模糊查询简单分析，得出sql语句

最近搞了一个登记系统,到了查询页面时候,有多的DropDownList控件,DropDownList里面很多选择可以选择，就是条件查询.但如何获得该查询的sql语句呢,我本来想到用 case,但考虑到与以前查询具体分数不同，它这里是有模糊查询的概念，譬如,当DropDownList里面有"全部"一项时,那么,case要分很细小,还是不妙啊，真实情况是

DropDownList3里面有 选择项  (全部,1,2,3,4,5)  DropDownList4里面有选择项(全部,a,b,c,d,e)那么,

考虑下,其实是数学的组合问题，简单演示在草稿上，如果两个DropDownList,则只有四种可能,要么是"全部"要么不是＂全部＂所以有以下结果

string sql;
        sql = "select * from file where 1=1";//一开始要赋给它,否则,编译时候不通过
        if(DropDownList3 .SelectedValue =="全部" && DropDownList4 .SelectedValue =="全部")
            sql = "select * from file where date_year='" + DropDownList1.SelectedValue + "'and subject ='" + DropDownList2.SelectedValue + "'";

       if (DropDownList3.SelectedValue == "全部" && DropDownList4.SelectedValue != "全部")
            sql = "select * from file where date_year='" + DropDownList1.SelectedValue + "'and subject ='" + DropDownList2.SelectedValue + "'and grade ='" + DropDownList4.SelectedValue + "'";

        if (DropDownList3.SelectedValue != "全部" && DropDownList4.SelectedValue == "全部")
            sql = "select * from file where date_year='" + DropDownList1.SelectedValue + "'and subject ='" + DropDownList2.SelectedValue + "'and person_shenfen ='" + DropDownList3.SelectedValue + "'";

        if (DropDownList3.SelectedValue != "全部" && DropDownList4.SelectedValue != "全部")
            sql = "select * from file where date_year='" + DropDownList1.SelectedValue + "'and subject ='" + DropDownList2.SelectedValue + "'and person_shenfen ='" + DropDownList3.SelectedValue + "'and grade ='" + DropDownList4.SelectedValue + "'";

这样就实现了sql语句的查询了．

如果有什么好的建议和意见，欢迎批评指正，谢谢．