SQL Interview Questions 2025

Introduction to SQL

Structured Query Language (SQL) has been the backbone of data management for over four decades, serving as the standard language for relational database management systems (RDBMS). Since its inception in the 1970s at IBM, SQL has evolved from a specialized tool into the universal language for data manipulation, powering everything from small business applications to massive enterprise systems and cutting-edge data science platforms.

What is SQL?

SQL (often pronounced "sequel") is a domain-specific language designed for managing data held in relational database management systems. Unlike general-purpose programming languages, SQL was specifically created to handle structured data, enabling users to define, manipulate, and control access to data in a relational database.

At its core, SQL follows a declarative programming paradigm—you specify what you want to accomplish, not how to accomplish it. The database engine determines the most efficient way to execute your query, abstracting away the complexities of data storage, indexing, and retrieval algorithms.

The Importance of SQL in Today's Data Ecosystem

In our data-driven world, SQL remains indispensable for several reasons:

  1. Universal Adoption: Virtually every major database platform—from traditional systems like Oracle, SQL Server, and MySQL to modern cloud-native solutions like Amazon Redshift, Google BigQuery, and Snowflake—supports SQL.

  2. Integration with Modern Technologies: Despite being developed decades ago, SQL continues to evolve and integrate with modern technologies. It interfaces seamlessly with big data frameworks like Hadoop (through Hive), streaming platforms (via systems like Kafka SQL), and even NoSQL databases (through SQL-like query layers).

  3. Foundation for Data Analytics: SQL serves as the foundation for data analytics and business intelligence. Tools like Tableau, Power BI, and Looker all rely on SQL to transform raw data into actionable insights.

  4. Essential for Data Engineering: Data engineers use SQL to build ETL (Extract, Transform, Load) pipelines, data warehouses, and data lakes, forming the backbone of an organization's data infrastructure.

  5. Critical for Software Development: Most web applications and enterprise software rely on SQL databases to store and retrieve application data, making SQL knowledge essential for backend developers.

  6. Accessible Entry Point to Data Science: SQL often serves as an entry point for aspiring data scientists, providing a relatively straightforward way to start working with structured data before moving to more complex analytical techniques.

The SQL Ecosystem

The SQL ecosystem encompasses various implementations and extensions:

  • Database Management Systems: Major SQL database systems include Oracle Database, Microsoft SQL Server, MySQL, PostgreSQL, SQLite, and many others. Each has its own dialect of SQL with unique features and optimizations.

  • SQL Variants: Different SQL implementations may have variations in syntax and features. Common variants include T-SQL (Microsoft), PL/SQL (Oracle), and SQL/PSM (SQL/Persistent Stored Modules).

  • SQL Standards: The ANSI/ISO SQL standard provides a common framework, though most database systems implement extensions beyond the standard.

  • NewSQL: A class of modern relational database systems that provide the scalability of NoSQL systems while maintaining the ACID guarantees of traditional SQL databases.

Core SQL Concepts

SQL is built around several fundamental concepts:

  1. Data Definition Language (DDL): Commands like CREATE, ALTER, and DROP for defining and modifying database structures.

  2. Data Manipulation Language (DML): Commands like SELECT, INSERT, UPDATE, and DELETE for querying and modifying data.

  3. Data Control Language (DCL): Commands like GRANT and REVOKE for controlling access to data.

  4. Transaction Control Language (TCL): Commands like COMMIT and ROLLBACK for managing transactions.

  5. Relational Model: Data organized in tables (relations) with rows (tuples) and columns (attributes), with relationships established through keys.

  6. ACID Properties: SQL databases traditionally follow ACID principles: Atomicity, Consistency, Isolation, and Durability.

  7. Query Optimization: SQL databases employ sophisticated query optimizers to determine the most efficient execution plan.

SQL in Modern Software Development

In modern development environments, SQL is often used in conjunction with:

  • ORM (Object-Relational Mapping): Frameworks like Hibernate, Entity Framework, and SQLAlchemy that abstract SQL operations through object-oriented interfaces.

  • Microservices: Where SQL databases might be used for specific services requiring transactional integrity.

  • DevOps Practices: Database version control, CI/CD for database changes, and infrastructure as code for database provisioning.

  • Data APIs: GraphQL and REST APIs that may translate client requests into SQL queries.

