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
CREATE TABLE Customers
(
    CustomerID    INTEGER         not null
        primary key,
    FirstName     TEXT not null,
    MiddleInitial TEXT null,
    LastName      TEXT not null
);
CREATE TABLE Employees
(
    EmployeeID    INTEGER         not null
        primary key,
    FirstName     TEXT not null,
    MiddleInitial TEXT null,
    LastName      TEXT not null
);
CREATE TABLE Products
(
    ProductID INTEGER            not null
        primary key,
    Name      TEXT    not null,
    Price     REAL null
);
CREATE TABLE Sales
(
    SalesID       INTEGER not null
        primary key,
    SalesPersonID INTEGER not null,
    CustomerID    INTEGER not null,
    ProductID     INTEGER not null,
    Quantity      INTEGER not null,
    foreign key (SalesPersonID) references Employees (EmployeeID)
            on update cascade on delete cascade,
        foreign key (CustomerID) references Customers (CustomerID)
            on update cascade on delete cascade,
        foreign key (ProductID) references Products (ProductID)
            on update cascade on delete cascade
);
