List the name of married employees with less than 20 vacation hours.
married employee refers to MaritalStatus = 'M'; less than 20 vacation hours refers to VacationHours<20
SELECT T1.FirstName, T1.LastName FROM Person AS T1 INNER JOIN Employee AS T2 ON T1.BusinessEntityID = T2.BusinessEntityID WHERE T2.MaritalStatus = 'M' AND T2.VacationHours < 20