What is the e-mail address of the employee who switched departments for the most times?
switched department the most refers to MAX(count(DepartmentID));
SELECT T2.EmailAddress FROM EmployeeDepartmentHistory AS T1 INNER JOIN EmailAddress AS T2 ON T1.BusinessEntityID = T2.BusinessEntityID GROUP BY T2.BusinessEntityID ORDER BY COUNT(T1.DepartmentID) DESC LIMIT 1