Linq語法詳細

閒言碎語

近期比較忙，但還是想寫點什麼，就分享一些基礎的知識給大家看吧，希望能幫助一些linq新手，如果有其它疑問，可以進右上角群，進行交流探討，謝謝。

開門見山

讀這篇文章之前，我先說下，每一種搜尋結果集，我都以三種方式變現出來，為啦更好的理解，希望不要嫌我囉嗦。

1.簡單的linq語法

            //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.帶where的查詢

            //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.簡單的函式計算（count，min，max，sum）

            //1
            ////獲取最大的rpId
            //var ss = (from r in db.Am_recProScheme
            //          select r).Max(p => p.rpId);
            ////獲取最小的rpId
            //var ss = (from r in db.Am_recProScheme
            //          select r).Min(p => p.rpId);
            //獲取結果集的總數
            //var ss = (from r in db.Am_recProScheme                  
            //         select r).Count();
            //獲取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)

            //如果取最後一個可以按倒敘排列再取值
            var ss = (from r in db.Am_recProScheme                     
                      select r).FirstOrDefault();

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

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

6.跳過前面多少條資料取餘下的資料

            //1
            var ss = (from r in db.Am_recProScheme
                      orderby r.rpId descending
                      select r).Skip(10); //跳過前10條資料，取10條之後的所有資料   
            //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.分頁資料查詢

            //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條到第20條資料                   

            //2 Take(10): 資料從開始獲取，獲取指定數量（10）的連續資料
            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，類似like '%%'

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

9.分組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,  //這個Key是recType
                         rpId = n.Sum(r => r.rpId), //組內rpId之和
                         MaxRpId = n.Max(r => r.rpId),//組內最大rpId
                         MinRpId = n.Min(r => r.rpId), //組內最小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.連線查詢 

            //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)";

 

如果你是linq新手，建議這十個常用的查詢標記學習，再加上檢視，基本上就能解決大部分的查詢問題啦。