How many employees work for AdvertureWorks that is single?
Employees refer to PersonType = 'EM'; Single refers to MaritalStatus = 's'
SELECT COUNT(T1.BusinessentityID) FROM Person AS T1 INNER JOIN Employee AS T2 ON T1.BusinessEntityID = T2.BusinessEntityID WHERE T1.PersonType = 'EM' AND T2.MaritalStatus = 'S'