本人转载:http://www.cnblogs.com/knowledgesea/p/3897665.html

闲言碎语

近期比较忙，但还是想写点什么，就分享一些基础的知识给大家看吧，希望能帮助一些linq新手，如果有其它疑问，可以进右上角群，进行交流探讨，谢谢。

开门见山

读这篇文章之前，我先说下，每一种搜索结果集，我都以三种方式变现出来，为啦更好的理解，希望不要嫌我啰嗦。

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";

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)";

12.排序

As Brannon says, it's OrderByDescending and ThenByDescending:

var query = from person in people            orderby person.Name descending, person.Age descending            select person.Name;

is equivalent to:

var query = people.OrderByDescending(person => person.Name)                  .ThenByDescending(person => person.Age)                  .Select(person => person.Name);

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