SQL's Future

Despite the rise of NoSQL databases and alternative data storage technologies, SQL continues to evolve and remain relevant:

  1. Adaptation to Big Data: SQL engines now efficiently process petabytes of data.

  2. Support for Semi-Structured Data: Modern SQL implementations can handle JSON, XML, and other semi-structured formats.

  3. Integration with AI/ML: SQL now includes built-in functions for machine learning and data science workflows.

  4. Cloud-Native Features: SQL databases are adapting to cloud-native architectures, offering serverless options, automatic scaling, and global distribution.

  5. Real-Time Analytics: SQL systems now support real-time analytics on streaming data.

Learning SQL: A Career Investment

Investing time in learning SQL offers significant career benefits:

  1. Versatility: SQL skills are transferable across industries and technologies.

  2. Longevity: SQL has remained relevant for decades and will likely continue to be important.

  3. High Demand: Database professionals consistently rank among the most sought-after tech roles.

  4. Foundation for Advanced Skills: SQL provides a foundation for more advanced data engineering and data science skills.

  5. Business Impact: SQL enables direct access to an organization's most valuable asset: its data.

Whether you're a developer, data analyst, business intelligence specialist, data scientist, or IT administrator, SQL proficiency remains one of the most valuable technical skills you can acquire in today's data-driven world.

The sections that follow provide a comprehensive list of SQL interview questions covering everything from basic concepts to advanced techniques, serving as both a learning resource and interview preparation guide.

SQL Interview Questions

Basics and SQL Fundamentals

  1. What is SQL and what are its primary uses?
  2. Explain the difference between SQL and MySQL.
  3. What are the different sublanguages in SQL?
  4. Explain the difference between DDL, DML, DCL, and TCL commands with examples.
  5. What are constraints in SQL? List and explain the common types.
  6. What is the difference between PRIMARY KEY and UNIQUE constraints?
  7. Explain the concept of NULL in SQL and how it differs from zero or an empty string.
  8. What is normalization? Explain the different normal forms.
  9. What is denormalization and when would you use it?
  10. Explain the concept of a transaction and its properties (ACID).
  11. What is the difference between SQL and NoSQL databases?
  12. Explain the difference between logical and physical database design.
  13. What are the various SQL standards and which one is most widely used?
  14. How does SQL handle data consistency across tables?
  15. What are the benefits of using SQL over file-based storage systems?
  16. Explain the concept of data independence in SQL.
  17. What are database schemas and how are they used?
  18. Explain the difference between OLTP and OLAP database systems.
  19. What is a database instance versus a database schema?
  20. How does SQL enforce data integrity at different levels?

Data Retrieval and Querying

  1. What is the difference between WHERE and HAVING clauses?
  2. Explain the different types of JOINs in SQL with examples.
  3. What is the difference between INNER JOIN and OUTER JOIN?
  4. How does a CROSS JOIN work and when would you use it?
  5. What is the difference between UNION and UNION ALL?
  6. Explain the difference between the LIKE and REGEXP operators.
  7. How do you use wildcards in SQL queries?
  8. What is the order of execution in an SQL query?
  9. Explain the GROUP BY clause and its purpose.
  10. What are aggregate functions? Give examples.
  11. What is the purpose of the DISTINCT keyword?
  12. How do you sort query results using ORDER BY?
  13. Explain the use of the OFFSET FETCH clause.
  14. What is the difference between INTERSECT and EXCEPT operators?
  15. How do you use the ALL, ANY, and SOME operators with subqueries?
  16. What is the purpose of the TOP clause in SQL Server or LIMIT in MySQL?
  17. How do you perform string manipulation in SQL queries?
  18. Explain how NULL values affect the results of joins and aggregations.
  19. What is the difference between a self-join and a recursive query?
  20. How do you use pattern matching effectively in SQL queries?
  21. What is the COALESCE function and how is it used?
  22. Explain the usage of NULLIF and ISNULL functions.
  23. How do you use conditional logic in SELECT statements?
  24. What is the purpose of the WITH TIES option in SQL Server?
  25. Explain how to implement pagination in SQL queries.

