Top SQL Interview Questions and Answers

A. SQL Interview Questions for Freshers

1. What is SQL?

SQL means Structured Query Language and is used to communicate with relational databases. It proposes a standardized way to interact with databases, allowing users to perform various operations on the data, including retrieval, insertion, updating, and deletion.

2. What are the different types of SQL commands?

SQL commands are grouped into categories:

• DDL (Data Definition Language): CREATE, ALTER, DROP, TRUNCATE
• DML (Data Manipulation Language): INSERT, UPDATE, DELETE
• DQL (Data Query Language): SELECT
• DCL (Data Control Language): GRANT, REVOKE
• TCL (Transaction Control Language): COMMIT, ROLLBACK, SAVEPOINT

3. What is the difference between Database, DBMS, and RDBMS?

• Database: Organized collection of data.
• DBMS: Software to manage databases (e.g., MS Access).
• RDBMS: A DBMS using relational tables with rows and columns, supporting relationships (e.g., MySQL, PostgreSQL, SQL Server).

4. What is a Primary Key?

A column or set of columns that uniquely identify each row. A primary key cannot contain duplicates or NULLs.

5. What is a foreign key?

Foreign key is a field in one table referencing the primary key in another. It establishes a relationship between the two tables, ensuring data consistency and enabling data retrieval across tables.

6. What is a Unique Key?

Often referred to as a unique constraint, a unique key guarantees that every value in a column (or a combination of columns) remains distinct and cannot be replicated within a table. In contrast to a primary key, a table has the flexibility to incorporate multiple unique keys.

7. What is NULL in SQL?

NULL represents missing or unknown data. It is not zero or an empty string. To check NULLs, use IS NULL or IS NOT NULL.

8. What is the difference between CHAR and VARCHAR?

• CHAR(n): Fixed length; pads unused space with blanks.
• VARCHAR(n): Variable length; stores only entered characters.

9. What are Data Types in SQL?

Common types include:

• Numeric → INT, DECIMAL, FLOAT
• String → CHAR, VARCHAR, TEXT
• Date/Time → DATE, TIME, TIMESTAMP
• Binary → BLOB

10. What is the difference between Primary Key and Unique Key?

• Primary Key: One per table, cannot be NULL.
• Unique Key: Multiple allowed, NULLs permitted (depends on DB engine).

11. What are Default and Check Constraints?

• DEFAULT: Assigns a default value if none is provided.
• CHECK: Ensures values meet a condition.

CREATE TABLE employees (

  id INT PRIMARY KEY,

  salary DECIMAL DEFAULT 30000,

age INT CHECK (age >= 18)

);

12. What is a Schema?

A schema is a logical container for database objects such as tables, views, and procedures. It organizes and groups related objects.

13. What is a Candidate Key?

A candidate key is a column (or set) that can uniquely identify rows. One candidate key becomes the primary key, while others remain alternatives.

14. What is a Composite Key?

A key formed by combining two or more columns to uniquely identify a row.

15. What is the difference between Super Key, Candidate Key, and Primary Key?

• Super Key: Any column set that uniquely identifies rows.
• Candidate Key: Minimal super key (no redundant attributes).
• Primary Key: Chosen candidate key used for row identification.

16. What is a Cartesian Join?

Also called a cross join, it returns the Cartesian product of two tables.

SELECT * FROM employees, departments;

17. What is the difference between SQL, MySQL, and SQL Server?

• SQL: The language for managing relational data.
• MySQL: An open-source RDBMS using SQL.
• SQL Server: Microsoft’s RDBMS product.
  
  

18. What is the difference between DELETE and TRUNCATE?

19. What is the difference between SQL and NoSQL?

• SQL: Relational, structured, ACID-compliant.
• NoSQL: Non-relational, schema-free, designed for scalability.
  
  

20. What are the advantages of SQL?

• Standardized language
• Portable across databases
• Supports complex queries and joins
• Ensures data integrity

In the following section, we take a look at the common intermediate SQL interview questions and answers so that you'll know what to expect from your interviewer.

B. SQL Interview Questions for Intermediate

21. What is a Table and a Field?

A table is a set of rows and columns. A field is a column representing an attribute.

22. What is the SELECT statement?

The SELECT statement retrieves data.

