys hold unique values. The number of primary keys in a table is limited to one, whereas the number of unique keys is unlimited.

Let’s go to the next SQL interview question.

What is the difference between clustered and non-clustered indexes in SQL?

Here is a table representing the key differences between clustered and non-clustered indexes:

Basis	Clustered Index	Non-Clustered Index
Required For	Filtering and storing records in memory physically	Making a logical order or sequence for data rows
Storage Methods	Data storage within the index’s leaf nodes	Data storage isn’t possible in the index’s leaf nodes
Access to Data	Fast	Slow
Size	Considerably large	Comparatively small
Extra Disk Space	Not Necessary	Needed to store indexes distinctly
Primary Feature	Enhances data retrieval performance	Must be created on columns utilized in joins
Key Type	The table’s primary key is, by default, a clustered index	It may be utilized with the distinct constraint on the table that serves the purpose of a composite key

Let’s go to the next SQL interview question.

What is Denormalization in a Database?

Database managers deploy the denormalization approach to boost the effectiveness of their database system. The normalization concept—which is described as properly organizing a database into tables for a specific purpose—is the foundation of the denormalization idea.

Let’s go to the next SQL query interview question.

What is the difference between DELETE and TRUNCATE statements in SQL?

TRUNCATE	DELETE
It is used to delete each and every row from the table.	This command is utilized for deleting or removing a row in the table.
This is a DDL command	This is a DML command
It is quicker than DELETE	It is slower than TRUNCATE
Doesn’t allow rollback of data	Allows rollback of data

Let’s go to the next SQL interview question.

What are the different types of SQL operators?

The following are the various types of SQL operators:

SQL Operators	Examples
Arithmetic Operators	+, -, *, /, etc.
Logical Operators	AND, IN, ISNULL, BETWEEN, NOT, ALL, etc.
Comparison Operators	=, != or <>, >=, !>, etc.
Bitwise Operators	NOT (~), AND (& symbol), OR (\|, ^), etc.
Compound Operators	+=, -=, /=, *=, etc.
String Operators	+= (String concatenation assignment), [^] (Character(s) not to match), etc.

Let’s go to the next SQL query interview question.

What is a view in SQL?

A view in SQL is a database object without any values. A portion of the data in the table is contained in this virtual table. It appears to be a real table with columns and rows, but since it is not physically there, it takes up less room. Although it doesn’t have any data of its own, it operates in a manner the same as the base table.

Let’s go to the next SQL interview question.

What are SQL comments?

The usage of SQL Comments allows for the clarification of certain SQL statement clauses as well as the blocking of SQL statement execution. In numerous database programming languages, comments play a significant role.

Let’s go to the next SQL interview question.

What are the different types of indexes in SQL?

SQL indexes are merely methods used for reducing the cost of the query. The performance of the query declines as the cost increases. The different types of indexes that SQL supports are as follows:

Clustered Index,
Bit-Map Index,
Composite Index,
Function-Based Index,
Unique Index,
Non-Clustered Index,
Normal Index, and
B-Tree Index

Let’s go to the next SQL query interview question.

What are SQL functions and their usage in SQL?

SQL functions are a series of SQL commands that carry out a certain operation. Functions promote code reuse.

You can construct a function to carry out the same action if you need to write long SQL scripts daily for it. Instead of writing a new SQL every time, you can simply call that function the following time.

Let’s go to the next SQL interview question.

List the different types of relationships in SQL.

There are various types of relationships in SQL, which are as follows:

One-To-One: This is a relationship between two tables where every single record in a table corresponds to only a single record in the other table.
One-To-Many & Many-To-One: These connections are the most common types of connections between records in two different tables.
Many-To-Many: It is the term used to describe a connection that necessitates multiple instances on both sides.
Self-Referencing Relationships: This technique should be used when a table must establish an association with itself.

Let’s go to the next SQL interview question.

What are basic SQL skills?

