Landing a job as an experienced SQL developer often involves acing a series of interviews, which frequently include questions to evaluate your proficiency in Structured Query Language. In this comprehensive guide, we delve into the top 100 SQL interview questions designed for experienced professionals, categorized by topic to help you prepare effectively.
Before we dive into the questions, remember that the key to acing any interview lies in practice, understanding the underlying concepts, and maintaining a calm and confident demeanor. Keep reading to discover a wealth of SQL interview questions and their answers.
Core SQL Concepts
Believe it or not, interviewers still expect you to nail the basics. Brush up on your fundamental SQL knowledge to impress your interviewer.
These questions test your understanding of fundamental SQL syntax, commands, and principles, such as SELECT, INSERT, UPDATE, DELETE, and Basic Join Operations. Make sure you're comfortable writing efficient and correct queries for these essential tasks.
Basic Queries
1. **How would you retrieve the first 10 rows from a table?** ```sql SELECT * FROM TableName LIMIT 10; ```
8. **Describe the difference between SELECT and SELECT DISTINCT.** - SELECT retrieves all unique values based on the conditions defined in the WHERE clause, while SELECT DISTINCT returns only distinct (unique) values from the selected columns.
DML Operations
21. **Write a query to update a specific record where the employee's salary is over 50000?** ```sql UPDATE Employees SET Salary = Salary * 1.10 WHERE Salary > 50000; ```
37. **Delete all rows from a specific table that are older than 30 days. What precautions should you take before running such a query?** - Always backup the table before running deletes. Run the query without the DELETE statement first to ensure the results are as expected. Then, comment out or remove the SELECT statement and uncomment or add the DELETE statement to execute the delete.
Advanced SQL
Mastering advanced SQL concepts will help you stand out and ace your interview. Brush up on these topics to show your interrogator just how skilled you are.
Topics in this section include Joins, Subqueries, Aggregate Functions, GROUP BY and HAVING clauses, Window Functions, and more. Use our questions and answers as a starting point for your preparation.
Joins and Subqueries
56. **Write a query to find the second-highest salary in the Employees table. Use a subquery.** ```sql SELECT MAX(Salary) FROM Employees WHERE Salary < (SELECT MAX(Salary) FROM Employees); ```
72. **Write a query to find the number of unique cities in the Employees table, along with the total number of employees from each city.** - You'll need to use a combination of SELECT, COUNT(), and GROUP BY statements.
Set Operations
87. **Explain the differences between UNION, INTERSECT, and EXCEPT. Write examples for each.** - UNION returns all rows that appear in either of the two SELECT statements. INTERSECT returns only the rows that appear in both SETs. EXCEPT returns only the rows present in the first SELECT statement but not in the second.
SQL Performance Tuning and Optimization
In this section, we'll delve into questions that assess your knowledge of SQL performance tuning and optimization strategies. Be prepared to discuss indexing, query optimization, and other best practices.
Familiarize yourself with these crucial topics, as an understanding of SQL performance not only helps you write efficient queries but also demonstrates your professional maturity as a developer.
Query Optimization Techniques
94. **What can you do to optimize a slow-running SELECT query?** - Having a proper index is the first line of optimization. If the query performs an ORDER BY operation, ensure that the column used for sorting is indexed. Additionally, ensure that functions (e.g., CAST or ABS) are not used in the WHERE clause, as they prevent the use of indexes.
Indexing
99. **How many indexes should you have on a table?** - There's no one-size-fits-all answer; it depends on the specific use case, number of rows in the table, and the access queries. However, a good rule of thumb is to have one index per highly selective column used in the WHERE clause. Too many indexes may impact write operations and consume excess storage space.
Finally, remember that SQL interviews are an opportunity for you to showcase your skills and thought process. Don't be afraid to explain your reasoning and ask clarifying questions about the requirements. Good luck, and happy coding!