jquery中如何判断在checkboxlist中至少有一个被选中？

Question

jquery中如何判断在一组checkbox中至少有一个被选中以阻止表单被提交？
Html如下：
<table id="ctl00_ContentPlaceHolder_ChkBoxListArchiveDept" border="0">
<tr>
<td><input id="ctl00_ContentPlaceHolder_ChkBoxListArchiveDept_0" type="checkbox" name="ctl00$ContentPlaceHolder$ChkBoxListArchiveDept$0" /><label for="ctl00_ContentPlaceHolder_ChkBoxListArchiveDept_0">a</label></td>
</tr><tr>
<td><input id="ctl00_ContentPlaceHolder_ChkBoxListArchiveDept_1" type="checkbox" name="ctl00$ContentPlaceHolder$ChkBoxListArchiveDept$1" /><label for="ctl00_ContentPlaceHolder_ChkBoxListArchiveDept_1">b</label></td>
</tr>
       ....略
</table>

Answer 1

<script src="js/jquery-1.3.2.js"></script>
<script language="javascript"> 
function check(){
var a = $(":checkbox:checked");
if(a.length < 1) alert("至少选一个")
}
</script> 
<input type="button" onclick="check();" value="click">
<table id="ctl00_ContentPlaceHolder_ChkBoxListArchiveDept" border="0"> 
<tr> 
<td> <input id="ctl00_ContentPlaceHolder_ChkBoxListArchiveDept_0" type="checkbox" name="ctl00$ContentPlaceHolder$ChkBoxListArchiveDept$0" /> <label for="ctl00_ContentPlaceHolder_ChkBoxListArchiveDept_0">a </label> </td> 
</tr> <tr> 
<td> <input id="ctl00_ContentPlaceHolder_ChkBoxListArchiveDept_1" type="checkbox" name="ctl00$ContentPlaceHolder$ChkBoxListArchiveDept$1" /> <label for="ctl00_ContentPlaceHolder_ChkBoxListArchiveDept_1">b </label> </td> 
</tr> 
</table>