C#中DataTable中的Compute方法使用收集
原文：https://blog.csdn.net/smartsmile2012/article/details/8309664
Compute函数的参数就两个：Expression，和Filter。
Expresstion是计算表达式，关于Expression的详细内容请看这里“http://msdn2.microsoft.com/zh-cn/library/system.data.datacolumn.expression(VS.80).aspx”。而Filter则是条件过滤器，类似sql的Where条件。
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 = "123"; System.Double b = 123; 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 = 1; i <= 10; 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'")[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
