Você também pode usar a função analítica RANK():
SELECT * FROM (
SELECT
Dept.DeptNo,
Dept.DName,
Emp.EName,
Emp.Sal,
RANK() OVER (PARTITION BY Dept.DeptNo ORDER BY Emp.Sal DESC) AS DeptSalRank
FROM Emp
INNER JOIN Dept ON Emp.DeptNo = Dept.DeptNo
)
WHERE DeptSalRank = 1
