方法一：

            DataTable dt =newDataTable();
            dt.Columns.Add(newDataColumn("C1",typeof(int)));
            dt.Columns.Add(newDataColumn("C2",typeof(string)));
            dt.Columns.Add(newDataColumn("C3",typeof(int)));
            dt.Rows.Add(1,"a",30);
            dt.Rows.Add(2,"a",40);
            dt.Rows.Add(3,"b",50);
            dt.Rows.Add(4,"b",60);
            dt.Rows.Add(5,"a",70);
            Console.WriteLine("a总和:"+dt.Compute("sum(C3)","C2='a'"));
            Console.WriteLine("b总和:"+dt.Compute("sum(C3)","C2='b'"));
/*
输出:
a总和:140
b总和:110
*/
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方法二：

            循环计算