常用linq语法

1.简单的linq语法

 //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.查询第一条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)";

建议这十个常用的查询标记学习，再加上视图、存储过程、函数，基本上就能解决大部分的查询问题啦。