Top 50 SQL Interview Questions and Answers (2025)

Structured Query Language (SQL) is the backbone of database management, making it a crucial skill for anyone pursuing a career in data analysis, software development, or database administration. 

Whether you’re a beginner stepping into the world of databases or an experienced professional preparing for an advanced role, knowing the right SQL interview questions and answers can give you a competitive edge.

In this guide, we’ll cover top SQL interview questions from basic to expert level, helping you understand key concepts like queries, joins, indexing, stored procedures, and database optimization. Mastering SQL not only enhances your problem-solving skills but also opens doors to high-paying jobs in top tech companies.

SQL is used across industries, from banking and healthcare to e-commerce and cloud computing. Employers seek candidates who can manage and manipulate data efficiently, making SQL a must-have skill.

Read on to explore essential SQL interview questions and answers, along with expert tips to help you confidently crack your next SQL interview!

SQL Interview Questions for Freshers (SQL Basic Questions)

If you're new to SQL and preparing for a job interview, understanding basic SQL interview questions is essential. These questions test your foundational knowledge of databases, queries, and key SQL concepts. 

Below are some of the top SQL interview questions for beginners:

1. What is SQL?

SQL (Structured Query Language) is a standard programming language used for managing and manipulating relational databases. It allows users to create, retrieve, update, and delete data. SQL is widely used in industries for handling structured data efficiently.

2. What are the different types of SQL commands?

SQL commands are divided into five types:

• DDL (Data Definition Language): CREATE, ALTER, DROP (for defining structure).
• DML (Data Manipulation Language): INSERT, UPDATE, DELETE (for modifying data).
• DQL (Data Query Language): SELECT (for retrieving data).
• TCL (Transaction Control Language): COMMIT, ROLLBACK, SAVEPOINT (for transaction management).
• DCL (Data Control Language): GRANT, REVOKE (for access control).

3. What is a primary key in SQL?

A primary key is a column or a set of columns that uniquely identifies each record in a table. It does not allow duplicate or NULL values. A table can have only one primary key, ensuring data integrity.

4. What is a foreign key?

A foreign key is a column or set of columns in one table that refers to the primary key of another table. It establishes a relationship between tables and ensures referential integrity. Foreign keys help maintain consistent and accurate data across multiple tables.

5. Explain the difference between DELETE and TRUNCATE commands.

• DELETE: Removes specific records based on a condition and can be rolled back using ROLLBACK.
• TRUNCATE: Removes all rows from a table but retains the structure, and it cannot be rolled back.
• DELETE is slower than TRUNCATE because it logs each deleted row.

6. What is the difference between SQL and MySQL?

SQL is a standard language used for database management, while MySQL is a relational database management system (RDBMS) that implements SQL. SQL defines how data is manipulated, whereas MySQL provides the tools to store, retrieve, and manage that data.

7. What is normalization in SQL?

Normalization is the process of organizing a database to reduce redundancy and improve efficiency. It divides large tables into smaller related tables and establishes relationships between them. This improves data integrity and reduces data anomalies.

8. What are joins in SQL? Name the different types.

Joins are used to retrieve data from multiple tables based on a related column. The different types of joins are:

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

9. What is the difference between WHERE and HAVING clauses?

• WHERE: Filters rows before performing aggregation (used with SELECT, UPDATE, DELETE).
• HAVING: Filters rows after aggregation (used with GROUP BY).
• WHERE works on individual records, while HAVING works on grouped data.

10. What is an index in SQL?

An index is a database object that speeds up data retrieval operations by creating a pointer to records. It works like a book index, allowing the database to find specific data quickly. However, indexes can slow down INSERT, UPDATE, and DELETE operations because they need to be updated.

11. What is the difference between UNIQUE and PRIMARY KEY constraints?

• PRIMARY KEY: Ensures uniqueness and does not allow NULL values. A table can have only one primary key.
• UNIQUE: Ensures uniqueness but allows NULL values. A table can have multiple unique constraints.

12. What is a stored procedure in SQL?

A stored procedure is a collection of SQL statements that are stored in the database and executed as a single unit. It helps improve performance by reducing repeated code execution and enhancing security by restricting direct access to data.

These SQL interview questions for beginners will help you build a strong foundation in SQL. 

Other Basic SQL Interview Questions (Asked in MNCs)

If you're preparing for SQL job interviews at top companies, understanding frequently asked SQL interview questions is essential. Below are some of the top SQL interview questions asked in MNCs, covering fundamental concepts to help you build confidence.

13. What is the difference between CHAR and VARCHAR data types?

CHAR is a fixed-length data type that stores strings of a defined length, while VARCHAR is a variable-length data type that stores strings with flexible lengths. CHAR is faster for fixed-size data, whereas VARCHAR is more space-efficient for variable-length text.