With the help of SQL expertise, data analysts can create, maintain, and retrieve data from relational databases – the one that organizes data into rows and columns. Thanks to this, users can also quickly obtain, manipulate, update, insert, and change data.

The most basic skills that an SQL expert should possess include the following:

Structuring a database
SQL System Skills like PostgreSQL and MySQL
SQL data analysis
OLAP skills
Database management
Creating SQL statements and clauses

Let’s go to the next SQL interview question.

What is normalization, and why is it important in database design?

Normalization enables seamless data structuring and organizing in a database. This includes developing tables and interlinking those together based on the principles and guidelines intended to protect the data and enhance the database’s adaptability by eliminating inconsistent reliance and duplication.

Let’s go to the next SQL interview question.

What is a primary key, and how is it different from a foreign key?

A primary key typically emphasizes the table’s uniqueness. It guarantees that the value in that particular column is distinct or unique. Typically, a foreign key is utilized to establish a connection between the two tables. A table can consist of only a single primary key.

Let’s go to the next SQL interview question.

What is the difference between SQL and NoSQL databases?

NoSQL	SQL
Distributed or Non-Relational Database System	RDBMS or Relational Database Management System
These types of databases are ideal for storing hierarchical data.	These types of databases are not suitable for storing hierarchical data.
Horizontally scalable	Vertically scalable
Dynamic schema	Predefined or static or fixed schema
Not suitable for complex queries	Ideal for complex queries
Adheres to CAP property	Adheres to the ACID property
Examples include GraphQL, MongoDB, Neo4j, etc.	Examples include PostgreSQL, MySQL, Oracle, etc.

Let’s go to the next SQL interview question.

What is the difference between a stored procedure and a function in SQL?

A function in SQL is obligated to return a value. However, the same is optional in Stored Procedure. While functions can only come with input parameters, stored procedures can have both output and input parameters.

Let’s go to the next SQL interview question.

What is a subquery in SQL, and how is it used?

Subqueries are a technique for carrying out tasks in a number of phases. They are sometimes referred to as nested or inner queries. For instance, you would need to perform each aggregation in a separate phase if you wished to compute the sums of numerous columns and then aggregate all of those figures.

Let’s go to the next SQL interview question.

What is PostgreSQL?

Eventually, PostgreSQL became a popular object-relational DBMS that can handle concurrent users and support variable workloads.

The worldwide development community constantly backs it, and it is now enterprise-level, versatile, reliable, and open-source. PostgreSQL has become quite well-liked among developers due to its fault-tolerance capability.

Let’s go to the next SQL interview question.

What is UNION, MINUS, and INTERSECT commands?

The UNION operator is leveraged to combine the outcomes of two tables. It is also used to remove any duplicate items.
The MINUS operator is used to return the rows of the first query. However, the rows of the second query aren’t returned.
The INTERSECT operator is used to combine the outcomes of both searches.

Before executing the above-mentioned SQL statements, it is crucial to meet some specific conditions. There must be an equal number of columns within the clause of every SELECT query.

FAQs

What is the difference between SQL and MySQL?

What are the different subsets of SQL?

SQL comes with four different subsets. These include the following:

DML or Data Manipulation Language: It is utilized to alter the database’s already-existing data. The group of commands includes INSERT, UPDATE, SELECT, and others.
DDL or Data Definition Language: This language describes the data structure, which includes operations like DROP, ALTER, CREATE, etc.
DCL or Data Control Language: It regulates access to the information kept in databases. This category of commands includes the verbs REVOKE and GRANT.
TCL or Transaction Control Language: It deals with the database’s transaction actions. This group of commands includes SAVEPOINT, SET TRANSACTION, ROLLBACK, COMMIT, and others.

Let’s go to the next SQL interview question.

What do you mean by DBMS? What are its different types?

