C# DataTable中平均值，方差，標準差等方法用法集錦(數值/字符串/運算符/表等操作)

Compute函數的參數就兩個：Expression，和Filter。

Expresstion是計算表達式，關於Expression的詳細內容請看這裏：

http://msdn2.microsoft.com/zh-cn/library/system.data.datacolumn.expression(VS.80).aspx

而Filter則是條件過濾器，類似sql的Where條件。

DataTable dt = new DataTable();
//嵌套的三元運算
object obj = dt.Compute("iif(1000=5,1000,iif(100>100,4001,2000))", null);
Response.Write(obj);
System.Data.DataTable table = new DataTable();
//計算常量，可以沒有初始化列
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;常數計算和filter無關
test = table.Compute("abs(1)", "");
Console.WriteLine(test);
//test=null，不知道爲什麼這個沒有報錯，而且返回null,其他的數學函數都會抱錯
test = table.Compute("2%2", "");
Console.WriteLine(test);
//test=0;
//其他函數參考下面的計算列
//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");
/**/
////拋出異常，這裏必須是聚合函數
//*************************************支持的聚合函數**********************//
//求數量
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)","");
/**/
////拋出異常，這裏不能是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;
//統計標準偏差
test = table.Compute("StDev(value)", "");
Console.WriteLine(test);
//test=3.02765035409749
//統計方差
test = table.Compute("Var(value)", "");
Console.WriteLine(test);
//test=9.16666666666667
//複雜計算
test = table.Compute("max(value)/sum(value)", "");
Console.WriteLine(test);
//test=0.181818181818182
/**/
/*******************************************計算列*************************/
System.Data.DataColumn column = new DataColumn("exp1", typeof(float));
table.Columns.Add(column);
//簡單計算
column.Expression = "value*2";
test = table.Select("id='id1'")[0]["exp1"];
Console.WriteLine(test);
//test=2;
//字符串函數
column.Expression = "len(id)";
test = table.Select("id='id1'")[0]["exp1"];
Console.WriteLine(test);
//test=3;
//字符串函數
column.Expression = "len(' '+id+' ')";
test = table.Select("id='id1'")[0]["exp1"];
Console.WriteLine(test);
//test=5;
//字符串函數
column.Expression = "len(trim(' '+id+' '))";
test = table.Select("id='id1'")[0]["exp1"];
Console.WriteLine(test);
//test=3;
//字符串函數
column.Expression = "substring(id,3,len(id)-2)";
test = table.Select("id='id1'")[0]["exp1"];
Console.WriteLine(test);
//test=1; //substring的起始字符位置爲1不是0
//類型轉換
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;
//相當於sqlserver的isnull
column.Expression = "isnull(value,10)";
test = table.Select("id='id1'")[0]["exp1"];
Console.WriteLine(test);
//test=1;
//三元運算符,相當於sqlserver的case when
column.Expression = "iif(value>5,1000,2000)";
test = table.Select("id='id1'")[0]["exp1"];
Console.WriteLine(test);
//test=2000;
//like運算符
column.Expression = "iif(id like '%1',1000,2000)";
test = table.Select("id='id1'")[0]["exp1"];
Console.WriteLine(test);
//test=1000;
//in運算符
column.Expression = "iif(id not in('id1'),1000,2000)";
test = table.Select("id='id1'")[0]["exp1"];
Console.WriteLine(test);
//test=2000;
//嵌套的三元運算
column.Expression = "iif(value>5,1000,iif(id like '%1',4000,2000))";
test = table.Select("id='id1'")[0]["exp1"];
Console.WriteLine(test);
//test=4000;
//客戶端計算所佔總數的百分比
column.Expression = "value/sum(value)";
test = table.Select("id='id1'")[0]["exp1"];
Console.WriteLine(test);
//test=0.01818182
//客戶端計算差值,比如nba常規賽的勝場差
column.Expression = "max(value)-value";
test = table.Select("id='id1'")[0]["exp1"];
Console.WriteLine(test);
//test=9
//***********************父子表計算*************************************/
//初始化子表,父子表關係
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);
}
//計算子表記錄數
column.Expression = "count(child(relation).value)";
test = table.Select("id='id1'")[0]["exp1"];
Console.WriteLine(test);
//test=10;
//計算父子表的百分比
column.Expression = "value/sum(child(relation).value)";
test = table.Select("id='id1'")[0]["exp1"];
Console.WriteLine(test);
//test=0.01818182;
//計算父子表的差值,比如父表爲庫存數量，子表爲訂購數量，計算得出需要補充的數量
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;
//比較遺憾的是沒有發現能夠計算同比和環比的方法，而且計算列無法作爲約束
//結束，DataTable可以讓你儘量發揮聰明才智來減少繁雜的sql語句並且減輕服務器計算符合