SQL, or Structured Query Language, is the foundational language used to interact with relational databases. Mastering SQL is crucial if you're aiming for a role that involves working with databases.

In 2024, SQL ranked among the top five most-used programming languages globally, with 51% of developers utilising it. Whether you're just starting your career or transitioning into a new role, the hiring process often includes SQL interview questions for freshers. These questions assess your understanding of the language, its features, and its practical applications.​

In this article, we'll explore some of the most common SQL interview questions for freshers, providing valuable tips and sample answers to help you stand out in your next interview. 

Overview of SQL

Before diving into SQL interview questions for freshers, it’s important to understand what SQL is and why it’s so important in data management.

SQL is a standard language used to communicate with relational databases. It’s used to create, manipulate, and query database data. Every time you use an app or website that pulls information from a database—whether it’s for displaying data, processing transactions, or managing user inputs—SQL is probably behind the scenes, making that happen.

There are various types of SQL commands:

• Data Query Language (DQL): Includes SELECT to retrieve data.
• Data Definition Language (DDL): Includes CREATE, ALTER, and DROP for defining database schemas.
• Data Manipulation Language (DML): Includes INSERT, UPDATE, and DELETE for manipulating data.
• Data Control Language (DCL): Includes GRANT and REVOKE to control user access.

As a fresher, you will need to be familiar with these basic components of SQL. Understanding the structure and purpose of each will help you navigate technical interviews more efficiently.

Ready to take your career to the next level? Book a free consultation with Topmate’s experts for career guidance tailored to SQL roles!

Now that you have a basic understanding of SQL, let's move on to some common SQL interview questions that freshers often face.

Basic and Intermediate SQL Interview Questions for Freshers

SQL is a powerful tool for managing and manipulating relational databases. Below are some basic SQL interview questions for freshers, with explanations and sample answers, which will help you get a solid foundation.

1. What is SQL?

Why it’s important: Understanding SQL is fundamental, as it is the primary language for database interaction.

Sample Answer

SQL stands for Structured Query Language. It is a standard programming language used to manage and manipulate relational databases. SQL allows users to create databases and insert, update, delete, and retrieve data from tables. It is used for querying data, performing analytics, and defining database structures and relationships.

2. What are SQL dialects? Give some examples.

Why it’s important: Knowing SQL dialects helps understand how different database systems implement SQL.

Sample Answer

SQL dialects refer to variations in SQL syntax and functionality across different database management systems (DBMS). While SQL is standardised, different DBMSs may implement slightly different syntaxes or additional features. Some common SQL dialects include:

• MySQL: A widely used open-source relational database.
• PostgreSQL: An advanced open-source relational database.
• SQL Server: Microsoft's relational database management system.
• Oracle SQL: Used by Oracle Database, offering advanced features for enterprise applications.

3. What are the main applications of SQL?

Why it’s important: Knowing SQL’s applications gives insight into how it’s used across industries for data management.

Sample Answer

SQL is used in various applications, including:

• Data Retrieval: Extracting information from databases for reports and analytics.
• Data Manipulation: Updating, inserting, and deleting records in the database.
• Database Management: Creating and managing tables, indexes, and relationships.
• Data Analysis: Performing calculations and aggregations using SQL functions like SUM(), AVG(), COUNT(), etc.

4. What is an SQL statement? Give some examples.

Why it’s important: Understanding SQL statements is crucial to interacting with databases and executing tasks.

Sample Answer

An SQL statement is a command instructing the database to perform a specific action, such as querying data or modifying the database. Examples include:

• SELECT: Retrieves data from the database.
• INSERT: Adds new records to a table.
• UPDATE: Modifies existing data in a table.
• DELETE: Removes records from a table.

5. What types of SQL commands (or SQL subsets) do you know?

Why it’s important: SQL commands are categorised into subsets that help organise tasks effectively.