14. What is a NULL value in SQL?

A NULL value in SQL represents missing or unknown data. It is different from zero or an empty string, as NULL means that no value has been assigned. SQL provides functions like IS NULL and IS NOT NULL to handle NULL values in queries.

15. What is the difference between INNER JOIN and OUTER JOIN?

INNER JOIN returns only the matching rows from both tables based on a common column. OUTER JOIN returns all records from one or both tables, including unmatched rows. OUTER JOIN is further divided into LEFT JOIN, RIGHT JOIN, and FULL JOIN, each determining how unmatched rows are handled.

16. What is a subquery in SQL?

A subquery is a query nested within another SQL query. It is used to retrieve data that will be used by the main query. Subqueries can be placed in SELECT, WHERE, or FROM clauses to filter or refine results dynamically.

17. What is the difference between GROUP BY and ORDER BY?

• GROUP BY is used to group rows based on common values, often with aggregate functions like SUM or COUNT.
• ORDER BY is used to sort results in ascending or descending order based on one or more columns.
• GROUP BY groups data, while ORDER BY arranges it in a specified order.

18. What is a view in SQL?

A view is a virtual table based on the result of a SQL query. It does not store data physically but displays data dynamically from existing tables. Views improve security by restricting direct table access and simplifying complex queries.

19. What are aggregate functions in SQL?

Aggregate functions perform calculations on a set of values and return a single result. Common aggregate functions include:

• SUM() – Calculates the total sum of a column.
• AVG() – Returns the average value.
• COUNT() – Counts the number of rows.
• MAX() / MIN() – Finds the highest and lowest values.

20. What is a self-join in SQL?

A self-join is a join where a table is joined with itself. It is used when comparing rows within the same table. A self-join requires using table aliases to differentiate between the original and duplicate instances of the table.

21. What is the difference between DELETE and TRUNCATE?

DELETE removes specific rows from a table based on a condition and can be rolled back. TRUNCATE removes all rows from a table without logging individual row deletions and cannot be rolled back. DELETE is slower as it logs changes, while TRUNCATE is faster as it resets the table structure.

22. What are the constraints in SQL?

Constraints are rules applied to table columns to maintain data integrity. Common constraints include:

• PRIMARY KEY – Uniquely identifies records.
• FOREIGN KEY – Links two tables.
• UNIQUE – Ensures unique values in a column.
• NOT NULL – Prevents NULL values.
• CHECK – Enforces a condition on data.

These SQL interview questions are commonly asked in MNC interviews. Keep practicing and reviewing more SQL interview questions and answers to ace your next SQL interview!

Intermediate Level SQL Interview Questions and Answers

These SQL interview questions and answers cover crucial concepts that are frequently asked in technical interviews. Mastering these will help both beginners and experienced professionals ace their SQL interviews.

23. What is a JOIN in SQL, and what are its types?

A JOIN is used to combine rows from multiple tables based on a related column. The main types are:

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

24. What is a subquery? Provide an example.

A subquery is a query within another SQL query that helps retrieve data dynamically. It can be used in SELECT, INSERT, UPDATE, or DELETE statements.
 Example:

SELECT name FROM employees  
WHERE department_id = (SELECT department_id FROM departments WHERE department_name = 'HR');

This returns employees who work in the HR department.

25. Explain the difference between UNION and UNION ALL.

UNION combines results from multiple queries and removes duplicates, while UNION ALL keeps duplicates. UNION takes longer as it sorts and filters duplicates, whereas UNION ALL is faster but may return repeated values.

26. Explain ACID properties in SQL.

ACID properties ensure reliable database transactions:

• Atomicity: Transactions are fully completed or not executed at all.
• Consistency: Data remains valid before and after a transaction.
• Isolation: Transactions do not interfere with each other.
• Durability: Completed transactions are permanently saved.

27. What is the difference between DELETE, TRUNCATE, and DROP?

• DELETE: Removes specific rows and can be rolled back.
• TRUNCATE: Removes all rows but keeps the table structure; cannot be rolled back.
• DROP: Deletes the entire table, including its structure.

28. What is an index in SQL, and what are its types?

An index in SOL improves query performance by speeding up data retrieval.

• Clustered Index: Sorts and stores data physically in a specific order.
• Non-Clustered Index: Creates a separate structure pointing to data locations without affecting physical storage.

29. What are stored procedures in SQL?

A stored procedure is a precompiled SQL query stored in the database. It helps improve performance, reduce network traffic, and enhance security by limiting direct table access.

30. What is the difference between a view and a table?

A table stores data physically, while a view is a virtual table based on an SQL query. Views simplify complex queries and enhance security by restricting data access.

