C#详解DataTable计算功能的实现

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^&^         
        } 
    } 
}