Sample Answer

SQL commands are divided into the following subsets:

• Data Query Language (DQL): SELECT (used to retrieve data from databases).
• Data Definition Language (DDL): CREATE, ALTER, DROP (used to define and modify database structures).
• Data Manipulation Language (DML): INSERT, UPDATE, DELETE (used to manipulate data within tables).
• Data Control Language (DCL): GRANT, REVOKE (used to control access permissions).
• Transaction Control Language (TCL): COMMIT, ROLLBACK, SAVEPOINT (used to manage transactions).

6. What is a database?

Why it’s important: Understanding databases is crucial because SQL operates within a database environment.

Sample Answer

A database is an organised collection of data that is stored and accessed electronically. It allows users to store, manage, and retrieve information efficiently. Databases are typically organised into tables, where data is structured into rows and columns. A well-designed database ensures fast querying, accuracy, and scalability.

7. What is a DBMS, and what types of DBMS do you know?

Why it’s important: Knowing DBMS concepts helps you understand how SQL interacts with different databases.

Sample Answer

DBMS stands for Database Management System, a software system that allows users to create, manage, and manipulate databases. Types of DBMS include:

• Hierarchical DBMS: Organises data in a tree-like structure (e.g., IBM Information Management System).
• Network DBMS: Uses a graph structure to represent relationships (e.g., Integrated Data Store).
• Relational database management systems (RDBMS): Store data in tables with rows and columns (e.g., MySQL, Oracle, SQL Server).
• NoSQL DBMS: Designed for unstructured data and scalable performance (e.g., MongoDB, Cassandra).

Is your resume standing out for SQL roles? Let Topmate’s industry professionals review your resume and LinkedIn to ensure it gets noticed by top recruiters.

8. What is RDBMS? Give some examples of RDBMS.

Why it’s important: Understanding RDBMS is key since most modern applications use relational databases.

Sample Answer

RDBMS stands for Relational Database Management System, a type of DBMS that stores data in a structured format using tables. RDBMS uses SQL to query and maintain the data. Examples include:

• MySQL: Popular open-source RDBMS.
• SQL Server: Microsoft’s relational database.
• Oracle: A robust enterprise-level relational database system.
• PostgreSQL: An advanced, open-source RDBMS with strong SQL compliance.

9. What are tables and fields in SQL?

Why it’s important: Tables and fields are the core components of any relational database, and understanding them is essential for working with SQL.

Sample Answer

‍

• Tables: A table is a collection of rows and columns used to store data. Each table contains a set of records that are structured into columns.
• Fields: Fields (also known as columns) are individual data elements within a table. Each field holds a specific data type, such as integers, strings, dates, etc.

10. What is an SQL query, and what types of queries do you know?

Why it’s important: Knowing the different types of queries allows you to interact with the database efficiently.

Sample Answer

An SQL query is a request made to the database to retrieve, modify, or manipulate data. Types of SQL queries include:

• Select Queries: Used to fetch data (e.g., SELECT * FROM employees).
• Action Queries: Used for actions like inserting, updating, or deleting data.
• Join Queries: Used to retrieve data from multiple tables based on relationships.
• Aggregate Queries: Used to perform calculations on data, like SUM, COUNT, AVG.

11. What is a subquery?

Why it’s important: Understanding subqueries helps you perform more complex operations within a query.

Sample Answer

A subquery is a query within another query. It is typically used to retrieve data that will be used in the outer query. Subqueries are enclosed in parentheses and can be used in SELECT, INSERT, UPDATE, or DELETE statements.

12. What types of SQL subqueries do you know?

Why it’s important: Knowing the different types of subqueries helps you handle various query scenarios.

Sample Answer

There are three main types of subqueries:

• Single-row Subquery: Returns a single value.
• Multiple-row Subquery: Returns more than one value.
• Correlated Subquery: Refers to columns of the outer query.

13. What is a constraint, and why use constraints? What SQL constraints do you know?

Why it’s important: Constraints ensure data integrity and consistency in the database.

Sample Answer

A constraint is a rule applied to data in a table to ensure that it meets certain conditions. Constraints are used to enforce data integrity and accuracy. Types of SQL constraints include:

• Primary Key: Ensures that each row in a table has a unique identifier.
• Foreign Key: Ensures referential integrity between tables.
• Unique: Ensures all values in a column are unique.
• Check: Ensures that values in a column meet specific criteria.
• Not Null: Ensures that a column cannot have NULL values.

