What is the highest pay rate of the employees who are exempt from collective bargaining?
employee exempt from collective bargaining refers to SalariedFlag = 1; highest pay rate refers to max(Rate)
SELECT T1.Rate FROM EmployeePayHistory AS T1 INNER JOIN Employee AS T2 ON T1.BusinessEntityID = T2.BusinessEntityID WHERE T2.SalariedFlag = 1 ORDER BY T1.Rate DESC LIMIT 1