A.SELECT。FROM Department LEFT JOIN Employee ON Department.ID =Employee.DepartmentID AND Employee.DepartmentID =NULL
B.SELECT。FROM Department LEFT JOIN Employee WHERE Department.ID =Employee.DepartmentID AND Employee.DepartmentID IS NULL
C.SELECT。FROM Department LEFT JOIN Employee ON Department.ID =Employee.DepartmentID WHERE Employee.DepartmentID IS NULL
D.SELECT。FROM Department LEFT JOIN Employee ON Department.ID =Employee.DepartmentID AND Employee.DepartmentID IS NULL