Advanced Querying

  1. What are subqueries and how do they work?
  2. What is the difference between correlated and non-correlated subqueries?
  3. Explain the EXISTS operator with an example.
  4. How do you use the CASE statement in SQL?
  5. What are Common Table Expressions (CTEs) and how are they useful?
  6. Explain window functions and their applications.
  7. What is the difference between ROW_NUMBER(), RANK(), and DENSE_RANK()?
  8. How would you find the Nth highest salary in a table?
  9. Explain how to pivot data in SQL.
  10. What are recursive queries and when would you use them?
  11. How do you use analytical functions like LAG and LEAD?
  12. What is the purpose of FIRST_VALUE and LAST_VALUE functions?
  13. Explain the use of GROUPING SETS, ROLLUP, and CUBE.
  14. How do you implement running totals and moving averages?
  15. What is a lateral join and when would you use it?
  16. How do you handle hierarchical data queries in SQL?
  17. Explain the concept of window frame specification in window functions.
  18. What are the EXCLUDE and RESPECT NULLS options in window functions?
  19. How do you implement gap and island problems in SQL?
  20. What is the difference between a derived table and a CTE?
  21. How do you use the WITH RECURSIVE clause in PostgreSQL?
  22. Explain how to perform complex aggregations using multiple window functions.
  23. How do you use NTILE() and PERCENT_RANK() functions?
  24. What are table-valued functions and how do they differ from scalar functions?
  25. How do you handle complex date-based calculations in SQL?

Data Manipulation and Modification

  1. Explain the difference between DELETE, TRUNCATE, and DROP.
  2. How do you insert data into a table from another table?
  3. What is the MERGE statement and how is it used?
  4. How do you update multiple columns in a single statement?
  5. Explain the concept of transactions in SQL.
  6. What are savepoints in a transaction?
  7. How do you handle error conditions in SQL transactions?
  8. What is the difference between implicit and explicit transactions?
  9. Explain how to use the OUTPUT clause with DML statements.
  10. What happens if a transaction is not explicitly committed or rolled back?
  11. How do you perform bulk insert operations?
  12. What are the different isolation levels in transactions?
  13. Explain the concept of transaction logs.
  14. How do you implement optimistic concurrency control?
  15. What are deadlocks and how can they be minimized?
  16. Explain the usage of INSTEAD OF triggers.
  17. How do you handle constraint violations during data modifications?
  18. What is the impact of referential integrity on DELETE operations?
  19. How do you perform conditional updates based on multiple criteria?
  20. Explain the concept of deferred constraints.
  21. What is the difference between physical and logical deletion of data?
  22. How do you implement soft deletes in SQL?
  23. Explain how to use the RETURNING clause in PostgreSQL.
  24. What are the best practices for large data modification operations?
  25. How do you log data changes in SQL tables?

Indexes and Performance

  1. What are indexes and why are they important?
  2. What is the difference between clustered and non-clustered indexes?
  3. What is a composite index and when would you use one?
  4. How do indexes impact INSERT, UPDATE, and DELETE operations?
  5. What are statistics in SQL and how do they affect query performance?
  6. Explain query optimization techniques in SQL.
  7. What is a query execution plan and how do you analyze it?
  8. How do you identify and resolve performance bottlenecks in SQL queries?
  9. What is index fragmentation and how do you address it?
  10. Explain the concept of query hints and when to use them.
  11. What is a covering index and when would you use it?
  12. Explain filtered indexes in SQL Server.
  13. How do you choose which columns to index?
  14. What is index selectivity and why is it important?
  15. How do you measure the performance impact of an index?
  16. What are bitmap indexes and when are they useful?
  17. Explain the concept of index tuning.
  18. What is the impact of data types on index performance?
  19. How do you handle index maintenance?
  20. What are spatial indexes and when would you use them?
  21. Explain the concept of included columns in indexes.
  22. How do indexes affect sorting operations?
  23. What is cardinality and how does it affect index performance?
  24. Explain the different index scan types (Index Seek vs. Index Scan).
  25. How do you diagnose index-related performance issues?

Views and Stored Procedures

  1. What is a view in SQL and what are its benefits?
  2. What is the difference between a simple view and a complex view?
  3. What are materialized views and when would you use them?
  4. What are stored procedures and their advantages?
  5. Explain the difference between stored procedures and functions.
  6. What are the various types of parameters in stored procedures?
  7. How do you handle errors in stored procedures?
  8. What is the difference between EXEC and sp_executesql?
  9. Explain the concept of dynamic SQL.
  10. What are triggers and when would you use them?
  11. How do you pass multiple values to a stored procedure?
  12. What is a table-valued parameter and how is it used?
  13. Explain the difference between AFTER and INSTEAD OF triggers.
  14. How do you create indexed views in SQL Server?
  15. What is the difference between a view and a derived table?
  16. How do you implement error handling in stored procedures?
  17. What are the security implications of using stored procedures?
  18. Explain the concept of nested stored procedures.
  19. How do you debug stored procedures?
  20. What is the maximum number of parameters a stored procedure can have?
  21. How do you handle transaction management within stored procedures?
  22. What are temporary stored procedures and when would you use them?
  23. Explain the scope of variables in stored procedures.
  24. How do you implement paging in stored procedures?
  25. What are the limitations of views in SQL?

