C#--详解DataTable的计算功能  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 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

//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^&^
   
   
   
  }

}
}