18. 3rd Highest Salary

SELECT DepartmentID,
        ThirdHighestSalary
    FROM
    (
        SELECT DISTINCT DepartmentID,
        (
            SELECT DISTINCT Salary FROM Employees
                WHERE DepartmentID = w.DepartmentID
                ORDER BY Salary DESC
                OFFSET 2 ROWS FETCH NEXT 1 ROW ONLY
        ) AS ThirdHighestSalary
          FROM Employees AS w
    ) AS t
 WHERE ThirdHighestSalary IS NOT NULL