Top SQL Interview Questions.

When preparing for a SQL (Structured Query Language) interview, it is critical to be well-versed in database concepts, SQL syntax, and the ability to write complicated queries. 

Top SQL interview questions covering a variety of areas are listed below:

1. What is SQL, and what is its use?

Explain the purpose of SQL and its role in managing and manipulating relational databases.

2. Differentiate between SQL and NoSQL databases.

Discuss the key differences between SQL (relational) and NoSQL (non-relational) databases.

3. Explain the primary key in a database.

Define what a primary key is and its importance in a relational database.

4. What is a foreign key?

Describe the concept of a foreign key and its role in establishing relationships between tables.

5. What is normalization?

Define normalization and explain its importance in database design.

6. What is denormalization?

Explain the concept of denormalization and when it might be used.

7. What are indexes, and why are they important?

Discuss the purpose of indexes in a database and how they improve query performance.

8. Write a SQL query to retrieve data from a table.

Provide a simple SELECT statement to retrieve data from a specific table.

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

Compare INNER JOIN and LEFT JOIN and explain when to use each.

10. Explain the GROUP BY clause.

Describe how the GROUP BY clause works and provide an example.

11. What is the HAVING clause used for?

Discuss the purpose of the HAVING clause in SQL.

12. Write a SQL query to find the second-highest salary from an Employee table.

Provide a query to find the second-highest salary from an Employee table.

13. How do you prevent SQL injection attacks?

Discuss strategies to prevent SQL injection attacks in database queries.

14. What is a subquery?

Define a subquery and provide an example of its usage.

15. Explain the difference between UNION and UNION ALL.

Differentiate between UNION and UNION ALL and discuss their use cases.

16. Write a SQL query to calculate the total count of rows in a table.

Provide a query to count the total number of rows in a specific table.

17. What is a stored procedure?

Define a stored procedure and discuss its advantages in database management.

18. How do you delete duplicate records from a table?

Provide a SQL query to remove duplicate records from a table.

19. Explain the concept of ACID properties in database transactions.

Discuss the four ACID properties (Atomicity, Consistency, Isolation, Durability) and their significance.

20. Write a SQL query to find the nth highest salary from an Employee table.

Provide a query to find the nth highest salary from an Employee table. 

"These questions cover a wide range of SQL principles, from fundamental to sophisticated, and will help you prepare for different levels of SQL interviews. It's critical to get experience writing SQL queries and to comprehend how to use these ideas in practical situations."