LinQ詳解

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.簡單的函數計算（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 好像不支持 Last() 
        var ss1 = db.Am_recProScheme.FirstOrDefault();
        //var ss1 = db.Am_recProScheme.First();          

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

6.跳過前面多少條數據取餘下的數據

        //1
        var ss = (from r in db.Am_recProScheme
                  orderby r.rpId descending
                  select r).Skip(10); //跳過前10條數據，取10條之後的所有數據   
        //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.分頁數據查詢

        //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條數據                   

        //2 Take(10): 數據從開始獲取，獲取指定數量（10）的連續數據
        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";


       //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.連接查詢

        //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)";