To collect and process data, a DBMS (Database Management System) interfaces with the user, other programs, and the database itself. A DBMS enables seamless communication between the databases and the users. Any sort of data, including photos, numbers, strings, etc., can be saved within the database and can be retrieved, edited, or deleted. Typically, there are two types of DBMS:

Relational Database Management System: Here, tables are used to store data. An example is MySQL.
Non-Relational Database Management System: It is characterized by the absence of the concepts of attributes, tuples, and relations. An example is MongoDB.

Let’s go to the next SQL interview question.

What is RDBMS? How is it different from DBMS?

RDBMS stands for Relational Database Management System. The primary distinction between DBMS and RDBMS is that relations can be established between the shared attributes of these tables. And that’s how RDBMS performs data storage in contrast to DBMS. RDBMS is the foundation of the majority of contemporary database management systems, including Amazon Redshift, IBM DB2, Oracle, Microsoft SQL Server, and MySQL. Let’s go to the next SQL query interview question.

Which are joins in SQL? Name the most commonly used SQL joins.

There are four types of SQL joins, which are as follows:

Inner Join: It fetches the records from both of the joined tables with values that match. Queries are often joined via an inner join.
```
SELECT *
FROM Table_A
JOIN Table_B;
SELECT *
FROM Table_A
INNER JOIN Table_B;
```
Left (Outer) Join: With a left outer join, each and every single row or record from the left are retrieved. Furthermore, the records that match those from the right are also retrieved.
```
SELECT *
FROM Table_A A
LEFT JOIN Table_B B
ON A.col = B.col;
```
Right (Outer) Join: The purpose of a right outer join is to get each and every row or record from the right and even the matching ones from the left.
```
SELECT *
FROM Table_A A
RIGHT JOIN Table_B B
ON A.col = B.col;
```
Full Join: This join method finds records that match either in the right or the left table.
```
SELECT *
FROM Table_A A
FULL JOIN Table_B B
ON A.col = B.col;
```

Let’s go to the next SQL query interview question.

What is the SELECT statement?

A SELECT statement retrieves 0–n rows from n–tables or n–views of a database. Most programs use the SELECT command, which is the most common DML or data manipulation language command. Due to the declarative nature of SQL as a programming language, SELECT queries only declare the result set rather than how to compute it. Let’s go to the next SQL query interview question.

What are some common clauses used with SELECT queries in SQL?

Here are some common clauses used with SELECT queries in SQL:

SQL WHERE: This clause is utilized to filter records in accordance with certain criteria, and it only keeps those that are required.
SQL ORDER BY: This clause is utilized to either sort the entries in DESC (descending) or ASC (ascending) order depending on one or more fields.
SQL GROUP BY: When combined with some aggregation procedures, this clause can summarize the database’s findings by grouping records with the same data.
SQL HAVING: This clause is used in conjunction with the GROUP BY clause to sort and filter records.

Let’s go to the next SQL interview question.

What is the difference between a primary key and a unique key?

What is the difference between clustered and non-clustered indexes in SQL?

Here is a table representing the key differences between clustered and non-clustered indexes:

Basis	Clustered Index	Non-Clustered Index
Required For	Filtering and storing records in memory physically	Making a logical order or sequence for data rows
Storage Methods	Data storage within the index’s leaf nodes	Data storage isn’t possible in the index’s leaf nodes
Access to Data	Fast	Slow
Size	Considerably large	Comparatively small
Extra Disk Space	Not Necessary	Needed to store indexes distinctly
Primary Feature	Enhances data retrieval performance	Must be created on columns utilized in joins
Key Type	The table’s primary key is, by default, a clustered index	It may be utilized with the distinct constraint on the table that serves the purpose of a composite key

Let’s go to the next SQL interview question.

What is Denormalization in a Database?

What is the difference between DELETE and TRUNCATE statements in SQL?

TRUNCATE	DELETE
It is used to delete each and every row from the table.	This command is utilized for deleting or removing a row in the table.
This is a DDL command	This is a DML command
It is quicker than DELETE	It is slower than TRUNCATE
Doesn’t allow rollback of data	Allows rollback of data

Let’s go to the next SQL interview question.

