实例讲解System.Data.DataTable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计算功能详解，转载请保留以下信息
//测试环境      vs2005,c#           
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;

//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。      
            
            
}

}
}