EXISTS的问题
刚接触SQL不太会用EXISTS
这有道题怎样解：
某公司SQL Server 2000一数据库中的Employees表用于存放所有员工的信息，Orders表用于存放所有的订单信息，Orders表有一外部键引用Employees表的主键，两表的关系如下图所示：

现在公司想统计一下最近一个月内没有任何销售订单的员工，则下列SQL语句中符合要求的有（ ）。（选择一项）

a)SELECT LastName, FirstName
FROM Employees AS e
WHERE EXISTS
(SELECT * FROM Orders
WHERE Orders.EmployeeID=e.EmployeeID
AND Orders.OrderDate > GetDate()-30)
b)SELECT LastName, FirstName
FROM Employees AS e
WHERE (SELECT * FROM Orders
WHERE Orders.EmployeeID=e.EmployeeID
AND Orders.OrderDate < GetDate()-30) > 0
c)SELECT LastName, FirstName
FROM Employees AS e
WHERE(SELECT * FROM Orders
WHERE Orders.EmployeeID=e.EmployeeID
AND Orders.OrderDate > GetDate()-30) < 0
d)SELECT LastName, FirstName
FROM Employees AS e
WHERE EXISTS
(SELECT * FROM Orders
WHERE Orders.EmployeeID=e.EmployeeID
AND Orders.OrderDate < GetDate()-30)

是不是应该选D？


[解决办法]
SELECT LastName, FirstName
FROM Employees AS e
WHERE NOT EXISTS
(SELECT * FROM Orders
WHERE Orders.EmployeeID=e.EmployeeID
AND Orders.OrderDate > GetDate()-30)
[解决办法]
用NOT EXISTS ，怎答案全是用EXISTS 的。