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SQL Interview Questions and Answers.
In the world of data-driven decision-making, SQL (Structured Query Language) stands out as the lingua franca for managing relational databases. Whether you are preparing to land your first job in data analytics or aiming to advance further in your tech career, proficiency in SQL is often a crucial requirement. This blog post delves into some of the most commonly asked SQL interview questions that you might encounter during a rigorous screening process. From basic queries to complex data manipulation and optimization problems, we cover a comprehensive range of topics that are designed to not only test your technical knowledge but also enhance your understanding of SQL’s powerful features. Prepare to unlock the secrets to acing your SQL interviews with our detailed explanations and insider tips.
How to prepare and Ace SQL Interview
1. Understand SQL Fundamentals
Ensure you have a strong grasp of SQL basics and advanced concepts:
- Joins and Subqueries: Know how to use inner, outer, left, and right joins effectively. Subqueries, both correlated and non-correlated, are crucial.
- Aggregation and Window Functions: Be proficient with functions like
SUM()
,AVG()
,COUNT()
,MIN()
,MAX()
, and understand window functions likeROW_NUMBER()
,RANK()
,DENSE_RANK()
, and how they differ. - Complex Conditions: Practice writing queries with multiple conditions and understand how to use
CASE
statements. - Group By and Having Clauses: These are essential for segmentation and conditional aggregates.
2. Practice LeetCode SQL Problems
- Daily Practice: Regular practice is key. Start with easier problems and gradually move to medium and hard problems.
- Focus on Problem Types: On LeetCode, problems are often categorized by the techniques they require (e.g., joins, window functions). Focus on mastering each category.
- Time Yourself: Practicing under time constraints can help simulate the pressures of a real interview.
3. Study Common Patterns and Techniques
- Pattern Recognition: Many SQL problems, especially on platforms like LeetCode, follow certain patterns. Identifying and learning these patterns can save time during the interview.
- Optimization: Learn how to optimize SQL queries for performance, which includes understanding indexes, avoiding unnecessary columns in SELECT and JOIN clauses, and minimizing subqueries.
4. Mock Interviews
- Peer Mock Interviews: Engage with peers or mentors who can conduct mock interviews. Platforms like Pramp or Interviewing.io offer free or paid mock interview services.
- Self-Review: If peers aren’t available, write down problems and solve them in a timed setting. Review your solutions against those provided by LeetCode for efficiency and accuracy.
5. Review SQL Theoretically
- Books and Online Resources: Books like “SQL Antipatterns” by Bill Karwin or online courses on platforms like Coursera, Udemy, or freeCodeCamp can provide deeper insights into efficient SQL coding practices.
- SQL Specifications: Sometimes, interviews can test specific SQL flavors (MySQL, PostgreSQL, SQL Server). Ensure you understand the particular SQL dialect expected in the interview.
6. Understand the Data
- Data Schemas: Before solving any problem, thoroughly understand the schema and relationships in the given database. This understanding is crucial for constructing correct and efficient queries.
7. Learn From Others
- Discuss Solutions: LeetCode and other forums allow users to discuss their solutions. Reviewing discussions can provide insights into different ways of solving the same problem and help uncover more efficient or elegant solutions.
8. Relax and Strategize During the Interview
- Clarify Questions: If a problem statement is unclear, don’t hesitate to ask for clarifications during the interview.
- Outline Your Approach: Before you start coding, briefly explain your approach to the interviewer. This can help them understand your thought process and guide you if you’re headed in the wrong direction.
Example Problem: Department Highest Salary
Problem Statement:
The Employee
table contains all employees. The Employee
table has columns Id, Name, Salary, and DepartmentId. There is also a Department
table that holds information about each department.

Write an SQL query to find the employees who have the highest salary in each of the departments.
Solution SQL Query:

SELECT d.Name AS Department, e.Name AS Employee, e.Salary
FROM Employee e
JOIN Department d ON e.DepartmentId = d.Id
WHERE (e.DepartmentId, e.Salary) IN (
SELECT DepartmentId, MAX(Salary)
FROM Employee
GROUP BY DepartmentId
);
Here’s a breakdown of this query:
- Subquery (Ranked): This part computes a rank for each employee within their department based on their salary in descending order.
PARTITION BY Department
ensures the ranking resets for each department. - Outer Query: The outer query selects the department, employee name, and salary from the ranked results, filtering to include only those entries where
Rank
is 3 or less, thereby ensuring that only the top three earners per department are selected.
This approach will efficiently retrieve the desired results if your database supports window functions, which is common in systems like PostgreSQL, MySQL (8.0+), SQL Server, and Oracle.
Discussion
This problem tests more advanced SQL concepts such as subqueries and the use of GROUP BY
with aggregate functions in conjunction with joins. It’s classified as a medium difficulty problem on LeetCode and helps in understanding how to manipulate and analyze data across multiple tables effectively. This type of query is very common in real-world scenarios where relational database management is required to generate reports or derive insights from the data.
Example Problem: Department Top 2 Highest Salary
Instead of displaying the top earner by department, display the top 2 earners by department
Solution Query:
You can use the DENSE_RANK()
or ROW_NUMBER()
window function. This allows you to assign a unique rank to each salary within its respective department, and then you can filter for the top three salaries.
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SELECT Department, Employee, Salary
FROM (
SELECT
d.Name AS Department,
e.Name AS Employee,
e.Salary,
DENSE_RANK() OVER (PARTITION BY e.DepartmentId ORDER BY e.Salary DESC) AS rank
FROM Employee e
JOIN Department d ON e.DepartmentId = d.Id
) ranked_employees
WHERE rank <= 3;
Here’s a breakdown of this query:
- Subquery (Ranked): This part computes a rank for each employee within their department based on their salary in descending order.
PARTITION BY Department
ensures the ranking resets for each department. - Outer Query: The outer query selects the department, employee name, and salary from the ranked results, filtering to include only those entries where
Rank
is 3 or less, thereby ensuring that only the top three earners per department are selected.
This approach will efficiently retrieve the desired results if your database supports window functions, which is common in systems like PostgreSQL, MySQL (8.0+), SQL Server, and Oracle.
Example Problem: Employees Earning More Than Their Managers
Problem Statement: The Employee
table holds all employees including their managers. Every employee has an Id, and there is also a column for the manager Id.

Write an SQL query to find the employees who earn more than their managers.
Solution SQL Query:
SELECT a.Name AS Employee
FROM Employee AS a, Employee AS b
WHERE a.ManagerId = b.Id AND a.Salary > b.Salary;
Explanation:
- The SQL query uses a self-join on the
Employee
table. It aliases the table asa
andb
wherea
represents the employees andb
represents the managers. - The
WHERE
clause matches each employee to their manager usinga.ManagerId = b.Id
and then checks if the employee’s salary is greater than the manager’s salary usinga.Salary > b.Salary
. - The result is the name of the employee who earns more than their manager.
Discussion
This question tests understanding of self-joins and basic comparison operations in SQL. It’s a relatively straightforward problem once you’re comfortable with the concept of joining a table to itself to compare records based on a relational key. It’s categorized under the “easy” level on LeetCode, but it encapsulates fundamental skills that can be built upon for more complex queries involving multiple joins, subqueries, and advanced SQL functions.
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- The Art of Unix Programming
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- Practices of an Agile Developer
- Don't Make Me Think
- Agile Software Development, Principles, Patterns, and Practices by Robert C. Martin
- Domain Driven Designs by Eric Evans
- The Design of Everyday Things by Donald Norman
- Modern C++ Design by Andrei Alexandrescu
- Best Software Writing I by Joel Spolsky
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- Computer Graphics: Principles and Practice in C (2nd Edition)
- Thinking in Java by Bruce Eckel
- The Elements of Computing Systems
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- The Annotated Turing
- Things That Make Us Smart by Donald Norman
- The Timeless Way of Building by Christopher Alexander
- The Deadline: A Novel About Project Management by Tom DeMarco
- The C++ Programming Language (3rd edition) by Stroustrup
- Patterns of Enterprise Application Architecture
- Computer Systems - A Programmer's Perspective
- Agile Principles, Patterns, and Practices in C# by Robert C. Martin
- Growing Object-Oriented Software, Guided by Tests
- Framework Design Guidelines by Brad Abrams
- Object Thinking by Dr. David West
- Advanced Programming in the UNIX Environment by W. Richard Stevens
- Hackers and Painters: Big Ideas from the Computer Age
- The Soul of a New Machine by Tracy Kidder
- CLR via C# by Jeffrey Richter
- The Timeless Way of Building by Christopher Alexander
- Design Patterns in C# by Steve Metsker
- Alice in Wonderland by Lewis Carol
- Zen and the Art of Motorcycle Maintenance by Robert M. Pirsig
- About Face - The Essentials of Interaction Design
- Here Comes Everybody: The Power of Organizing Without Organizations by Clay Shirky
- The Tao of Programming
- Computational Beauty of Nature
- Writing Solid Code by Steve Maguire
- Philip and Alex's Guide to Web Publishing
- Object-Oriented Analysis and Design with Applications by Grady Booch
- Effective Java by Joshua Bloch
- Computability by N. J. Cutland
- Masterminds of Programming
- The Tao Te Ching
- The Productive Programmer
- The Art of Deception by Kevin Mitnick
- The Career Programmer: Guerilla Tactics for an Imperfect World by Christopher Duncan
- Paradigms of Artificial Intelligence Programming: Case studies in Common Lisp
- Masters of Doom
- Pragmatic Unit Testing in C# with NUnit by Andy Hunt and Dave Thomas with Matt Hargett
- How To Solve It by George Polya
- The Alchemist by Paulo Coelho
- Smalltalk-80: The Language and its Implementation
- Writing Secure Code (2nd Edition) by Michael Howard
- Introduction to Functional Programming by Philip Wadler and Richard Bird
- No Bugs! by David Thielen
- Rework by Jason Freid and DHH
- JUnit in Action
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