31. What is normalization? Name its types.

Normalization reduces redundancy and improves data consistency. The main types are:

• 1NF: Eliminates duplicate columns and ensures atomicity.
• 2NF: Removes partial dependencies.
• 3NF: Eliminates transitive dependencies.

32. What is the difference between RANK(), DENSE_RANK(), and ROW_NUMBER()?

• RANK(): Assigns ranks but skips numbers for duplicate values.
• DENSE_RANK(): Assigns ranks without skipping numbers for duplicates.
• ROW_NUMBER(): Assigns a unique sequential number to each row.

33. What is a trigger in SQL?

A trigger is an automatic database action executed when a specific event (INSERT, UPDATE, DELETE) occurs. It helps maintain data consistency by enforcing business rules.

34. What is a cursor in SQL?

A cursor allows row-by-row processing of query results. It is useful for handling individual records but is slower compared to set-based operations.

35. What is the difference between HAVING and WHERE clauses?

• WHERE: Filters records before aggregation and applies to individual rows.
• HAVING: Filters grouped records after aggregation using functions like SUM() or COUNT().

36. What is the difference between a candidate key and a primary key?

A candidate key is any column or set of columns that can uniquely identify a record. A primary key is a selected candidate key that uniquely identifies rows and cannot have NULL values.

37. What is the COALESCE function in SQL?

The COALESCE() function returns the first non-null value from a list.
 Example:

SELECT COALESCE(NULL, 'Data Science', 'SQL'); -- Output: 'Data Science'

It ensures NULL values are replaced with meaningful data.

These SQL interview questions will help you prepare for intermediate-level queries and real-world database scenarios.

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SQL Interview Questions for Experienced

These SQL interview questions and answers cover advanced topics that are frequently asked in technical interviews. Mastering these concepts will help experienced professionals tackle complex database-related challenges.

38. Explain different isolation levels in SQL.

Isolation levels define how transactions interact with each other to maintain data integrity. The four main isolation levels are:

• Read Uncommitted: Allows dirty reads, where uncommitted changes are visible.
• Read Committed: Prevents dirty reads but allows non-repeatable reads.
• Repeatable Read: Prevents dirty and non-repeatable reads but allows phantom reads.
• Serializable: The strictest level, preventing all concurrency issues by locking rows.

39. How does a clustered index work?

A clustered index determines the physical order of data in a table. Each table can have only one clustered index, as the data is stored in sorted order based on the indexed column. It improves search performance for range queries but slows down insertions and updates.

40. What is a window function in SQL?

A window function performs calculations across a set of table rows related to the current row. Unlike aggregate functions, it does not group results but retains individual row data. Examples include RANK(), DENSE_RANK(), ROW_NUMBER(), and SUM() OVER().

41. What is the difference between a CTE and a temporary table?

A Common Table Expression (CTE) is a temporary result set within a query using the WITH clause. It improves readability and recursion handling. A temporary table is created in the database temporarily and persists until manually dropped or the session ends.

42. What are materialized views in SQL?

A materialized view is a stored query result that is periodically refreshed. Unlike regular views, which fetch real-time data, materialized views store data physically, improving query performance for complex calculations.

43. What is the difference between NOLOCK and READ COMMITTED in SQL?

• NOLOCK: Allows reading data without acquiring locks, leading to dirty reads.
• READ COMMITTED: Ensures only committed data is read, preventing dirty reads but allowing non-repeatable reads.

44. What is sharding in SQL databases?

Sharding is a database partitioning method where large datasets are split across multiple servers to improve performance and scalability. Each shard operates independently, reducing the load on a single database instance.

45. What is database normalization, and when should denormalization be used?

Normalization organizes data efficiently to eliminate redundancy and improve consistency. Denormalization is used when performance is prioritized over redundancy, such as in reporting databases where fewer joins improve query speed.

46. Explain the difference between an OLTP and an OLAP system.

• OLTP (Online Transaction Processing): Handles real-time transactions, optimized for quick inserts and updates (e.g., banking systems).
• OLAP (Online Analytical Processing): Designed for complex queries and reporting, optimized for read-heavy operations (e.g., data warehouses).

47. How does indexing impact SQL performance?

Indexing speeds up data retrieval but slows down inserts, updates, and deletes due to additional maintenance. Choosing the right index type (clustered, non-clustered, composite, or unique) is crucial for optimizing SQL performance.

48. What is the difference between EXISTS and IN operators?

• EXISTS: Checks for the existence of records in a subquery and stops once a match is found, making it more efficient for large datasets.
• IN: Compares a value against a list of values and is better suited for small, predefined lists.