SELECT name, salary

FROM employees

WHERE department = 'HR'

ORDER BY salary DESC;

23. What are SQL Constraints?

Rules applied to maintain data integrity. Common ones include PRIMARY KEY, FOREIGN KEY, UNIQUE, CHECK, and NOT NULL.

24. What is Normalization?

Normalization reduces redundancy.

• 1NF: Atomic values
• 2NF: No partial dependency
• 3NF: No transitive dependency
• BCNF: Every determinant is a candidate key
  
  

25. What is the difference between WHERE and HAVING?

26. What are Indexes?

Indexes speed up lookups. Types include clustered, non-clustered, composite, unique, and covering indexes.

27. What is GROUP BY?

GROUP BY groups rows with same values, used with aggregates.

28. What are Aggregate Functions?

Functions that summarize data: SUM(), COUNT(), AVG(), MIN(), MAX().

29. What is an Alias?

An alias renames a column or table temporarily.

30. What is a View?

A view is a saved query result presented as a virtual table.

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31. What are Common Table Expressions (CTEs)?

A CTE is a temporary result set defined with WITH used in queries.

WITH dept_avg AS (

  SELECT department_id, AVG(salary) AS avg_sal

  FROM employees

  GROUP BY department_id

)

SELECT * FROM employees e

JOIN dept_avg d ON e.department_id = d.department_id

WHERE e.salary > d.avg_sal;

32. What is a Recursive CTE?

A recursive CTE references itself, often used for hierarchical data.

33. What are Wildcards in SQL?

Wildcards match patterns in strings:

• % → matches zero or more characters
• _ → matches exactly one character
  
  

34. What is the difference between BETWEEN and IN?

• BETWEEN: Checks if value is within a range.
• IN: Checks if value is among specified list.

35. Difference between IN and EXISTS

• IN: Compares a value against a list or subquery.
• EXISTS: Checks if subquery returns any rows.

36. Difference between ANY and ALL

• ANY: True if condition holds for at least one value.
• ALL: True if condition holds for all values.

37. Difference between View and Materialized View

• View: Virtual, recalculated every time.
• Materialized View: Stores results physically, refreshed periodically.

38. What is a Temporary Table?

A table created for intermediate storage, often session-specific.

39. What is a Subquery?

A query nested inside another. Can be simple or correlated.

40. What is a Trigger?

Code that runs automatically on events like insert, update, or delete.

41. What is a Stored Procedure?

A precompiled set of SQL statements stored in the database.

42. What is a Function?

A reusable program that returns a single value or a table.

43. What is the difference between UNION and UNION ALL?

• UNION: Combines results, removes duplicates.
• UNION ALL: Combines results, keeps duplicates.
  
  

44. Difference between RANK(), DENSE_RANK(), and ROW_NUMBER()

• RANK(): Skips ranks for ties
• DENSE_RANK(): No gaps in ranking
• ROW_NUMBER(): Assigns unique numbers
  
  

45. What are ACID Properties?

Atomicity, Consistency, Isolation, Durability are key properties to ensure reliable transactions.

C. SQL Interview Questions for Experienced

46. Explain different isolation levels in SQL

Isolation levels define the visibility of data changes that one transaction makes to other concurrent transactions. There are four commonly used isolation levels in SQL:

• READ UNCOMMITTED: At this isolation level, transactions can read changes made by other transactions even if those changes have not been committed. While this provides the highest concurrency level, it also introduces the risk of encountering dirty reads.
• READ COMMITTED: In this level, transactions can only read committed data, avoiding dirty reads. However, it may still suffer from non-repeatable reads and phantom reads.
• REPEATABLE READ: Transactions at this level ensure that any data read during the transaction remains unchanged throughout the transaction's lifetime. It prevents non-repeatable reads but may still allow phantom reads.
• SERIALIZABLE: This represents the utmost isolation level, guaranteeing absolute isolation between transactions. While it eradicates all concurrency problems, locking mechanisms may reduce its efficiency.

47. How does a clustered index work, and how is it different from a non-clustered index?

A clustered index defines the actual storage order of rows within a table, allowing for only one clustered index per table and directly influencing the on-disk data organization. Conversely, a non-clustered index does not impact the physical arrangement of data and can coexist with multiple indexes within the same table.

