比如说有styledesc这个字段,数据要求的是50位,char形,可不可以只限制他50位,其它的不限制,
输入什么字符都可以的.只要是char形,只要数据库允许就行

这样,安全写入数据库操作,正常读出并显示在不同的场合,应用.

应该是怎样做呢?

下面是我总结的几点.非常有可能不对,请指正.
如果对用户的输入是可以任意字符,(除了某字段特定的输入限制条件,如输入长度,输入类型==).
就是输入尽可能不作限制.

对一字符串str,他输出的方向有以下几种:
1.输出至HTML中,Function fn_chk_to_html(Str)
2.输出至Script中(如javascript),Function fn_chk_to_script(Str)
3.输出至sql语句中,而且这条SQL语句是用''两个单引括起字符串Str的.
Function fn_chk_to_sql_mark(Str)
4.输出至sql语句中,而且这条SQL语句是没有用单引括起Str的.Function fn_chk_to_sql_go(Str)
5.输出至URL        .Function fn_chk_to_url(Str)

<%Function fn_chk_to_html(Str)
'如<input text="<%=request("styledesc")%>">的情况下
 If Isnull(Str) Then
  ChkSql = ""
  Exit Function
 End If
 Str = trim(Str)
 Str = Replace(Str, Chr(0), "",1,-1,1)
 Str = Replace(Str, """", """,1,-1,1)
 Str = Replace(Str, "'", "'",1,-1,1)
 Str = Replace(Str, "<","<",1,-1,1)
 Str = Replace(Str, ">",">",1,-1,1)
 Str = Replace(Str, vbCrlf, "<br>",1,-1,1)
 fn_chk_to_html = Str
End Function
%>

<%Function fn_chk_to_script(Str)
'如 response.write "<script>alert('"&request("styledesc")&"');</script>"的情况下
 If Isnull(Str) Then
  ChkSql = ""
  Exit Function
 End If
 Str = trim(Str)
 Str = Replace(Str, "\", "\\",1,-1,1)
 Str = Replace(Str, """", "\""",1,-1,1)
 Str = Replace(Str, "'", "\'",1,-1,1)
 Str = Replace(Str,Chr(13),"\n",1,-1,1)
 fn_chk_to_script = Str
End Function
%>
<%
Function fn_chk_to_sql_mark(Str)
'如 sql="select * from style where styledesc like '"&request("styledesc")&"'"的情况下
 If Isnull(Str) Then
  ChkSql = ""
  Exit Function
 End If
 Str = trim(Str)
 Str = Replace(Str, "'", "''",1,-1,1)
 fn_chk_to_sql_mark = Str
End Function
%>

<%Function fn_chk_to_sql_go(Str)
'如sql = "select * from "&request("table")的情况下.??
 If Isnull(Str) Then
  ChkSql = ""
  Exit Function
 End If
 Str = trim(Str)
 Str = Replace(Str, Chr(0), "",1,-1,1)
 Str = Replace(Str, """", """,1,-1,1)
 Str = Replace(Str, "'", "'",1,-1,1)
 Str = Replace(Str, "<","<",1,-1,1)
 Str = Replace(Str, ">",">",1,-1,1)
 Str = Replace(Str, "[", "[",1,-1,1)
 Str = Replace(Str, "]", "]",1,-1,1)
 Str = Replace(Str, "\", "\",1,-1,1)
 Str = Replace(Str, "*", "*",1,-1,1)
 Str = Replace(Str, "%", "%",1,-1,1)
 Str = Replace(Str, ";", ";",1,-1,1)
 Str = Replace(Str, vbCrlf, "<br>",1,-1,1)
 Str = Replace(Str, "--", "--")
 fn_chk_to_sql_go = Str
 End Function
%>

<%Function fn_chk_to_url(Str)

'如 str="<img src='showimg.asp?id="&request("id")&"'>"的情况下
 If Isnull(Str) Then
  ChkSql = ""
  Exit Function
 End If
 Str = trim(Str)
 Str = server.URLEncode(Str)
 fn_chk_to_sql_mark = Str
End Function
%>