20 Most Essential SQL Query Interview Questions & Answers

Structured Query Language (SQL) continues to be an essential skill for data professionals. SQL can be your ticket to getting ready for your job or next step (senior data) goal. This blog maps out the top 20 SQL query interview questions that recruiters often ask, along with their answers at the end, to help you prepare now!

These SQL query interview questions and answers are curated to simulate real-world interviewing situations and provided with theory and practical examples.

1. What is SQL?

SQL is an acronym for Structured Query Language. The language is meant to work with databases, i.e., to get, add, change, or remove data. This query is the most frequently asked in SQL interviews, as it basically explains what SQL is.

2. What are the different types of SQL commands?

The significant SQL interview questions usually start with an understanding of the types of commands available:

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

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

WHERE filters rows before grouping, whereas HAVING filters them after grouping. This is an old and most-asked SQL query in an interview.

4. What is a JOIN? State its types.

A JOIN joins rows from two or more tables based on a related column between them. Most asked SQL queries in interviews also have:

• INNER JOIN
  
  
• LEFT JOIN
  
  
• RIGHT JOIN
  
  
• FULL JOIN
  
  

5. What is a Primary Key?

A Primary Key uniquely identifies every record within a table. This is one of the top 20 SQL query interview questions because it is critical for database design.

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

Both enforce uniqueness. However, a Primary Key does not allow nulls and a Unique Key does allow nulls.

7. What are Aggregate Functions in SQL?

They provide a single value derived from a set of values:

• COUNT()
  
  
• SUM()
  
  
• AVG()
  
  
• MAX()
  
  
• MIN()
  
  

These are important SQL queries for interview because they’re commonly used in analytics.

8. How do you find duplicate records in a table?

SELECT column_name, COUNT(*)

FROM table_name

GROUP BY column_name

HAVING COUNT(*) > 1;

This is a favorite among SQL query interview questions and answers for practical assessments.

9. How can you delete duplicate rows but keep one?

DELETE FROM employees

WHERE id NOT IN (

  SELECT MIN(id)

  FROM employees

  GROUP BY email

);

This question ranks high in most asked SQL queries in interview lists due to its complexity.

10. What is a Subquery?

A Subquery is a query within a query. Subqueries are usually used in the top 20 SQL query interview questions when testing how well someone understands nested logic.

11. What is the difference between IN and EXISTS?

IN: Runs the subquery and returns the result to the outer query.

EXISTS: Return TRUE if the subquery returns any rows.

This is a tricky but important SQL query for interviews.

12. What is a View?

A View is a virtual table based on the output of an SQL query. A view is often used for security and simplifying data, and it regularly appears in SQL query interview questions.

13. How do you retrieve the second-highest salary?

SELECT MAX(salary)

FROM employees

WHERE salary < (

  SELECT MAX(salary) FROM employees

);

This is a question that requires logical problem-solving, and it is among the top 20 SQL queries interview questions.

14. What is Constraints in SQL?

Constraints are rules that are enforced on the data in tables (ex: NOT NULL, CHECK, UNIQUE). This is one of the SQL query interview questions and answers that shows knowledge of data integrity.

15. What is Normalization?

Normalization is a way to avoid data redundancy. It is a typical theory-based question for interviewing on important SQL queries.

16. What is the use of GROUP BY?

GROUP BY aggregates rows sharing the same values. It’s usually combined with aggregate functions, and it is important in most common SQL queries asked in interview rounds.

17. What is the difference of TRUNCATE and DELETE?

DELETE: deletes specific rows, can be roll backed.

TRUNCATE: permanently deletes all rows, cannot be rollbacked.

This is often found within SQL query interview questions and answers to test deeper knowledge.

18. What is a Self Join?

A Self Join is just a regular join, but the table is joined with itself. This is one of the important SQL queries for an interview that tests relationships.

19. How do you fetch the first N records?

SELECT * FROM employees LIMIT 5;

Or for SQL Server:

SELECT TOP 5 * FROM employees;

This is simple but it covers the most asked SQL queries in the interview coding rounds.

20. How do you use CASE in SQL?

SELECT name,

  CASE

    WHEN salary > 50000 THEN ‘High’

    ELSE ‘Low’

  END AS salary_range

FROM employees;

This is a reasoning-based query that is frequently found among the top 20 SQL query interview questions.

Final Thoughts

Interview preparation involves knowledge of both concepts and practical applications. The SQL query interview questions and answers provided below are handpicked to represent actual use cases and interviewee expectations.

From fundamentals such as JOINs to complex queries with subqueries and aggregation, these vital SQL interview questions for prep will provide you with a solid advantage. Bookmark this guide and practice these top 20 SQL query interview questions prior to your next technical round!