• Clustered Index: When you create a clustered index on a table, the table's rows are physically rearranged to match the order of the indexed column(s). This makes range queries efficient but may slow down insert/update operations.
• Non-clustered Index: Non-clustered indexes are separate data structures that store a copy of a portion of the table's data and point to the actual data rows. They improve read performance but come with some overhead during data modification.

48. Discuss SQL Server Reporting Services

SQL Server Reporting Services is a reporting tool provided by Microsoft for creating, managing, and delivering interactive, tabular, graphical, and free-form reports. SSRS allows users to design and generate reports from various data sources, making it a valuable asset for businesses needing comprehensive reporting capabilities.

49. What are CTEs (Common table expressions)?

Common Table Expressions (CTEs) serve as momentary result sets that you can mention within SQL statements, typically found within SELECT, INSERT, UPDATE, or DELETE operations. They're established using the `WITH` keyword and are instrumental in streamlining intricate queries by dividing them into more digestible components.

50. Explain the MERGE statement

The SQL MERGE statement executes insertions, updates, or deletions on a target table, guided by the outcomes of a source table or query. It consolidates the functionalities of several individual statements (INSERT, UPDATE, DELETE) into one comprehensive statement, rendering it particularly valuable for achieving data synchronization between tables.

51. How do you use a window function in SQL?

Window functions are employed to perform computations on a group of table rows associated with the current row. They enable the generation of result sets containing aggregated data while retaining the distinct details of each row. Typical window functions encompass ROW_NUMBER(), RANK(), DENSE_RANK(), and SUM() OVER().

52. What is a pivot table, and how do you create one in SQL?

A pivot table is a technique for rotating or transposing rows into columns to better analyze and summarize data. You can create pivot tables in SQL using the `PIVOT` operator to convert row-based data into a column-based format.

53. Describe the process of database mirroring

Database mirroring is a high-availability solution in SQL Server that involves creating and maintaining redundant database copies on separate servers. It ensures data availability and disaster recovery by automatically failing over to the mirror server in case of a primary server failure.

54. Explain the concept of table partitioning

Partitioning a table involves breaking down a sizable table into smaller, more easily handled segments known as partitions. This method can enhance SQL query efficiency by permitting the Server to focus solely on pertinent partitions while executing queries. Typically, partitioning uses a column characterized by a high cardinality, such as date or region.

55. How do you handle transactions in distributed databases?

Handling transactions in distributed databases involves ensuring the ACID (Atomicity, Consistency, Isolation, Durability) properties across multiple databases or nodes. This can be achieved through distributed transaction management protocols like Two-Phase Commit (2PC) or by using distributed database systems designed for this purpose.

56. What is the use of the explain plan?

The EXPLAIN plan is a valuable feature in numerous relational database management systems. This tool offers a comprehensive view of the database engine's strategy for executing a query, encompassing details such as the selected execution plan, join techniques, index utilization, and projected costs. Database administrators (DBAs) and developers rely on EXPLAIN plans to enhance the performance of their queries.

57. Discuss SQL Server integration services (SSIS)

Microsoft provides SQL Server Integration Services as a powerful ETL (Extract, Transform, Load) tool. It enables data integration from various sources, data transformation as needed, and data loading into destination systems like data warehouses or databases.

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58. What are indexed views?

Indexed or materialized views are precomputed result sets stored as physical tables in the database. They improve query performance by allowing the database engine to access pre-aggregated or pre-joined data directly from the indexed view, reducing the need for complex query processing.

59. Explain the concept of database sharding

Database sharding is a horizontal partitioning technique that distributes data across multiple database instances or servers. It's commonly used in large-scale systems to improve scalability and performance. Each shard contains a subset of the data, and a sharding strategy determines how data is distributed.

60. How do you manage large-scale databases for performance?

Managing large-scale databases for performance involves various strategies, including proper indexing, partitioning, query optimization, hardware optimization, and caching. Monitoring and fine-tuning the database ensures optimal performance as data volumes grow.

61. What is a materialized view?

Materialized views are a type of database component designed to maintain the outcomes of a query in the form of a tangible table. These views undergo periodic updates to ensure that the stored data remains current. They enhance the efficiency of database queries, particularly for intricate or frequently executed ones.

