请问这个SQL怎么会错呢？？？

Question

Valid_num- DateDiff('Dd',BeginDate,now)<=0

Answer 1

SELECT UserName,trueName,UserPoint,addDate,UserEmail FROM Users WHERE Valid_num- DateDiff(d,BeginDate,now)<=0

Answer 2

SELECT UserName,trueName,UserPoint,addDate,UserEmail FROM Users WHERE Valid_num- DateDiff(dd,BeginDate,now)<=0

Answer 3

也不会，会这样错误
　SELECT UserName,trueName,UserPoint,addDate,UserEmail FROM Users WHERE Valid_num- DateDiff(dd,BeginDate,now)<=0 
Microsoft OLE DB Provider for SQL Server 错误 '80040e14' 列名 'now' 无效。