原文： c#中datatable中的compute方法使用收集
compute函数的参数就两个：expression，和filter。
expresstion是计算表达式，关于expression的详细内容请看这里“http://msdn2.microsoft.com/zh-cn/library/system.data.datacolumn.expression(vs.80).aspx”。而filter则是条件过滤器，类似sql的where条件。
datatable dt = new datatable();
//嵌套的三元运算 牛叉到五体投地
object obj = dt.compute("iif(1000=5,1000,iif(100>100,4001,2000))", null);
response.write(obj);
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);
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报错，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
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