Department Top Three Salaries

Department Top Three Salaries

1. What is this problem about?

The Department Top Three Salaries interview question is an advanced version of the "Highest Salary" problem. You need to identify employees who earn one of the top three unique salaries in their department. If multiple employees earn the same salary, they all count toward that specific rank. This Department Top Three Salaries coding problem is a "Hard" SQL task because it involves multi-level filtering and ranking.

2. Why is this asked in interviews?

Tech giants like Apple and Meta use this question to test a candidate's mastery of complex SQL queries. It evaluates your understanding of how to handle "Top-N" queries within categories. It’s a classic Database interview pattern that checks if you can handle duplicate values (ties) correctly while limiting results.

3. Algorithmic pattern used

This problem is best solved using the DENSE_RANK() Window Function.

• DENSE_RANK() assigns ranks without gaps (e.g., 1, 1, 2, 3). This is crucial because the problem asks for the "top three unique salaries."
• The query structure involves a Common Table Expression (CTE) or a subquery where you calculate the rank OVER(PARTITION BY departmentId ORDER BY salary DESC).
• The outer query then filters for rank <= 3.

4. Example explanation

IT Department Salaries: [90k, 90k, 85k, 80k, 70k].

1. Rank 1: 90k (Two employees).
2. Rank 2: 85k (One employee).
3. Rank 3: 80k (One employee). Result: All employees earning 90k, 85k, or 80k are included. The employee earning 70k is excluded.

5. Common mistakes candidates make

• Using RANK() instead of DENSE_RANK(): RANK() would skip numbers (e.g., 1, 1, 3), potentially excluding the third highest salary if there's a tie at the top.
• Forgetting unique values: The problem often specifies "unique" salaries, meaning you must count distinct values rather than distinct employees.
• Subquery syntax: Writing overly complex correlated subqueries that are difficult to debug and inefficient to run.

6. Interview preparation tip

Always clarify if "Top 3" refers to the top three people or the top three unique values. In database contexts, this distinction dictates whether you use ROW_NUMBER() or DENSE_RANK().