Data Integrity and Constraints

  1. How do you enforce data integrity in SQL?
  2. Explain referential integrity and how it's maintained.
  3. What is cascading in foreign key constraints?
  4. How do you implement check constraints?
  5. What are DEFAULT constraints and how are they used?
  6. Explain the concept of table partitioning.
  7. What is the difference between horizontal and vertical partitioning?
  8. How do you implement unique constraints across multiple columns?
  9. What are the limitations of constraints in SQL?
  10. How do constraints affect performance?
  11. What is domain integrity and how is it enforced?
  12. Explain the concept of assertion constraints.
  13. How do you implement conditional constraints?
  14. What is the impact of constraints on DML operations?
  15. Explain how to use computed columns with constraints.
  16. What are filtered unique constraints in SQL Server?
  17. How do you implement complex business rules using constraints?
  18. What is the difference between an enforced and a trusted constraint?
  19. How do you handle constraint violations programmatically?
  20. Explain how to implement cross-database constraints.
  21. What are temporal tables and how do they maintain data integrity?
  22. How do you implement data validation beyond basic constraints?
  23. What is the difference between primary key and unique key constraints?
  24. How do you handle foreign key constraints in data warehousing?
  25. Explain the concept of deferrable constraints in PostgreSQL.

Transactions and Concurrency

  1. What are the different transaction isolation levels in SQL?
  2. Explain the problems that can occur in concurrent transactions (dirty reads, non-repeatable reads, phantom reads).
  3. What is deadlock and how can it be prevented?
  4. Explain optimistic vs. pessimistic locking.
  5. What is row versioning in SQL?
  6. How does READ COMMITTED SNAPSHOT isolation level work?
  7. What is lock escalation and when does it occur?
  8. How do you handle long-running transactions?
  9. What is the difference between implicit and explicit transactions?
  10. Explain how to use the WITH NOLOCK hint and its implications.
  11. What is lock granularity and how does it affect performance?
  12. How do you monitor locks in a database system?
  13. What is a transaction log and how is it used in recovery?
  14. Explain the concept of two-phase commit.
  15. How do you handle distributed transactions?
  16. What is the impact of isolation levels on concurrency and throughput?
  17. Explain how row versioning affects tempdb usage.
  18. What are the different lock types (shared, exclusive, update, etc.)?
  19. How do you handle blocking transactions?
  20. What is a transaction savepoint and how is it used?
  21. Explain the concept of transaction chaining.
  22. How do you implement retry logic for handling deadlocks?
  23. What is a transaction schedule and serializability?
  24. How do you implement optimistic concurrency in applications?
  25. What are the best practices for transaction management in high-concurrency environments?

Database Design

  1. What are entity-relationship diagrams (ERDs) and how are they used?
  2. Explain the different types of relationships in database design (one-to-one, one-to-many, many-to-many).
  3. How do you implement a many-to-many relationship in SQL?
  4. What is cardinality in database design?
  5. Explain the concept of database normalization and its levels.
  6. What are the advantages and disadvantages of normalization?
  7. When would you denormalize a database schema?
  8. How do you design for performance versus design for maintainability?
  9. What are best practices for naming conventions in SQL?
  10. How do you handle temporal data in database design?
  11. What is a star schema versus a snowflake schema?
  12. How do you model hierarchical data in a relational database?
  13. What are slowly changing dimensions and their types?
  14. Explain the concept of database sharding.
  15. What is indexing strategy in database design?
  16. How do you handle historical data versus current data?
  17. What is the role of surrogate keys versus natural keys?
  18. How do you design databases for scalability?
  19. What is the impact of normalization on query performance?
  20. Explain how to model polymorphic associations in SQL.
  21. What are design patterns for handling multi-tenant databases?
  22. How do you design for data warehousing versus operational databases?
  23. What are the considerations for international data in database design?
  24. Explain how to handle soft deletes at the database level.
  25. What are the best practices for designing audit trails?

