C#中Datatable的Compute的用法（转载）

        DataTable dt = new DataTable();


        //嵌套的三元运算（iif判断）
        object obj = dt.Compute("iif(1000=5,1000,iif(100>100,4001,2000))", null);
        Response.Write(obj);//计算常量，可以没有初始化列
        object test = table.Compute("1+1", "");
        Console.WriteLine(test);
  
        //计算绝对值
        test = table.Compute("abs(1)", "");
        Console.WriteLine(test);
      
   
        //求余数
        test = table.Compute("2%2", "");
        Console.WriteLine(test);


        //*************************************支持的聚合函数**********************//

        //求数量
        test = table.Compute("count(id)", "false");
        Console.WriteLine(test);
        //test=0;

        test = table.Compute("count(id)", "true");
        Console.WriteLine(test);
        //test=10;



        //求和
        test = table.Compute("sum(value)", "");
        Console.WriteLine(test);
        //test=55;

        //test = table.Compute("sum(id)","");
        /**/
        ////抛出异常，这里不能是string


        //平均
        test = table.Compute("avg(value)", "");
        Console.WriteLine(test);
        //test=5;


        //最小
        test = table.Compute("min(value)", "");
        Console.WriteLine(test);
        //test=1;

        //最大
        test = table.Compute("max(value)", "");
        Console.WriteLine(test);
        //test=10;

        //统计标准偏差
        test = table.Compute("StDev(value)", "");
        Console.WriteLine(test);
        //test=3.02765035409749

        //统计方差
        test = table.Compute("Var(value)", "");
        Console.WriteLine(test);
        //test=9.16666666666667


        //复杂计算
        test = table.Compute("max(value)/sum(value)", "");
        Console.WriteLine(test);
        //test=0.181818181818182




        /*******************************************计算列*************************/

        System.Data.DataColumn column = new DataColumn("exp1", typeof(float));
        table.Columns.Add(column);


        //简单计算
        column.Expression = "value*2";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=2;

        //字符串函数
        column.Expression = "len(id)";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=3;

        //字符串函数
        column.Expression = "len(' '+id+' ')";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=5;

        //字符串函数
        column.Expression = "len(trim(' '+id+' '))";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=3;

        //字符串函数
        column.Expression = "substring(id,3,len(id)-2)";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=1; //substring的起始字符位置为1不是0

        //类型转换
        column.Expression = "convert(substring(id,3,len(id)-2),'System.Int32')*1.6";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=1.6;

        //相当于sqlserver的isnull
        column.Expression = "isnull(value,10)";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=1;

        //三元运算符,相当于sqlserver的case when
        column.Expression = "iif(value>5,1000,2000)";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=2000;

        //like运算符
        column.Expression = "iif(id like '%1',1000,2000)";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=1000;

        //in运算符
        column.Expression = "iif(id not in('id1'),1000,2000)";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=2000;

        //嵌套的三元运算
        column.Expression = "iif(value>5,1000,iif(id like '%1',4000,2000))";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=4000;


        //客户端计算所占总数的百分比
        column.Expression = "value/sum(value)";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=0.01818182


        //客户端计算差值,比如nba常规赛的胜场差
        column.Expression = "max(value)-value";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=9


        //***********************父子表计算*************************************/
        //初始化子表,父子表关系
        DataTable tableChild = new DataTable();

        tableChild.Columns.Add("id", typeof(string));
        tableChild.Columns.Add("value", typeof(int));

        System.Data.DataSet ds = new DataSet();
        ds.Tables.Add(tableChild);
        ds.Tables.Add(table);
        DataRelation relation = new DataRelation("relation", table.Columns["id"], tableChild.Columns["id"]);
        ds.Relations.Add(relation);

        for (int i = 1; i <= 10; i++)
        {
            System.Data.DataRow dRow = tableChild.NewRow();
            dRow["id"] = "id1";
            dRow["value"] = i;
            tableChild.Rows.Add(dRow);
        }


        //计算子表记录数
        column.Expression = "count(child(relation).value)";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=10;



        //计算父子表的百分比
        column.Expression = "value/sum(child(relation).value)";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=0.01818182;


        //计算父子表的差值,比如父表为库存数量，子表为订购数量，计算得出需要补充的数量
        column.Expression = "iif(value-sum(child(relation).value)>0,0,value-sum(child(relation).value))";
        test = table.Select("id='id1'")[0]["exp1"];
        Console.WriteLine(test);
        //test=-54;

        //比较遗憾的是没有发现能够计算同比和环比的方法，而且计算列无法作为约束
        //结束，DataTable可以让你尽量发挥聪明才智来减少繁杂的sql语句并且减轻服务器计算符合

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