System.Data.DataTable的奇异之处

http://blog.csdn.net/jinjazz/archive/2008/04/08/2261721.aspx

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using System; using System.ComponentModel; using System.Data; using System.Windows.Forms; namespace WindowsApplication1 { public partial class Form1 : Form { public Form1() { InitializeComponent(); } private void button1_Click(object sender,EventArgs e) { // System.Data.DataTable计算功能详解，转载请保留以下信息 // 测试环境 vs2005,c# // 作者: jinjazz // 作者blog: http://blog.csdn.net/jinjazz/ 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); // 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抱错，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 table.Rows.Clear(); // 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,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',2000)"; test = table.Select("id='id1'")[0]["exp1"]; Console.WriteLine(test); // test=1000; // in运算符 column.Expression = "iif(id not in('id1'),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,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,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^&^ } } }

（编辑：李大同）

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