49. What is a deadlock in SQL, and how can it be prevented?

A deadlock occurs when two transactions hold resources the other needs, preventing progress. It can be avoided by ensuring consistent transaction order, using shorter transactions, and implementing deadlock detection mechanisms.

50. What is partitioning in SQL, and what are its types?

Partitioning splits large tables into smaller, manageable pieces for better performance. Types include:

• Range Partitioning: Based on a value range (e.g., sales data by year).
• List Partitioning: Based on predefined list values (e.g., region-based partitions).
• Hash Partitioning: Uses a hash function for even distribution.

These SQL interview questions for experienced professionals will help you tackle high-level database challenges in MNC interviews. Keep practicing and optimizing queries for better performance!

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SQL Fundamentals & Key Concepts

SQL is the backbone of relational databases, enabling efficient data management, retrieval, and manipulation. Whether you're a beginner or an experienced professional, understanding SQL fundamentals is essential for working with structured data. 

Below are the key concepts that form the foundation of SQL and help in writing optimized queries:

1. Database Structure & Organization

SQL is used to manage Relational Database Management Systems (RDBMS), where data is stored in tables, rows, and columns. Each table has a primary key to uniquely identify records and a foreign key to establish relationships between tables, ensuring data integrity.

2. SQL Commands & Data Manipulation

SQL operations are categorized into DDL (Data Definition Language), DML (Data Manipulation Language), DCL (Data Control Language), and TCL (Transaction Control Language). Common commands include SELECT, INSERT, UPDATE, and DELETE, which allow users to manage and manipulate data efficiently.

3. Data Integrity & Optimization

To maintain data accuracy, SQL uses constraints like NOT NULL, UNIQUE, CHECK, and FOREIGN KEY. Normalization minimizes redundancy, while denormalization improves query performance. Indexes (Clustered & Non-clustered) optimize search speed and enhance database efficiency.

4. Transactions & ACID Properties

Transactions in SQL ensure data reliability through Atomicity, Consistency, Isolation, and Durability (ACID) properties. SQL commands like COMMIT, ROLLBACK, and SAVEPOINT help in managing transactions and preventing data loss during failures.

5. Advanced SQL Features

SQL offers powerful features like Joins to combine data from multiple tables, Views for simplified data access, and Stored Procedures & Triggers for automating tasks. Window Functions enable complex calculations over a set of rows, improving data analysis capabilities

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SQL Interview Preparation Tips

Preparing for an SQL interview requires a strong understanding of database concepts, practical problem-solving skills, and familiarity with real-world applications. Whether you are a beginner or an experienced professional.

Following these key strategies can help you confidently tackle SQL interview questions:

1. Master the Fundamentals

• Ensure you have a solid grasp of SQL basics, including database structures, SQL commands (DDL, DML, DCL, TCL), and key concepts like primary keys, foreign keys, indexes, constraints, and normalization. 
• Understanding ACID properties and transaction management is also essential.

2. Practice SQL Queries Regularly

• Hands-on experience is crucial for SQL interviews. Work on writing and optimizing SELECT, INSERT, UPDATE, DELETE, and JOIN queries.
• Solve real-world database problems using GROUP BY, HAVING, subqueries, window functions, and aggregate functions to enhance your query-building skills.

3. Understand Query Optimization Techniques

• Employers often test your ability to write efficient SQL queries. Learn about indexing, query execution plans, proper use of joins, and reducing redundancy.
• Avoid common performance pitfalls like unnecessary subqueries or improper indexing.

4. Work on Real-World SQL Scenarios

• Many interview questions focus on practical use cases in database management.
• Gain experience by working on sample databases like Sakila, Northwind, or AdventureWorks.
• Practice solving business-related queries involving large datasets and performance tuning.

5. Revise Advanced SQL Concepts

• For senior roles, interviewers may ask about stored procedures, triggers, views, recursive queries, CTEs (Common Table Expressions), and window functions.
• Ensure you understand how these advanced SQL features are applied in database management and optimization.

By following these SQL interview preparation tips, you can improve your confidence and problem-solving abilities, making you well-prepared to handle both basic and advanced SQL interview questions.

Conclusion

Mastering SQL is essential for anyone looking to build a career in database management, data analysis, or software development. This comprehensive list of SQL interview questions and answers equips you with the knowledge needed to tackle technical interviews with confidence. 

Whether you're a fresher learning the basics or an experienced professional refining your skills, understanding SQL concepts like queries, joins, indexing, and stored procedures will give you a strong competitive edge.

As industries continue to rely on data-driven decision-making, SQL remains a valuable and in-demand skill. By consistently practicing and staying updated with the latest advancements in SQL, you can unlock exciting job opportunities in top tech companies.

Looking to strengthen your SQL skills further? Explore our SQL Courses and advance your career with expert-led training!

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