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 like ROW_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.

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 as a and b where a represents the employees and b represents the managers.
• The WHERE clause matches each employee to their manager using a.ManagerId = b.Id and then checks if the employee’s salary is greater than the manager’s salary using a.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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