Advanced SQL Concepts

  1. What are CTEs (Common Table Expressions) and how are they different from temporary tables?
  2. Explain the PIVOT and UNPIVOT operators.
  3. What are JSON functions in SQL and how are they used?
  4. How do you handle hierarchical data in SQL?
  5. What is the difference between a deterministic and non-deterministic function?
  6. Explain how to use ROLLUP, CUBE, and GROUPING SETS.
  7. What are the uses of the APPLY operator (CROSS APPLY and OUTER APPLY)?
  8. How do you implement paging in SQL queries?
  9. What are the differences between SQL in various database systems (MySQL, PostgreSQL, SQL Server, Oracle)?
  10. Explain SQL injection and how to prevent it.
  11. What are XML functions in SQL and how are they used?
  12. Explain the concept of OLAP cubes and MDX.
  13. How do you implement full-text search in SQL?
  14. What are spatial data types and functions?
  15. Explain how to use the MERGE statement for upserting data.
  16. What are table partitioning strategies?
  17. How do you implement change data capture (CDC) in SQL?
  18. What is columnstore indexing and when should it be used?
  19. Explain memory-optimized tables and natively compiled stored procedures.
  20. How do you implement data masking for sensitive information?
  21. What are the advantages and limitations of using SQL for ETL processes?
  22. Explain how to use SEQUENCE objects instead of identity columns.
  23. What are temporal tables and how are they implemented?
  24. How do you handle versioning in database schemas?
  25. What are the benefits of using Graph tables in SQL Server?

Data Types and Functions

  1. What are the common data types in SQL?
  2. Explain the differences between VARCHAR and CHAR data types.
  3. How do you handle date and time data in SQL?
  4. What are the different string manipulation functions in SQL?
  5. Explain numeric and mathematical functions in SQL.
  6. How do you convert between different data types in SQL?
  7. What are collations and how do they affect string comparisons?
  8. How do you handle NULL values in functions and calculations?
  9. What are user-defined functions and when would you use them?
  10. Explain aggregate functions and their limitations.
  11. What is the difference between VARCHAR and NVARCHAR data types?
  12. How do you handle Unicode data in SQL?
  13. Explain the use of the CAST and CONVERT functions.
  14. What are the differences between DATETIME and DATETIME2 data types?
  15. How do you handle time zone conversions in SQL?
  16. What are the limitations of numeric data types?
  17. Explain how to use the FORMAT function for displaying data.
  18. What are binary data types and when would you use them?
  19. How do you handle complex mathematical calculations in SQL?
  20. What is the difference between scalar and table-valued functions?
  21. Explain how to use CLR integration for custom data types.
  22. What are hierarchyid and spatial data types?
  23. How do you optimize storage with data compression?
  24. What are the differences between functions like ISNULL, COALESCE, and NULLIF?
  25. Explain how to use window functions with different data types.

Security and Administration

  1. How do you manage user permissions in SQL?
  2. What is the difference between authentication and authorization in database security?
  3. Explain the concept of roles in SQL security.
  4. How do you encrypt sensitive data in SQL databases?
  5. What are the best practices for SQL database backup and recovery?
  6. How do you monitor database performance?
  7. What tools would you use for SQL query optimization?
  8. Explain how to implement row-level security.
  9. What are database auditing techniques?
  10. How do you handle database migrations and schema changes?
  11. What is the principle of least privilege and how do you apply it?
  12. Explain transparent data encryption (TDE).
  13. How do you handle SQL Server login security versus database user security?
  14. What are the different types of database backups and their recovery models?
  15. How do you implement always-on availability groups?
  16. What is database mirroring versus replication?
  17. Explain how to handle database corruption.
  18. What are the best practices for SQL Server log management?
  19. How do you implement disaster recovery for SQL databases?
  20. What are security considerations for stored procedures and views?
  21. How do you implement data masking and column-level encryption?
  22. What is the difference between contained databases and traditional databases?
  23. Explain how to use extended events for monitoring.
  24. What are resource governor features and how do they help in multi-tenant environments?
  25. How do you manage large-scale database deployments?

