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.
Ensure you have a strong grasp of SQL basics and advanced concepts:
SUM()
, AVG()
, COUNT()
, MIN()
, MAX()
, and understand window functions like ROW_NUMBER()
, RANK()
, DENSE_RANK()
, and how they differ.CASE
statements.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.
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
);
PARTITION BY Department
ensures the ranking resets for each department.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.
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.
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;
PARTITION BY Department
ensures the ranking resets for each department.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.
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.
SELECT a.Name AS Employee
FROM Employee AS a, Employee AS b
WHERE a.ManagerId = b.Id AND a.Salary > b.Salary;
Employee
table. It aliases the table as a
and b
where a
represents the employees and b
represents the managers.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
.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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