List the first names of employees with trading quantity for more than 500.
trading quantity for more than 500 refers to Quantity > 500;
SELECT DISTINCT T1.FirstName FROM Employees AS T1 INNER JOIN Sales AS T2 ON T1.EmployeeID = T2.SalesPersonID WHERE T2.Quantity > 500
database name:db_id sales
Customers table Customers Cols: Customer ID dtype integer, First Name dtype text, Middle Initial dtype text, Last Name dtype text, 
Employees table Employees Cols: Employee ID dtype integer, First Name dtype text, Middle Initial dtype text, Last Name dtype text, 
Products table Products Cols: Product ID dtype integer, Name dtype text, Price dtype real, 
Sales table Sales Cols: Sales ID dtype integer, SalesPerson ID dtype integer, Customer ID dtype integer, Product ID dtype integer, Quantity dtype integer, 