TOP 50 SQL Interview Queries with Solutions

Basic SQL Queries (1-10)

1. Fetch "FIRST_NAME" from Worker table using the alias name as <WORKER_NAME>

SELECT first_name AS WORKER_NAME
FROM worker;

2. Fetch "FIRST_NAME" from Worker table in upper case

SELECT UPPER(first_name)
FROM worker;

3. Fetch unique values of DEPARTMENT from Worker table

SELECT DISTINCT department
FROM worker;

4. Print the first three characters of FIRST_NAME from Worker table

SELECT SUBSTRING(first_name, 1, 3)
FROM worker;

5. Find the position of the alphabet ('b') in the first name column 'Amitabh'

SELECT INSTR(first_name, 'B')
FROM worker
WHERE first_name = 'Amitabh';

6. Print FIRST_NAME after removing white spaces from the right side

SELECT RTRIM(first_name)
FROM worker;

7. Print DEPARTMENT after removing white spaces from the left side

SELECT LTRIM(first_name)
FROM worker;

8. Fetch unique values of DEPARTMENT and print their length

SELECT DISTINCT department, LENGTH(department)
FROM worker;

9. Print FIRST_NAME after replacing 'a' with 'A'

SELECT REPLACE(first_name, 'a', 'A')
FROM worker;

10. Print FIRST_NAME and LAST_NAME in a single column COMPLETE_NAME with a space separator

SELECT CONCAT(first_name, ' ', last_name) AS COMPLETE_NAME
FROM worker;

Sorting and Filtering (11-20)

11. Print all Worker details ordered by FIRST_NAME Ascending

SELECT *
FROM worker
ORDER BY first_name;

12. Print all Worker details ordered by FIRST_NAME Ascending and DEPARTMENT Descending

SELECT *
FROM worker
ORDER BY first_name, department DESC;

13. Print details for Workers with first name as "Vipul" and "Satish"

SELECT *
FROM worker
WHERE first_name IN ('Vipul', 'Satish');

14. Print details of workers excluding first names, "Vipul" and "Satish"

SELECT *
FROM worker
WHERE first_name NOT IN ('Vipul', 'Satish');

15. Print details of Workers with DEPARTMENT name as "Admin*"

SELECT *
FROM worker
WHERE department LIKE 'Admin%';

16. Print details of Workers whose FIRST_NAME contains 'a'

SELECT *
FROM worker
WHERE first_name LIKE '%a%';

17. Print details of Workers whose FIRST_NAME ends with 'a'

SELECT *
FROM worker
WHERE first_name LIKE '%a';

18. Print details of Workers whose FIRST_NAME ends with 'h' and contains six alphabets

SELECT *
FROM worker
WHERE first_name LIKE '_____h';

19. Print details of Workers whose SALARY is between 100000 and 500000

SELECT *
FROM worker
WHERE salary BETWEEN 100000 AND 500000;

20. Print details of Workers who joined in Feb'2014

SELECT *
FROM worker
WHERE YEAR(joining_date) = 2014 AND MONTH(joining_date) = 02;

Aggregation and Grouping (21-25)

21. Fetch the count of employees working in the department 'Admin'

SELECT department, COUNT(*)
FROM worker
WHERE department = 'Admin';

22. Fetch worker full names with salaries between 50000 and 100000

SELECT CONCAT(first_name, ' ', last_name)
FROM worker
WHERE salary BETWEEN 50000 AND 100000;

23. Fetch the number of workers for each department in descending order

SELECT department, COUNT(worker_id) AS no_of_worker
FROM worker
GROUP BY department
ORDER BY no_of_worker DESC;

24. Print details of Workers who are also Managers

SELECT w.*
FROM worker AS w
INNER JOIN title AS t ON w.worker_id = t.worker_ref_id
WHERE t.worker_title = 'Manager';

25. Fetch titles with more than one worker

SELECT worker_title, COUNT(*) AS count
FROM title
GROUP BY worker_title
HAVING count > 1;

Advanced Queries (26-35)

26. Show only odd rows from the worker table

SELECT *
FROM worker
WHERE MOD(WORKER_ID, 2) <> 0;

27. Show only even rows from the worker table

SELECT *
FROM worker
WHERE MOD(WORKER_ID, 2) = 0;

28. Clone a table from another table

CREATE TABLE worker_clone LIKE worker;
INSERT INTO worker_clone SELECT * FROM worker;
SELECT * FROM worker_clone;

29. Fetch intersecting records of two tables

SELECT worker.*
FROM worker
INNER JOIN worker_clone USING(worker_id);