Want to ace your SQL interview? Practice with mock technical and HR interviews conducted by Topmate’s experts. Get real-time feedback and sharpen your skills for success.

14. What is a join? And what types of joins do you know?

Why it’s important: Joins are essential for retrieving data from multiple related tables.

Sample Answer

A join is an SQL operation used to combine rows from two or more tables based on a related column. Types of joins include:

• INNER JOIN: Returns rows when there is a match in both tables.
• LEFT JOIN: Returns all rows from the left table and matching rows from the right table.
• RIGHT JOIN: Returns all rows from the right table and matching rows from the left table.
• FULL OUTER JOIN: Returns all rows when a match exists in either table.

15. What is an index? What types of indexes do you know?

Why it’s important: Understanding indexes helps optimize query performance, especially when dealing with large datasets.

Sample Answer

An index is a database object that speeds up the retrieval of rows from a table by creating a sorted copy of selected columns. It improves query performance significantly, particularly for SELECT queries. Types of indexes include:

• Unique Index: Ensures all values in the indexed column are unique.
• Composite Index: Uses multiple columns to form an index for more complex queries.
• Full-text Index: This special index is used to efficiently search for large text fields.
• Clustered Index: Alters the physical storage of data in the table based on the index order.
• Non-clustered Index: Creates a separate structure to point to the data in the table.

16. What is a schema?

Why it’s important: A schema organises data, making it easier to understand and manage relationships between tables.

Sample Answer

A schema is the logical design of a database, outlining how data is organised and related. It defines tables, columns, relationships, and constraints within the database. A schema provides a blueprint for the structure and ensures consistency across the database. In most RDBMS, a schema is a container for database objects such as tables, views, and indexes.

17. What is an SQL comment?

Why it’s important: Comments in SQL document code, improve readability, and facilitate future developers' maintenance.

Sample Answer

A SQL comment is used to add notes or explanations within SQL code. The database does not execute it. Comments are useful for clarifying code and providing context to others or yourself for future reference. SQL comments come in two types:

• Single-line comment: Begins with --. Example: -- This is a comment.
• Multi-line comment: Enclosed between /* and */. Example: /* This is a multi-line comment */.

18. What is an SQL operator?

Why it’s important: SQL operators perform operations on data in SQL statements, essential for querying and manipulating data.

Sample Answer

An SQL operator is a symbol that performs specific operations on one or more expressions. Operators are essential for querying and modifying data. Types of SQL operators include:

• Arithmetic Operators: Such as +, -, *, /, and % to perform mathematical operations.
• Comparison Operators: Such as =, !=, <, >, <=, and >= for comparing values.
• Logical Operators: Such as AND, OR, and NOT for combining conditions.
• BETWEEN: Used to filter values within a specific range.
• IN: Used to specify multiple possible values for a column.
• LIKE: Used for pattern matching in string values.

19. What is a clause?

Why it’s important: Clauses help define the structure of SQL queries, allowing you to specify the conditions and operations you want to perform.

Sample Answer

A clause in SQL is an SQL query component that defines a specific operation or condition. Clauses are used to structure the query and specify what data to retrieve, how to filter it, and how to display the results. Common clauses include:

• SELECT: Specifies the columns to retrieve.
• FROM: Specifies the table from which to retrieve the data.
• WHERE: Filters the data based on conditions.
• ORDER BY: Sorts the results in a specific order.
• GROUP BY: Groups of rows that have the same values are divided into summary rows.
• HAVING: Filters groups created by GROUP BY.

Looking to land your dream job in SQL? Topmate connects you with professionals from leading tech companies like Google, Microsoft, and Amazon. Get your referral today!

20. What is a NULL value? How is it different from zero or a blank space?

Why it’s important: Understanding NULL is crucial for properly handling missing or undefined data in databases.

