Top Asked SQL Interview Questions

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. 

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.

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. 

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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. 

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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. 

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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;

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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.

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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. 

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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.

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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:

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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. 

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

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What are the different types of SQL operators?

The following are the various types of SQL operators: 

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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.

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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. 

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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 

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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. 

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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.

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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 

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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. 

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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. 

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What is the difference between SQL and NoSQL databases?

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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. 

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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.

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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. 

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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.