Write a query to find the second highest salary from each department in a table.

Instruction: Assuming a table with employee details including salaries and department IDs, construct a SQL query to find the second highest salary in each department.

Context: This question challenges the candidate to use advanced SQL techniques to solve common but non-trivial problems involving sorting and ranking within groups.

Official Answer

Thank you for posing such a thought-provoking question. It's one that showcases the importance of being adept at SQL, especially in roles that require a deep understanding of data manipulation and analysis to derive meaningful insights, such as a Data Analyst position. In my experience, especially during my tenure at leading tech companies, I've had to tackle similar queries to inform strategic decisions. Let's dive into the question, ensuring we're on the same page.

We aim to write a SQL query to identify the second highest salary within each department from a given employee details table. For the sake of clarity, let's assume our table is named EmployeeDetails and it includes columns for EmployeeID, Salary, DepartmentID, among others. The crux of the solution involves utilizing SQL's window functions to partition the data by department and then rank the salaries within each partition.

Here's how I would approach the solution:

WITH RankedSalaries AS (
  SELECT
    DepartmentID,
    Salary,
    DENSE_RANK() OVER (PARTITION BY DepartmentID ORDER BY Salary DESC) AS SalaryRank
  FROM EmployeeDetails
)
SELECT DepartmentID, Salary
FROM RankedSalaries
WHERE SalaryRank = 2;

In this query, we start by creating a Common Table Expression (CTE) named RankedSalaries. Within this CTE, we select the DepartmentID, Salary, and use the DENSE_RANK() window function to assign a rank to each salary within its respective department, ordered in descending order. This ranking is crucial as it allows us to treat multiple occurrences of the same salary within a department equitably, ensuring that we truly identify the second highest salary, even in cases where the highest salary is drawn by more than one employee.

Following the CTE, we select the DepartmentID and Salary from RankedSalaries, filtering to include only those records where SalaryRank equals 2. This effectively gives us the second highest salary in each department, as per our initial requirement.

To elaborate on the metrics and assumptions: the DENSE_RANK() function is pivotal here as it doesn't skip ranks if there are ties. For instance, if two employees share the highest salary within a department, they would both receive a rank of 1, and the next highest salary after them would be ranked as 2, which is exactly what we're looking for.

Navigating through such SQL queries not only demonstrates technical acumen but also an ability to think critically about data structures and the logic behind data retrieval. It's a skill I've honed over the years, solving real-world problems and contributing to data-driven decision-making processes. This framework, while tailored here for a Data Analyst role, can be easily adapted for any position requiring SQL expertise, with minor adjustments to align with specific job responsibilities. It underscores my approach to problem-solving: clarify the question, define assumptions, provide a precise and logical solution, and explain the methodology, all while ensuring the answer is accessible and engaging.

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