LINQ的语法实例

发布网友发布时间：2022-04-27 13:17

共2个回答

懂视网时间：2022-04-09 02:41

  //1
  var ss = from r in db.Am_recProScheme
   select r;
  //2
  var ss1 = db.Am_recProScheme;
  //3
  string sssql = "select * from Am_recProScheme";

2.带where的查询

  //1
  var ss = from r in db.Am_recProScheme
   where r.rpId > 10
   select r;
  //2
  var ss1 = db.Am_recProScheme.Where(p => p.rpId > 10);
  //3
  string sssql = "select * from Am_recProScheme where rpid>10";

3.简单的函数计算（count，min，max，sum）

  //1
  ////获取最大的rpId
  //var ss = (from r in db.Am_recProScheme
  //  select r).Max(p => p.rpId);
  ////获取最小的rpId
  //var ss = (from r in db.Am_recProScheme
  //  select r).Min(p => p.rpId);
  //获取结果集的总数
  //var ss = (from r in db.Am_recProScheme   
  //  select r).Count();
  //获取rpId的和
  var ss = (from r in db.Am_recProScheme
   select r).Sum(p => p.rpId);


  //2
  //var ss1 = db.Am_recProScheme.Max(p=>p.rpId);
  //var ss1 = db.Am_recProScheme.Min(p => p.rpId);
  //var ss1 = db.Am_recProScheme.Count() ;
  var ss1 = db.Am_recProScheme.Sum(p => p.rpId);
  Response.Write(ss);

  //3
  string sssql = "select max(rpId) from Am_recProScheme";
   sssql = "select min(rpId) from Am_recProScheme";
   sssql = "select count(1) from Am_recProScheme";
   sssql = "select sum(rpId) from Am_recProScheme";

4.排序order by desc/asc

  var ss = from r in db.Am_recProScheme
   where r.rpId > 10
   orderby r.rpId descending //倒序
   // orderby r.rpId ascending //正序
   select r;

  //正序
  var ss1 = db.Am_recProScheme.OrderBy(p => p.rpId).Where(p => p.rpId > 10).ToList();
  //倒序
  var ss2 = db.Am_recProScheme.OrderByDescending(p => p.rpId).Where(p => p.rpId > 10).ToList();

  string sssql = "select * from Am_recProScheme where rpid>10 order by rpId [desc|asc]";

5.top(1)

  //如果取最后一个可以按倒叙排列再取值
  var ss = (from r in db.Am_recProScheme   
   select r).FirstOrDefault();

  //（）linq to ef 好像不支持 Last() 
  var ss1 = db.Am_recProScheme.FirstOrDefault();
  //var ss1 = db.Am_recProScheme.First();  

  string sssql = "select top(1) * from Am_recProScheme";

6.跳过前面多少条数据取余下的数据

  //1
  var ss = (from r in db.Am_recProScheme
   orderby r.rpId descending
   select r).Skip(10); //跳过前10条数据，取10条之后的所有数据 
  //2 
  var ss1 = db.Am_recProScheme.OrderByDescending(p => p.rpId).Skip(10).ToList();
  //3
  string sssql = "select * from (select ROW_NUMBER()over(order by rpId desc) as rowNum, * from [Am_recProScheme]) as t where rowNum>10";

7.分页数据查询

  //1
  var ss = (from r in db.Am_recProScheme
   where r.rpId > 10
   orderby r.rpId descending
   select r).Skip(10).Take(10); //取第11条到第20条数据   

  //2 Take(10): 数据从开始获取，获取指定数量（10）的连续数据
  var ss1 = db.Am_recProScheme.OrderByDescending(p => p.rpId).Where(p => p.rpId > 10).Skip(10).Take(10).ToList();
  //3
  string sssql = "select * from (select ROW_NUMBER()over(order by rpId desc) as rowNum, * from [Am_recProScheme]) as t where rowNum>10 and rowNum<=20";

8.包含，类似like ‘%%‘

  //1
  var ss = from r in db.Am_recProScheme
   where r.SortsText.Contains("张")
   select r;
  //2
  var ss1 = db.Am_recProScheme.Where(p => p.SortsText.Contains("张")).ToList();
  //3
  string sssql = "select * from Am_recProScheme where SortsText like ‘%张%‘";

9.分组group by

  //1
  var ss = from r in db.Am_recProScheme
   orderby r.rpId descending
   group r by r.recType into n
   select new
   {
    n.Key, //这个Key是recType
    rpId = n.Sum(r => r.rpId), //组内rpId之和
    MaxRpId = n.Max(r => r.rpId),//组内最大rpId
    MinRpId = n.Min(r => r.rpId), //组内最小rpId
   };
  foreach (var t in ss)
  {
  Response.Write(t.Key + "--" + t.rpId + "--" + t.MaxRpId + "--" + t.MinRpId);
  }
  //2
  var ss1 = from r in db.Am_recProScheme
   orderby r.rpId descending
   group r by r.recType into n
   select n;
  foreach (var t in ss1)
  {
  Response.Write(t.Key + "--" + t.Min(p => p.rpId));
  }
  //3
  var ss2 = db.Am_recProScheme.GroupBy(p => p.recType);
  foreach (var t in ss2)
  {
  Response.Write(t.Key + "--" + t.Min(p => p.rpId));
  }
  //4
  string sssql = "select recType,min(rpId),max(rpId),sum(rpId) from Am_recProScheme group by recType";

