DataTable中Compute计算函数

 DataTable dt = new DataTable();
 //嵌套的三元运算 牛叉到五体投地
 object obj = dt.Compute("iif(1000=5,1000,iif(100>100,4001,2000))", null);
 Response.Write(obj);

 System.Data.DataTable table = new DataTable();
 //计算常量，可以没有初始化列
 object test = table.Compute("1+1", "");
 Console.WriteLine(test);

 string a = "";
 System.Double b = ;
 decimal c = 123m;
 Console.WriteLine(Convert.ToDecimal(a));
 //test=2;

 test = table.Compute("1+1", "false");
 Console.WriteLine(test);
 //test=2;常数计算和filter无关

 test = table.Compute("abs(1)", "");
 Console.WriteLine(test);
 //test=null，不知道为这个什么没有报错，而且返回null,其他的数学函数都会抱错

 test = table.Compute("2%2", "");
 Console.WriteLine(test);
 //test=0;
 //其他函数参考下面的计算列

 //初始化datatale
 table.Columns.Add("id", typeof(string));
 table.Columns.Add("value", typeof(int));
 for (int i = ; i <= ; i++)
 {
 System.Data.DataRow dRow = table.NewRow();
 dRow["id"] = "id" + i.ToString();
 dRow["value"] = i;
 table.Rows.Add(dRow);
 }

 //test = table.Compute("value+1", "true");
 /**/
 ////抛出异常，这里必须是聚合函数

 //*************************************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'")[]["exp1"];
 Console.WriteLine(test);
 //test=2;

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

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

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

 //字符串函数
 column.Expression = "substring(id,3,len(id)-2)";
 test = table.Select("id='id1'")[]["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'")[]["exp1"];
 Console.WriteLine(test);
 //test=1.6;

 //相当于sqlserver的isnull
 column.Expression = "isnull(value,10)";
 test = table.Select("id='id1'")[]["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'")[]["exp1"];
 Console.WriteLine(test);
 //test=2000;

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

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