Sample Answer

A NULL value in SQL represents missing, undefined, or unknown data. It is not the same as zero or a blank space:

• Zero (0) is a numeric value.
• Blank space is a string with no characters.
• NULL represents the absence of a value. In SQL, NULL is used to indicate that a data value is missing or not applicable. When performing operations or comparisons, NULL requires special handling, such as using the IS NULL or IS NOT NULL syntax.

21. What is the difference between SQL and NoSQL?

Why it’s important: Understanding the difference between SQL and NoSQL is important for choosing the right database for specific applications.

Sample Answer

SQL (Structured Query Language) databases are relational databases that store data in tables with fixed schemas. They use structured query languages to define and manipulate data. Examples include MySQL, SQL Server, and PostgreSQL.

On the other hand, NoSQL (Not Only SQL) databases are non-relational databases that store data in flexible formats such as key-value pairs, document stores, or graph databases. Examples include MongoDB, Cassandra, and Redis.

Key differences:

• SQL: Structured, tabular data with predefined schemas and strong consistency.
• NoSQL: Flexible, unstructured data models, scalability, and often used for big data and real-time web apps.

22. What are some common challenges when working with SQL databases?

Why it’s important: Knowing common challenges helps you prepare for issues arising during database management and querying.

Sample Answer

Some common challenges when working with SQL databases include:

• Performance issues: Querying large datasets can slow down performance.
• Database locking: When multiple users access the database simultaneously, locks can cause delays.
• Data consistency: Ensuring data integrity when dealing with multiple transactions.
• Scalability: Handling an increasing volume of data efficiently.
• Complex queries: Writing complex queries with multiple joins can be time-consuming and prone to errors.

23. What is an alias?

Why it’s important: Aliases make SQL queries more readable and concise by assigning temporary names to tables or columns.

Sample Answer

An alias is a temporary name given to a table or column in a SQL query. It helps improve query readability and makes complex queries easier to write. You can create an alias using the AS keyword. For example:

• Column Alias: SELECT first_name AS name FROM employees;
• Table Alias: SELECT e.name FROM employees AS e;

24. What is the DISTINCT statement, and how do you use it?

Why it’s important: The DISTINCT statement is crucial for retrieving unique values from a dataset, especially when working with large amounts of data.

Sample Answer

The DISTINCT statement is used to remove duplicate values from the result set. It ensures that unique values are returned for the selected columns. For example:

• SELECT DISTINCT column_name FROM table_name; This query will return unique values from the specified column in the table.

25. How to create a table?

Why it’s important: Creating tables correctly ensures that your data is organised, is easily queried, and adheres to the integrity rules that allow for accurate and reliable data processing in your applications.

Sample Answer

Creating a table in SQL involves defining its name and structure, including columns and their respective data types, such as integer, text, or date. You can also apply constraints like primary keys for uniqueness or foreign keys for table relationships. For example, when creating a table for employee data, you define columns like employee ID, name, and salary, selecting appropriate data types for each. Establishing a primary key on employee ID ensures uniqueness. Proper table creation ensures data integrity and easy retrieval. It also sets the stage for efficient database management.

Once you’re comfortable with basic and intermediate questions, it’s time to tackle the more challenging SQL interview questions you might face.

Advanced SQL Interview Questions for Freshers

Advanced SQL questions dive deeper into complex topics like joins, subqueries, indexing, and performance optimization. Mastering these topics can set you apart from other candidates in competitive interview settings.

1. What is a function in SQL, and why are functions used?

Why it’s important: Functions in SQL help encapsulate business logic or reusable code, simplifying queries and improving code efficiency.

Sample Answer

In SQL, a function is a stored program that takes input parameters, performs operations, and returns a single value. Functions allow for code reusability, making it easier to manage and organise code by breaking it into manageable chunks. They also improve query performance, as they can perform calculations directly in SQL rather than fetching data into an application for processing. For example, functions like AVG(), COUNT(), and SUM() are used to perform calculations directly in the query.

2. What types of SQL functions do you know?

Why it’s important: Knowing different types of SQL functions allows you to apply the right function based on the requirements of your queries and operations.