30. Show records from one table that another table does not have

SELECT worker.*
FROM worker
LEFT JOIN worker_clone USING(worker_id)
WHERE worker_clone.worker_id IS NULL;

31. Show the current date and time

SELECT CURDATE();
SELECT NOW();

32. Show the top 5 records with highest salary

SELECT *
FROM worker
ORDER BY salary DESC
LIMIT 5;

33. Determine the 5th highest salary

SELECT *
FROM worker
ORDER BY salary DESC
LIMIT 4,1;

34. Determine the 5th highest salary without using LIMIT

SELECT salary
FROM worker w1
WHERE 4 = (
    SELECT COUNT(DISTINCT (w2.salary))
    FROM worker w2
    WHERE w2.salary >= w1.salary
);

35. Fetch employees with the same salary

SELECT w1.*
FROM worker w1, worker w2
WHERE w1.salary = w2.salary AND w1.worker_id != w2.worker_id;

Subqueries and Complex Operations (36-50)

36. Show the second highest salary using a sub-query

SELECT MAX(salary)
FROM worker
WHERE salary NOT IN (SELECT MAX(salary) FROM worker);

37. Show one row twice in results

SELECT * FROM worker
UNION ALL
SELECT * FROM worker
ORDER BY worker_id;

38. List worker_id who does not get bonus

SELECT worker_id
FROM worker
WHERE worker_id NOT IN (SELECT worker_ref_id FROM bonus);

39. Fetch the first 50% records from the worker table

SELECT *
FROM worker
WHERE worker_id <= (SELECT COUNT(worker_id)/2 FROM worker);

40. Fetch departments with less than 4 people

SELECT department, COUNT(department) AS depCount
FROM worker
GROUP BY department
HAVING depCount < 4;

41. Show all departments with the number of employees

SELECT department, COUNT(department) AS depCount
FROM worker
GROUP BY department;

42. Show the last record from the worker table

SELECT *
FROM worker
WHERE worker_id = (SELECT MAX(worker_id) FROM worker);

43. Fetch the first row of the worker table

SELECT *
FROM worker
WHERE worker_id = (SELECT MIN(worker_id) FROM worker);

44. Fetch the last five records from the worker table

(SELECT *
FROM worker
ORDER BY worker_id DESC
LIMIT 5)
ORDER BY worker_id;

45. Print names of employees with the highest salary in each department

SELECT w.department, w.first_name, w.salary
FROM (SELECT MAX(salary) AS maxsal, department
      FROM worker
      GROUP BY department) temp
INNER JOIN worker w ON temp.department = w.department AND temp.maxsal = w.salary;

46. Fetch three highest salaries using a co-related subquery

SELECT DISTINCT salary
FROM worker w1
WHERE 3 >= (
    SELECT COUNT(DISTINCT salary)
    FROM worker w2
    WHERE w1.salary <= w2.salary
)
ORDER BY w1.salary DESC;

-- Alternative solution
SELECT DISTINCT salary
FROM worker
ORDER BY salary DESC
LIMIT 3;

47. Fetch three lowest salaries using a co-related subquery

SELECT DISTINCT salary
FROM worker w1
WHERE 3 >= (
    SELECT COUNT(DISTINCT salary)
    FROM worker w2
    WHERE w1.salary >= w2.salary
)
ORDER BY w1.salary DESC;

48. Fetch nth highest salary from the worker table

SELECT DISTINCT salary
FROM worker w1
WHERE n >= (
    SELECT COUNT(DISTINCT salary)
    FROM worker w2
    WHERE w1.salary <= w2.salary
)
ORDER BY w1.salary DESC;

49. Fetch departments with their total salary amounts

SELECT department, SUM(salary) AS depSal
FROM worker
GROUP BY department
ORDER BY depSal DESC;

50. Fetch names of workers with the highest salary

SELECT first_name, salary
FROM worker
WHERE salary = (SELECT MAX(Salary) FROM worker);

Conclusion

SQL remains one of the most valuable technical skills in today's data-driven world. Whether you're preparing for an interview, refreshing your knowledge, or learning SQL for the first time, understanding these concepts will provide a strong foundation for working with relational databases.

Remember that the best way to learn SQL is through practice. Try implementing these concepts in a real database system, work through sample problems, and apply your knowledge to real-world scenarios. With persistence and practice, you can become proficient in SQL and unlock the full potential of relational data.

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DBMS Interview Questions

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