C# LINQ基本语法

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.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";


           //多字段
var result = (from item in data
group item by new { item.Name, item.Type } into items
select new
{
    items.Key.Name,
    items.Key.Type,
    Cnt = items.Count()
}).ToList();


var s = data.GroupBy(p => new { p.Type, p.Name }).Select(p=>new {
          p.Key.Type,
          p.Key.Name,
          cnt=p.Count()
}).ToList();

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)";

转载自：https://www.cnblogs.com/knowledgesea/p/3897665.html