Sample Answer

SQL functions are classified into two main types:

• Aggregate functions: These perform calculations on multiple rows and return a single result, such as SUM(), COUNT(), AVG(), and MAX().
• Scalar functions: These operate on a single value and return a single result, such as LENGTH(), LOWER(), UPPER(), and CONCAT(). Functions help make complex operations simpler and more efficient within SQL queries.

3. What aggregate functions do you know?

Why it’s important: Aggregate functions are crucial for summarising and analyzing data within a dataset. Mastering them is essential for any SQL developer.

Sample Answer

Aggregate functions are used to calculate a set of values to return a single summary value. Common aggregate functions include:

• COUNT(): Returns the number of rows that match a specified condition.
• SUM(): Adds up all the numeric values in a specified column.
• AVG(): Computes the average value of a numeric column.
• MIN(): Returns the smallest value from a specified column.
• MAX(): Returns the largest value from a specified column.

These functions are extremely helpful in grouping and summarising data for reports and analysis.

4. What scalar functions do you know?

Why it’s important: Scalar functions are used for manipulating individual values and are fundamental in making data more readable and usable in SQL queries.

Sample Answer

Scalar functions operate on a single value and return a single result. Some commonly used scalar functions include:

• CONCAT(): Combines two or more strings into a single string.
• UPPER(): Converts a string to uppercase.
• LOWER(): Converts a string to lowercase.
• LENGTH(): Returns the length of a string.
• ROUND(): Rounds a number to a specified number of decimal places.

These functions are useful for cleaning and formatting data, improving its presentation and usability.

5. What are case manipulation functions? Give some examples.

Why it’s important: Case manipulation functions help standardise text data, making it easier to compare and process within SQL queries.

Sample Answer

Case manipulation functions in SQL are used to modify the case of string data. Common case manipulation functions include:

• UPPER(): Converts all characters in a string to uppercase.
• LOWER(): Converts all characters in a string to lowercase.
• INITCAP(): Capitalises the first letter of each word in a string.

These functions are useful when comparing or processing textual data without case sensitivity.

6. What are character manipulation functions? Give some examples.

Why it’s important: Character manipulation functions allow SQL developers to clean and transform text data, improving data quality and usability.

Sample Answer

Character manipulation functions are used to perform operations on strings. Some common character manipulation functions include:

• SUBSTRING(): Extracts a substring from a string.
• TRIM(): Removes leading and trailing spaces from a string.
• REPLACE(): Replaces occurrences of a substring within a string.
• CONCAT(): Concatenates two or more strings into one string.

These functions help modify or extract data from strings for better formatting and reporting.

7. What is the difference between local and global variables?

Why it’s important: Understanding the scope of variables (local vs. global) is critical for managing data efficiently in your SQL queries and functions.

Sample Answer

The difference between local and global variables lies in their scope:

• Local variables: These are variables that are defined within a function or block of code and can only be accessed within that function.
• Global variables: These are defined outside any function or block and can be accessed anywhere in the SQL script or program.

Local variables are useful for temporary storage within a function, while global variables are useful for passing data between or across the application.

8. What is the difference between SQL and PL/SQL?

Why it’s important: SQL and PL/SQL have different capabilities, and understanding them helps you choose the right tool for solving database-related tasks.

Sample Answer

SQL (Structured Query Language) is a standard language used for querying and manipulating data in databases. PL/SQL (Procedural Language/SQL) is an extension of SQL used for writing procedural code that can execute complex logic, such as loops and conditions. PL/SQL allows for the use of variables, error handling, and stored procedures, while SQL is primarily used for querying and modifying data.

PL/SQL is generally used to write code involving multiple SQL statements and complex logic.

9. What is the difference between LEFT JOIN and LEFT OUTER JOIN?

Why it’s important: Understanding joins is crucial for combining data from multiple tables in relational databases.

Sample Answer

LEFT JOIN and LEFT OUTER JOIN are functionally identical in SQL. Both return all rows from the left table and matching rows from the right table. If there’s no match, the result is NULL on the right side.