62. Discuss the strategies for database backup and recovery

Ensuring data availability and disaster recovery relies on implementing vital backup and recovery strategies. These strategies encompass various methods, such as full backups, differential backups, transaction log backups, and regular testing of restoration stored procedures.

63. What are the best practices for securing a SQL database?

Securing a SQL database involves implementing access controls, encryption, auditing, and regular security assessments. Best practices include using strong authentication, limiting permissions, and keeping database systems and software up to date.

64. Explain the concept of database replication

Replication is copying and synchronizing data from one database to another. It ensures data availability, load balancing, and disaster recovery. Common replication types include snapshot, transactional, and merge replication.

65. How do you monitor SQL Server performance?

Monitoring SQL Server performance involves tracking key performance metrics, setting up alerts for critical events, and analyzing performance bottlenecks. Tools like SQL Server Profiler and Performance Monitor are commonly used.

66. What is a database warehouse?

A database warehouse is a centralized repository that stores data from various sources for analytical and reporting purposes. It is optimized for querying and analysis and often contains historical data.

67. Explain the use of full-text search in SQL

Full-text search in SQL allows users to search for text-based data within large text fields or documents. It uses advanced indexing and search algorithms to provide efficient and accurate text-searching capabilities.

68. How do you manage database concurrency?

Database concurrency involves a database system's capability to manage multiple concurrent transactions while upholding data integrity. To achieve this, various techniques such as locking mechanisms, optimistic concurrency control, and isolation levels are employed to oversee and regulate database concurrency.

69. What are the challenges in handling big data in SQL?

Handling big data in SQL involves dealing with large volumes of data that exceed the capabilities of traditional database systems. Challenges include data storage, processing, scalability, and efficient querying. Solutions may include distributed databases and big data technologies like Hadoop and Spark.

70. How do you implement high availability in SQL databases?

High availability in SQL databases ensures that the database remains accessible and operational despite failures. Techniques like clustering, replication, and failover mechanisms help achieve high availability.

71. Explain the use of XML data type in SQL Server

The XML data type allows for storing, retrieving, and manipulating XML data. It also provides support for querying XML documents using XQuery and is commonly used in applications that deal with XML data structures.

72. Discuss the concept of NoSQL databases and their interaction with SQL

NoSQL databases are non-relational databases for handling large volumes of unstructured or semi-structured data. They interact with SQL databases through various integration methods, such as data pipelines, ETL processes, and API-based data transfers.

73. What is a spatial database?

A spatial database stores and queries geometric and geographic data, such as maps, GPS coordinates, and spatial objects. It provides specialized functions and indexing methods to support spatial queries and analysis.

74. How do you migrate a database from one SQL server to another?

Database migration involves moving a database from one SQL server or platform to another. This undertaking demands careful planning, data transfer, schema conversion, and thorough testing to ensure a smooth transition while minimizing the potential for data loss or system downtime.

75. Discuss advanced optimization techniques for SQL queries

Advanced optimization techniques for SQL queries include using query hints, indexing strategies, query rewriting, and understanding the query execution plan. Profiling tools and performance monitoring are essential for identifying and resolving performance bottlenecks.

Alongside these questions, interviews also test your ability to solve real-world coding challenges and apply SQL in scenario-based situations. Many candidates are also asked to contrast SQL vs NoSQL systems, reflecting the diverse data environments companies use today. Just as important, interviewers often look out for common mistakes, like misusing WHERE and HAVING or overlooking performance tuning, that can undermine otherwise strong answers.

D. Query-based SQL Interview Questions and Answers

76. Find the Second Highest Salary

SELECT MAX(salary)

FROM employees

WHERE salary < (SELECT MAX(salary) FROM employees);

This is a classic interview question. The inner query identifies the maximum salary, while the outer query selects the maximum salary that is lower than that, giving the second highest.

77. Find the Nth Highest Salary

SELECT DISTINCT salary

FROM employees e1

WHERE N-1 = (

  SELECT COUNT(DISTINCT salary)

  FROM employees e2

  WHERE e2.salary > e1.salary

);

This query counts how many distinct salaries are higher than the current row’s salary. If that number equals N-1, the row corresponds to the Nth highest salary.

