Mastering SQL: 30 Essential Interview Questions and Comprehensive Answers

Whether you're gearing up for a job interview, brushing up on your SQL skills, or just eager to deepen your understanding of database management, this blog is your go-to resource. Join us on a journey to master SQL as we explore fundamental concepts, query optimization, transaction management, and more. Let's unlock the power of databases together! Here are 30 SQL interview questions along with their answers:

1. What is SQL?

- Answer: SQL stands for Structured Query Language. It is a programming language designed for managing and manipulating relational databases.

2. Explain the difference between SQL and MySQL.

- Answer: SQL is a language used for managing relational databases, while MySQL is an open-source relational database management system that uses SQL as its language.

3. What is a primary key?

- Answer: A primary key is a unique identifier for a record in a table. It ensures that each record can be uniquely identified and is used to establish relationships between tables.

4. Explain the difference between INNER JOIN and LEFT JOIN.

- Answer: INNER JOIN returns only the matching rows in both tables, while LEFT JOIN returns all rows from the left table and the matching rows from the right table.

5. What is normalization?

- Answer: Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity.

6. What is the purpose of the GROUP BY clause?

- Answer: The GROUP BY clause is used to group rows that have the same values in specified columns into summary rows, like calculating aggregate functions (e.g., COUNT, SUM, AVG).

7. Explain the difference between DELETE and TRUNCATE.

- Answer: DELETE is used to remove specific rows from a table based on a condition, while TRUNCATE is used to remove all rows from a table without any condition.

8. What is a foreign key?

- Answer: A foreign key is a field in a table that refers to the primary key in another table. It establishes a link between the two tables.

9. What is a stored procedure?

- Answer: A stored procedure is a precompiled collection of one or more SQL statements that can be executed as a single unit. It is stored in the database and can be called by name.

10. Explain the difference between UNION and UNION ALL.

- Answer: UNION combines the results of two SELECT statements and removes duplicate rows, while UNION ALL includes all rows, including duplicates.

11. What is the purpose of the HAVING clause?

- Answer: The HAVING clause is used in conjunction with the GROUP BY clause to filter the results based on a specified condition for grouped data.

12. What is ACID in the context of databases?

- Answer: ACID stands for Atomicity, Consistency, Isolation, and Durability, which are properties that ensure the reliability of database transactions.

13. Explain the difference between a view and a table.

- Answer: A table is a collection of data, while a view is a virtual table that is based on the result of a SELECT query. Views do not store data themselves but provide a way to represent data from one or more tables.

14. What is the purpose of the ORDER BY clause?

- Answer: The ORDER BY clause is used to sort the result set of a query in ascending or descending order based on one or more columns.

15. How do you eliminate duplicate values from a SELECT query result?

- Answer: Use the DISTINCT keyword in the SELECT statement to eliminate duplicate values.

16. Explain the difference between a clustered and a non-clustered index.

- Answer: A clustered index determines the physical order of data in a table, while a non-clustered index does not affect the physical order but provides a logical order for faster retrieval.

17. What is a self-join?

- Answer: A self-join is a regular join, but the table is joined with itself. It is used to combine rows with related data in the same table.

18. What is the purpose of the LIKE operator in SQL?

- Answer: The LIKE operator is used in a WHERE clause to search for a specified pattern in a column.

19. Explain the purpose of the COMMIT and ROLLBACK statements.

- Answer: COMMIT is used to save the changes made during the current transaction, while ROLLBACK is used to undo the changes made during the current transaction.

20. What is the difference between a unique key and a primary key?

- Answer: Both enforce uniqueness, but a table can have multiple unique keys, while it can have only one primary key.

21. What is a subquery?

- Answer: A subquery is a query nested within another query. It can be used to retrieve data that will be used in the main query as a condition.

22. Explain the purpose of the BETWEEN operator.

- Answer: The BETWEEN operator is used to filter the result set within a specified range. It is inclusive of the specified values.

23. What is the purpose of the COUNT() function?

- Answer: The COUNT() function is used to count the number of rows in a result set or the number of occurrences of a specific value in a column.

24. Explain the concept of a transaction in SQL.

- Answer: A transaction is a sequence of one or more SQL statements that are executed as a single unit. It follows the ACID properties and ensures data consistency.

25. How do you add a new record to a table?

- Answer: Use the INSERT INTO statement to add a new record to a table.

26. What is the purpose of the MAX() and MIN() functions?

- Answer: The MAX() function returns the maximum value in a column, while the MIN() function returns the minimum value.

27. Explain the difference between a candidate key, a primary key, and a super key.

- Answer: A super key is any set of columns that uniquely identifies a row, a candidate key is a minimal super key, and a primary key is a selected candidate key.

28. What is the purpose of the SQL CASE statement?

- Answer: The CASE statement is used to perform conditional logic within a SQL query, similar to the IF-THEN-ELSE statement in other programming languages.

29. Explain the purpose of the ROW_NUMBER() function.

- Answer: The ROW_NUMBER() function is used to assign a unique number to each row within a partition of a result set, typically used for ranking.

30. How can you update data in a table using SQL?

- Answer: Use the UPDATE statement to modify existing records in a table based on a specified condition.

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