The term OUTER in LEFT OUTER JOIN is optional and does not change the functionality of the join. It is used for clarity and is not required for the query to work.

10. What is indexing in SQL, and how does it improve performance?

Why it’s important: Indexing is vital for improving the speed of data retrieval operations in large databases.

Sample Answer

An index in SQL is a data structure that improves the speed of data retrieval operations on a table. It works similarly to an index in a book, allowing the database to quickly locate the desired rows without scanning the entire table. Indexes can significantly improve query performance, particularly on large datasets. However, creating too many indexes can slow down write operations.

Common types of indexes include clustered and non-clustered indexes. They can be created on columns frequently used in search conditions, such as WHERE clauses.

11. What is a stored procedure, and how is it different from a function?

Why it’s important: Stored procedures and functions are essential for encapsulating repetitive tasks and business logic in databases.

Sample Answer

A stored procedure is a set of SQL statements saved and executed together to perform a task, such as inserting, updating, or deleting data. Unlike functions, stored procedures do not return a value. They can execute multiple SQL commands, handle exceptions, and be called in SQL queries.

Conversely, a function is similar but always returns a single value and is usually used for computations or data transformations in queries.

12. What set operators do you know?

Why it’s important: Set operators are used to combine results from multiple SELECT queries, allowing you to compare and merge data from different sources.

Sample Answer

Set operators allow you to combine the results of two or more SELECT statements. The most common set operators are:

• UNION: Combines results from two SELECT queries and removes duplicates.
• INTERSECT: Returns only the common rows between two SELECT queries.
• EXCEPT (or MINUS in some databases): Returns rows from the first query that do not exist in the second query.

These operators help in working with data from multiple sources more efficiently.

13. What is the difference between primary and unique keys?

Why it’s important: Primary and unique keys ensure data integrity and uniqueness in relational databases.

Sample Answer

• Primary Key: A primary key uniquely identifies each record in a table and cannot contain NULL values. A table can have only one primary key.
• Unique Key: A unique key also ensures that all values in a column are distinct, but unlike a primary key, it can contain NULL values. A table can have multiple unique keys.

Both primary and unique keys ensure no duplicate values exist in specific columns.

14. What is a composite primary key?

Why it’s important: Composite primary keys are used when a single column cannot uniquely identify a record; they ensure data integrity in cases where multiple attributes are needed to form a unique identifier.

Sample Answer

A composite primary key is a primary key that consists of two or more columns in a table. It is used when no single column can uniquely identify records. For example, in an order details table, you may use a combination of order_id and product_id as a composite primary key to ensure that each record is uniquely identified by the combination of these two columns. This approach helps enforce data integrity when a single column doesn’t suffice.

15. What is a view, and why use it?

Why it’s important: Views help simplify complex queries, encapsulate business logic, and provide secure access to certain subsets of data in a database.

Sample Answer

A view is a virtual table in SQL created by a SELECT query. It doesn’t store data but instead presents data from one or more tables. Views can simplify complex queries, limit access to sensitive data, and provide a customised representation of the database. For example, a view can be created to show only selected columns from a table or filter data based on certain criteria, making it easier for users to query specific subsets of data without altering the original tables.

16. Can we create a view based on another view?

Why it’s important: Views based on other views allow you to modularise your SQL queries, making them more maintainable and easier to manage.

Sample Answer

Yes, we can create a view based on another view. This is known as nested views. A nested view allows you to use an existing view as a source for another view, which can simplify the structure of complex queries. For example, if you have a view that combines data from multiple tables, you can create another view based on that first view to filter further or aggregate the data. However, nested views can affect performance, so using them wisely is essential.

17. Can we still use a view if the original table is deleted?

Why it’s important: Understanding how views interact with underlying tables is essential to managing data integrity and ensuring your queries do not break when a table is deleted.

Sample Answer

No, if the original table a view uses is deleted, the view will become invalid and no longer return any results. Since views depend on the underlying tables, removing a table will disrupt the data flow for any views that rely on it. It is crucial to carefully manage the lifecycle of tables and views to avoid unexpected disruptions, especially in a production environment.