78. Employees Without Managers

SELECT *

FROM employees

WHERE manager_id IS NULL;

Some employees, like CEOs or department heads, may not report to anyone. This query returns all rows where manager_id is NULL.

79. Count Employees by Job Title

SELECT job_title, COUNT(*) AS total

FROM employees

GROUP BY job_title;

GROUP BY clusters employees by job title, and COUNT(*) counts rows in each group. The result shows how many employees hold each role.

80. Find Duplicate Salaries

SELECT salary, COUNT(*)

FROM employees

GROUP BY salary

HAVING COUNT(*) > 1;

This groups records by salary and filters with HAVING to return only salaries that appear more than once. It’s useful for identifying duplicates in a dataset.

81. Customers With No Orders

SELECT c.id, c.name

FROM customers c

WHERE NOT EXISTS (

  SELECT 1 FROM orders o WHERE o.customer_id = c.id

);

The NOT EXISTS clause checks for the absence of related records. This query lists customers who have never placed an order.

82. Employees Hired in the Last 30 Days

SELECT *

FROM employees

WHERE hire_date >= CURRENT_DATE - INTERVAL '30' DAY;

Using date arithmetic, this query filters employees whose hire dates fall within the last 30 days. It’s a practical way to track recent hires.

83. Running Total of Salaries

SELECT employee_id, salary,

       SUM(salary) OVER (ORDER BY employee_id) AS running_total

FROM employees;

The window function SUM() OVER creates a cumulative total of salaries ordered by employee ID. This is often used in reporting and analytics.

84. Top 3 Earners Per Department

SELECT *

FROM (

  SELECT e.*, RANK() OVER (PARTITION BY department_id ORDER BY salary DESC) AS rnk

  FROM employees e

) t

WHERE rnk <= 3;

Explanation:
RANK() OVER (PARTITION BY ...) assigns ranks within each department. Filtering where rnk <= 3 returns the top three earners in every department.

85. Find Customers Who Ordered This Year but Not Last Year

SELECT DISTINCT c.id, c.name

FROM customers c

JOIN orders o ON c.id = o.customer_id

WHERE YEAR(o.order_date) = 2025

AND c.id NOT IN (

  SELECT customer_id

  FROM orders

  WHERE YEAR(order_date) = 2024

);

Explanation:
This query identifies customers with orders in 2025 but excludes those who also ordered in 2024. It’s useful for tracking new or returning customers year-over-year.

Popular SQL Server Interview Questions

86. What is the difference between CAST and CONVERT functions in SQL?

In SQL job interviews, you're often asked about data type conversion, and that’s where CAST() and CONVERT() come in. Both are used to change a value from one data type to another, like turning a string into a date or a number. 

The key difference is that CAST() follows the SQL standard, making it more portable across different databases. CONVERT() is specific to SQL Server and provides more formatting options, especially useful when handling date and time formats. 

During SQL interview preparation, it’s smart to know when to use each depending on the system you're working with.

87. How do you design a database schema for a large-scale application?

The first step is understanding the application requirements clearly. Then, normalize the data to remove redundancy, define clear relationships using foreign keys, and create indexes for faster lookups. 

Also, consider how the system will handle growth, through partitioning, sharding, or caching strategies. A good schema is not just about structure; it’s about designing with performance, scalability, and clarity in mind.

88. How do you implement Data Security and Encryption in SQL?

Data protection often comes up during SQL job interviews, and it's an essential part of working with sensitive databases. 

To implement security, use techniques like hashing passwords with HASHBYTES() or encrypting fields using ENCRYPTBYKEY() in SQL Server. Add role-based access controls so only authorized users can access or modify confidential information. 

Don’t forget to encrypt connections using SSL/TLS. Understanding how to secure data, both in transit and at rest, is a key part of SQL interview preparation, especially for roles involving large-scale applications or regulated industries.

Did You Know? 🔍

Professionals with advanced SQL skills, such as database administrators and database architects, receive a median annual pay of $117,450, and the job growth outlook is 8% in the coming years. (Source: U.S. Bureau of Labor Statistics)

SQL Interview Questions at a Glance

Interviewers usually assess how well you understand SQL, from simple commands to how you apply it in real situations. Let’s look at some of the most common SQL interview questions based on different experience levels:

A. SQL Interview Questions for Freshers

• General SQL Concepts

As a beginner, the interviewer is likely to ask you to elucidate on what the abbreviation SQL stands for and how it can be applied to daily data management. They may ask SQL interview questions about databases, data types that might be stored in a database, and whether you've used any tools or engines in your coursework or job.

