Write a SQL query to retrieve the department, employee name, employee ID, and salary of the employee with the second highest salary in each department using dense_rank.

The query you provided is almost correct but missing a closing parenthesis and an alias for the subquery. Here is the corrected version:

SELECT department, employee_name, employee_id, salary

FROM (

SELECT e.department,

     e.employee_name,
     e.employee_id,
     s.salary,
     DENSE_RANK() OVER (PARTITION BY e.department ORDER BY s.salary DESC) AS rnk

FROM employee e

JOIN salary s USING (employee_id)

) AS sub

WHERE rnk = 2;

Explanation: The inner SELECT joins employee and salary, then assigns a rank per department. The outer SELECT filters for rank 2, giving the second highest salary per department. Complexity is dominated by the sort in the window function, O(n log n).