18. What types of SQL relationships do you know?

Why it’s important: SQL relationships are fundamental for establishing connections between tables, which is key to creating a normalized database schema and ensuring referential integrity.

Sample Answer

There are three main types of SQL relationships:

• One-to-One (1:1): Each record in the first table is related to one and only one record in the second table. For example, each employee may have one personal email address.
• One-to-Many (1:N): One record in the first table is related to multiple records in the second table. For example, a single customer may place multiple orders.
• Many-to-Many (N:N): Multiple records in the first table are related to multiple records in the second. For example, students can enroll in various courses, each with numerous students.

These relationships help structure data effectively and ensure consistency across different tables.

Unsure about the next step in your SQL career? Connect with Topmate’s mentors, who can guide you in mastering SQL roles and advancing your tech career.

19. What are the possible values of a BOOLEAN data field?

Why it’s important: BOOLEAN data types are used to store true/false values, and understanding how they are represented and used is essential for logic and decision-making in SQL queries.

Sample Answer

In SQL, a BOOLEAN data type typically holds two possible values:

• TRUE: Represented by 1 in some databases.
• FALSE: Represented by 0 in most SQL implementations. 

Some databases might use additional values, such as NULL, to represent unknown or missing data. It’s important to check the specific database documentation for how BOOLEAN values are handled, as this can differ slightly between systems.

20. What is normalization in SQL, and why is it used?

Why it’s important: Normalization is a key process in database design to reduce redundancy, improve data integrity, and streamline query performance.

Sample Answer

Normalization organises the database to eliminate redundancy and dependency by dividing large tables into smaller, related tables. The primary goal of normalization is to reduce data redundancy and ensure data integrity. The process involves applying a series of rules, known as normal forms (1NF, 2NF, 3NF), to structure data in a way that makes it efficient for querying and reduces the chances of data anomalies. For example, separating customer and order data into two different tables prevents the repetition of customer information across multiple order records.

21. What is denormalization in SQL, and why is it used?

Why it’s important: Denormalization is sometimes necessary when performance issues arise, especially for read-heavy databases where complex joins can slow query response times.

Sample Answer

Denormalisation combines tables that were previously separated during normalisation, often to optimise query performance. It introduces redundancy by storing data in multiple places. Denormalization is useful when read performance is a priority over data integrity, such as when queries frequently join multiple tables, leading to slower performance. By denormalising, you reduce the need for complex joins and improve query speed but at the cost of increased data redundancy and potential inconsistencies.

22. What is the difference between renaming a column and giving an alias to it?

Why it’s important: Understanding the difference helps you use SQL effectively in queries, particularly when making data more readable or changing data representations for specific use cases.

Sample Answer

Renaming a column refers to permanently changing the name of a column in the database schema, which will reflect across all queries that reference that column. In contrast, aliasing a column in SQL provides a temporary name used only for the duration of a specific query. Aliases are typically used to make queries more readable or to give a column a more meaningful name for reporting or analysis purposes. For example, SELECT name AS "Employee Name" temporarily changes the name column to "Employee Name" within the query result.

23. What is the difference between nested and correlated subqueries?

Why it’s important: Understanding the distinction is key to writing efficient and accurate queries when you need to use subqueries in your SQL operations.

Sample Answer

A nested subquery is a query independent of the outer query and can be executed independently. It is placed inside another query and doesn’t reference any column from the outer query. In contrast, a correlated subquery refers to the outer query and depends on its values for execution. For example, in a correlated subquery, the inner query references a column from the outer query in its condition. Correlated subqueries are generally slower than nested subqueries because they are executed repeatedly for each row processed by the outer query.

24. What is the difference between clustered and non-clustered indexes?

Why it’s important: Indexing improves query performance by organising data efficiently, and knowing the types of indexes helps you choose the right one for your use case.

