这个存储过程快把我搞疯了

使用这个存储过程能读出数据
CREATE PROCEDURE sp_selPage @count int,@page int,@tablename char(20),@fieldname char(20)
as DECLARE @SQL varchar(800)
SET @SQL='select top '+Cast(@count as char) +' * from '+@tablename+' where '+@fieldname+' not in
(select top '+CAST(@count*(@page-1) AS CHAR)+' '+@fieldname+' from '+@tablename+' order by '+@fieldname+' desc)
order by '+@fieldname+' desc'
EXEC(@SQL)
GO

可我加了个条件后怎么就读不出来了呢？如下：
CREATE PROCEDURE sp_selPage_a @count int,@page int,@tablename char(20),@fieldname char(20),@classID bigint
as DECLARE @SQL varchar(800)
SET @SQL='select top '+Cast(@count as char) +' * from '+@tablename+' where classname='+@classID+' and '+@fieldname+' not in
(select top '+CAST(@count*(@page-1) AS CHAR)+' '+@fieldname+' from '+@tablename
+' where classname='+@classID+' order by '+@fieldname+' desc)
order by '+@fieldname+' desc'
EXEC(@SQL)
GO