• SQL Commands and Usage

You should be comfortable with basic commands like SELECT, INSERT, UPDATE and DELETE. They may give you a couple of tasks to do, say, filter data with WHERE and order the data with ORDER BY. These types of basic SQL interview questions would be included to ensure you have a good sense of how SQL behaves.

• Keys and Rules in Tables

You should be prepared to explain what the term primary key, unique key or foreign key is. These terms are important for keeping the data structured the right way. They may also ask how constraints like NOT NULL or UNIQUE assist in keeping data clean.

• Joins and Handling Missing Data

You’ll likely get questions on combining data from different tables using joins like INNER JOIN or LEFT JOIN. Interviewers also ask how you’d deal with missing values in the data, so knowing how NULL works is useful.

B. SQL Interview Questions for Intermediate

• Working with Groups and Totals

You will likely be presented with questions about calculating totals or averages using SUM, AVG or COUNT. Employers are assessing if you know how to group data using GROUP BY, which is common in reports.

• Joins and Inside Queries

You may be asked questions about joining tables together and using queries inside other queries (subqueries); all are common methods of data analysis in projects, so the interviewers are looking for your ability to easily write those queries easily.

• Combining Results

You might be asked to join results using UNION or find differences using other methods. These types of SQL interview questions and answers show how you handle more than one SQL query result at a time.

• Improving Speed

Some questions may touch on how to make queries run faster. You don’t need to go too deep, but having a basic idea of things like indexes and keeping queries clean can help you stand out.

C. SQL Interview Questions for Experienced

In a senior-level technical SQL interview, you can expect questions that test your knowledge of SQL syntax, database design, query optimization, and problem-solving abilities using SQL. Other technical SQL interview question topics include:

• Enhancing Queries

If you've been working with SQL for any length of time, you can count on being asked about improving and speeding up your queries. SQL interview questions posed to inexperienced candidates will be more focused on how they deal with the slowdown of very large databases while keeping their processes sped up and efficient.

• Managing Data Changes

In addition, employers would like to see how you handle situations requiring more than one user to manage the same data. You might be asked broader committed questions like how you save a change, when you can apply a rollback, or how you address public safety with data revisions.

• Using Stored Procedures

Interviews might also involve complying with, or making use of, stored procedures and triggers which are good solutions for saving reusable SQL processes and automating some of the processes. Generally, interviewers want to learn if you utilize either in your practice.

• Solving Business Problems

When it comes to SQL interview questions for professionals/individuals with 5 years of experience, questions are less about definitions and more about scenarios in your past. So be prepared to talk about how you might have resolved a data problem, fixed or enhanced a report you previously created, or how you might have informed the decision-making process at a team level with SQL data.

Conclusion

Proper preparation for SQL interview questions is important to making it through technical interviews, whether you are a fresh graduate or someone with years of experience. The best way to feel more comfortable with actual interviews is simply by practicing common SQL interview questions, starting with easy ones and then moving on to the harder ones. 

To improve your skills further, consider joining Simplilearn’s SQL Certification Course. It covers everything from basic to advanced concepts and is a great option for anyone serious about acquiring foundational SQL knowledge to help prepare for interviews.

FAQs

1. How can I start learning SQL?

Join Simplilearn’s SQL Certification Course. It covers basics, offers hands-on practice, and is great for beginners.

2. How should I prepare for an SQL interview?

Practice real queries, revise key concepts like SELECT and JOINs, and review common SQL interview questions and answers.

3. What are the most common mistakes candidates make in SQL interviews?

Some frequent mistakes include:

• Using SELECT* instead of fetching required columns.
• Forgetting how to handle NULL values correctly (IS NULL vs = NULL).
• Mixing up WHERE and HAVINGS with aggregates.
• Writing queries without considering indexes or performance.
• Overlooking transaction controls like COMMIT and ROLLBACK.

Avoiding these shows that you not only know SQL syntax but also follow real-world best practices.