10.连接查询

  //1
  var ss = from r in db.Am_recProScheme
   join w in db.Am_Test_Result on r.rpId equals w.rsId
   orderby r.rpId descending
   select r;
  //2
  var ss1 = db.Am_recProScheme.Join(db.Am_Test_Result, p => p.rpId, r => r.rsId, (p, r) => p).OrderByDescending(p => p.rpId).ToList();
  //3
  string sssql = "select r.* from [Am_recProScheme] as r inner join [dbo].[Am_Test_Result] as t on r.[rpId] = t.[rsId] order by r.[rpId] desc";

11.sql中的In

  //1
  var ss = from p in db.Am_recProScheme
    where (new int?[] { 24, 25,26 }).Contains(p.rpId)
    select p;
  foreach (var p in ss)
  {
  Response.Write(p.Sorts);
  }
  //2
  string st = "select * from Am_recProScheme where rpId in(24,25,26)";

Linq语法详细(三种方式：linq、Lambda、SQL语法)

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热心网友时间：2022-04-08 23:49

C#3.0 LINQ 查询语法
首先来看一个很简单的LINQ查询例子，查询一个int数组中小于5的数字，并按照大小顺序排列：
static void Main(string[] args)
{
int[] arr = new int[] { 8, 5, 89, 41, 1, 2, 3, 65, 1 };
var m = from n in arr where n < 5 orderby n descending select n;//小于5，并且倒叙排列显示
foreach (var n in m)
{
Console.WriteLine(n);
}
Console.ReadLine();
}
上述代码除了LINQ查询语法外，其他都是我们所熟悉的语法，而LINQ查询语法跟SQL查询语法很相似，除了先后顺序。
Q：为何 LINQ 查询语法是以 from 关键字开头的，而不是以 select 关键字开头的？select 开头这种写法跟SQL的写法更接近，更易懂呀？
A：简单来说，为了IDE的智能感知（Intellisence）这个功能，select 关键字放在后面了。
编程语言以 select 开头写LINQ查询语法不是没出现过，你如果使用过2005年的VB9 CTP 版本，那时候VB9的LINQ查询语法就是 select 关键字在前面，但是 select 关键字在前面，在做智能感知（Intelisence）时候就很头大。经过微软IDE组的权衡，确定了把 from 关键字放在最前面。
那时候 VB9 LINQ的查询语法还是 select 参数在最前面。不过后来 VB9 测试版改成了跟 C# 一样的做法， from 关键字放在最前面了。
更详细的解释，来自装配脑袋
假设你要书写这样的代码：Select p. Name, p.Age From p In persons Where xxx ，代码是一个个字符输入的。
我们在写到 p in persons 之前，p 的类型是无法推测的，所以写 Select p. 的时候，Name之类的属性不会弹出智能提示来。
这样就需要先去写 From 这句，再回来写 Select。
微软IDE组经过反复考虑决定，还不如就把 Select 写到后面了。于是编程语言中的写法就确定这样来写了。
VB9 的这个变化可以参看这篇博客:
Select/From vs. From/Select revisited...
我们再来看一个稍稍复杂的LINQ查询：
在我们列举的语言字符串中，我们希望按照字符长短，分类列举出来，实现代码如下：
static void Main(string[] args)
{
string[] languages = { Java, C#, C++, Delphi, VB.net, VC.net, C++Builder, Kylix, Perl, Python };
var query = from item in languages
group item by item.Length into lengthGroups
orderby lengthGroups.Key
select lengthGroups;
foreach (var item in query)
{
Console.WriteLine(strings of length{0}, item.Key);
foreach (var val in item)
{
Console.WriteLine(val);
}
}
Console.ReadLine();
}
其中的 into 关键字表示将前一个查询的结果视为后续查询的生成器，这里是跟 group by 一起使用的。
LINQ中的Group by不要跟 SQL 中的Group by 混淆，SQL由于是二维结构，Group by 的一些逻辑受二维结构的约束，无法像 LINQ 中的Group by 这么灵活。
事实上，LINQ的查询语法存在以下两种形式：
查询方法方式：（Methord Syntax）
主要利用System.Linq.Enumerable类中定义的扩展方法和Lambda表达式方式进行查询
查询语句方式：（Query Syntax）一种更接近SQL语法的查询方式，可读性更好。