What are the different types of SQL operators?

The following are the various types of SQL operators:

SQL Operators	Examples
Arithmetic Operators	+, -, *, /, etc.
Logical Operators	AND, IN, ISNULL, BETWEEN, NOT, ALL, etc.
Comparison Operators	=, != or <>, >=, !>, etc.
Bitwise Operators	NOT (~), AND (& symbol), OR (\|, ^), etc.
Compound Operators	+=, -=, /=, *=, etc.
String Operators	+= (String concatenation assignment), [^] (Character(s) not to match), etc.

Let’s go to the next SQL query interview question.

What is a view in SQL?

What are SQL comments?

What are the different types of indexes in SQL?

SQL indexes are merely methods used for reducing the cost of the query. The performance of the query declines as the cost increases. The different types of indexes that SQL supports are as follows:

Clustered Index,
Bit-Map Index,
Composite Index,
Function-Based Index,
Unique Index,
Non-Clustered Index,
Normal Index, and
B-Tree Index

Let’s go to the next SQL query interview question.

What are SQL functions and their usage in SQL?

SQL functions are a series of SQL commands that carry out a certain operation. Functions promote code reuse. You can construct a function to carry out the same action if you need to write long SQL scripts daily for it. Instead of writing a new SQL every time, you can simply call that function the following time. Let’s go to the next SQL interview question.

List the different types of relationships in SQL.

There are various types of relationships in SQL, which are as follows:

One-To-One: This is a relationship between two tables where every single record in a table corresponds to only a single record in the other table.
One-To-Many & Many-To-One: These connections are the most common types of connections between records in two different tables.
Many-To-Many: It is the term used to describe a connection that necessitates multiple instances on both sides.
Self-Referencing Relationships: This technique should be used when a table must establish an association with itself.

Let’s go to the next SQL interview question.

What are basic SQL skills?

Structuring a database
SQL System Skills like PostgreSQL and MySQL
SQL data analysis
OLAP skills
Database management
Creating SQL statements and clauses

Let’s go to the next SQL interview question.

What is normalization, and why is it important in database design?

What is a primary key, and how is it different from a foreign key?

What is the difference between SQL and NoSQL databases?

NoSQL	SQL
Distributed or Non-Relational Database System	RDBMS or Relational Database Management System
These types of databases are ideal for storing hierarchical data.	These types of databases are not suitable for storing hierarchical data.
Horizontally scalable	Vertically scalable
Dynamic schema	Predefined or static or fixed schema
Not suitable for complex queries	Ideal for complex queries
Adheres to CAP property	Adheres to the ACID property
Examples include GraphQL, MongoDB, Neo4j, etc.	Examples include PostgreSQL, MySQL, Oracle, etc.

Let’s go to the next SQL interview question.

What is the difference between a stored procedure and a function in SQL?

What is a subquery in SQL, and how is it used?

What is PostgreSQL?

Founded in 1986, PostgreSQL was initially known as Postgres. It was created to make systems fault-resistant and support data integrity. This helped developers in creating enterprise-level applications. Eventually, PostgreSQL became a popular object-relational DBMS that can handle concurrent users and support variable workloads. The worldwide development community constantly backs it, and it is now enterprise-level, versatile, reliable, and open-source. PostgreSQL has become quite well-liked among developers due to its fault-tolerance capability. Let’s go to the next SQL interview question.

What is UNION, MINUS, and INTERSECT commands?

The UNION operator is leveraged to combine the outcomes of two tables. It is also used to remove any duplicate items.
The MINUS operator is used to return the rows of the first query. However, the rows of the second query aren’t returned.
The INTERSECT operator is used to combine the outcomes of both searches.

Before executing the above-mentioned SQL statements, it is crucial to meet some specific conditions. There must be an equal number of columns within the clause of every SELECT query.

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