Sample Answer

A clustered index determines the physical order of data in a table and stores the table rows based on the indexed column’s values. A table can have only one clustered index. In contrast, a non-clustered index stores a separate structure from the data and holds pointers to the data rows. A table can have multiple non-clustered indexes. Clustered indexes are faster for retrieval but slower for insertion and updates, while non-clustered indexes allow for more flexibility and can be used for various columns.

25. What is the difference between the DELETE and TRUNCATE statements?

Why it’s important: Understanding these commands helps you effectively manipulate and clean your database while considering data safety and performance.

Sample Answer

The DELETE statement is a Data Manipulation Language (DML) command that removes rows from a table based on a specified condition and can be rolled back. TRUNCATE is a Data Definition Language (DDL) command that removes all rows from a table but does not log individual row deletions, making it faster. TRUNCATE cannot be rolled back in many databases and does not fire triggers. Unlike DELETE, which allows conditional deletion, TRUNCATE deletes all records in the table without exceptions.

After understanding the advanced concepts, let’s go over some practical tips to help you tackle SQL interview questions effectively.

Tips for Tackling SQL Interview Questions

When preparing for SQL interview questions for freshers, it's essential to focus on the following:

1. Master the Basics

Understand the fundamental SQL concepts like syntax, joins, and data types. This will form the foundation for answering most SQL interview questions for freshers.

2. Understand Query Optimisation

Be familiar with writing efficient queries. Practice minimising resource usage and optimising queries, especially when working with large datasets.

3. Focus on Real-World Scenarios

Be prepared to solve practical problems using SQL. Interviewers often ask you to write queries that meet specific requirements or conditions.

4. Know Subqueries and Nested Queries

Subqueries are commonly used in SQL. Ensure you're comfortable with nested queries and their use in filtering and manipulating data.

Got a job offer? Let Topmate’s experts help you negotiate your salary and maximize your compensation package with proven negotiation strategies.

5. Prepare for Scenario-Based Questions

Scenario-based questions assess your problem-solving abilities. Practice writing queries based on real-world business problems and data needs.

Focusing on these tips will equip you to tackle SQL interview questions for freshers and demonstrate your proficiency in the language. Next, let’s explore some valuable resources that can further help you in your SQL interview preparation.

Resources for SQL Interview Preparation

Preparing for an SQL interview requires a deep understanding of both basic and advanced concepts. Whether you're a fresher or an experienced professional, having access to the right resources can make all the difference. Here are some of the Topmate resources that can help you prepare effectively for SQL interviews in 2025:

1. 30 Essential SQL Questions

Get a solid foundation in SQL with this list of 30 essential SQL questions that cover all the key concepts and operations. Perfect for anyone preparing for an SQL interview.

2. PL/SQL MCQ Questions

Test your knowledge of PL/SQL with a range of multiple-choice questions that will help sharpen your skills and prepare you for both technical and HR interviews. Dive into the collection of PL/SQL MCQs here.

3. SQL Interview Questions

This resource provides an extensive list of SQL interview questions, designed to help you navigate the complexities of SQL during your interview. From basics to advanced, it covers a wide range of topics. 

4. Data Engineering Mock Interview - SQL, Python

If you're preparing for a data engineering role, this mock interview covering SQL and Python is ideal. Practice your problem-solving skills in a simulated interview environment and improve your chances of success. Check out the Data Engineering Mock Interview here.

By practicing these SQL questions and reviewing the provided answers, you’ll be better equipped to handle a variety of interview scenarios. Get ready to showcase your SQL skills with confidence!

Conclusion

As you prepare your SQL interview questions for freshers, remember that strong foundational knowledge is key. You can approach your interview confidently by focusing on the basics and understanding how SQL commands interact with data.

SQL is one of the most in-demand skills in the tech industry, especially for data roles. Practicing SQL interview questions for freshers and improving your problem-solving skills increases your chances of success. Consistent practice, mastering core SQL concepts, and being able to apply them in practical scenarios will set you apart from the competition.

Topmate connects you with professionals who offer career mentorship, interview prep, and salary negotiation tips. Start your journey toward a successful career in SQL by booking a mock interview with us today!

Reach out to us for further assistance!