最简单的 asp.net 不用 ajax 亦可做到,
两个 DropDownList1、DropDownList2,数据来源各为
SqlDataSource1、SqlDataSource2,
SqlDataSource2 的 SELECT WHERE 条件,设置为 DropDownList1 的选项,
另设 DropDownList1 的 AutoPostBack 属性为 True (每次选完后会 refresh 页面)。
http://www.cnblogs.com/downmoon/archive/2010/06/15/1758675.html
方法很多,asp.net ajax,js ajax,ajax.net...
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