Based on the lastet payrate of each employee, calculate the average hourly payrate for each department.
latest payrate refers to max(RateChangeDate); average hourly payrate = divide(sum(Rate), count(BusinessEntityID)) for each DepartmentID
SELECT AVG(T1.Rate) FROM EmployeePayHistory AS T1 INNER JOIN EmployeeDepartmentHistory AS T2 ON T1.BusinessEntityID = T2.BusinessEntityID INNER JOIN Department AS T3 ON T2.DepartmentID = T3.DepartmentID WHERE T1.RateChangeDate = ( SELECT MAX(T1.RateChangeDate) FROM EmployeePayHistory AS T1 INNER JOIN Department AS T2 ON T1.BusinessEntityID = T2.DepartmentID )