为什么会提示语法错误 (操作符丢失) 在查询表达式 'id<' 中。？错在哪里?

<%
sql="select top 1 title,id from news where id<"&nid&" order by id desc"
set rs=conn.execute(sql)
if rs.eof then
response.write "上一篇文章:没有了"
else
response.write "上一篇文章: <a href=new.asp?id="&rs("id")&">"&rs("title")&"</a>"
end if
sql="select top 1 title,id from news where id>"&nid&" order by id"
set rss=conn.execute(sql)
if rs.eof then
response.write "下一篇文章:没有了"
else
response.write "下一篇文章:<a href=new.asp?id="&rs("id")&">"&rs("title")&"</a>"
end if
%>

nid没有赋值