Jag kan inte testa detta, men det borde fungera:
;WITH DepartmentsSalary AS
(
SELECT department_ID, AVG(Salary) AvgSalary, MIN(Salary) MinSalary
FROM employees
GROUP BY department_ID
)
SELECT department_ID, MinSalary
FROM ( SELECT department_ID, AvgSalary, MAX(AvgSalary) OVER() MaxSalary, MinSalary
FROM DepartmentsSalary) D
WHERE MaxSalary = AvgSalary