C# LINQ語法詳解

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)

//如果取最后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余下的數據

//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條之后的所有數據   
//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";

10.sql中的In

//Linq
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);
}
//SQL
string st = "select * from Am_recProScheme where rpId in(24,25,26)";

11.內連接 INNER JOIN

//Linq
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;
//Lambda
var ss1 = db.Am_recProScheme.Join(db.Am_Test_Result, p => p.rpId, r => r.rsId, (p, r) => p).OrderByDescending(p => p.rpId).ToList();
//SQL
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";

12.左連接 LEFT JOIN

//兩個DataTable關聯，查找只在第一個表中的數據
static void Main(string[] args)
{
    DataTable dt1 = new DataTable("Ta");
    DataTable dt2 = new DataTable("Tb");
    dt1.Columns.Add("ID", typeof(int));
    dt1.Columns.Add("Name", typeof(string));
    dt1.Rows.Add(1, "小明");
    dt1.Rows.Add(2, "小紅");
    dt1.Rows.Add(3, "小黑");
    dt2.Columns.Add("ID", typeof(int));
    dt2.Columns.Add("Name", typeof(string));
    dt2.Rows.Add(1, "小黃");
    dt2.Rows.Add(2, "小紅");
    dt2.Rows.Add(3, "小強");
    //方法一：Linq
    //var query = from q1 in dt1.AsEnumerable()
    //            join q2 in dt2.AsEnumerable()
    //            on q1.Field<string>("Name") equals q2.Field<string>("Name") into tmp
    //            from q3 in tmp.DefaultIfEmpty()
    //            where q3 == null
    //            select new
    //            {
    //                ID = q1.Field<int>("ID"),
    //                Name = q1.Field<string>("Name")
    //            };
    //方法二：Lambda
    var query = dt1.AsEnumerable().GroupJoin(
        dt2.AsEnumerable(),
        x => x.Field<string>("Name"),
        y => y.Field<string>("Name"),
        (x, y) => y.DefaultIfEmpty(). Where(w => w == null).
        Select(z => new { ID = x.Field<int>("ID"), Name = x.Field<string>("Name") })
        ).SelectMany(x => x);
    foreach (var item in query)
    {
        Console.WriteLine($"ID={item.ID}    Name={item.Name}");
    }
    Console.Read();
}

實例用法：

//數據庫 + 自定義名稱 =new 數據庫
mydbDataContext con = new mydbDataContext();
//模糊查詢表達式中用.Contains
con.car.Where(r=>r.name.Contains(TextBox1.Text.Trim())).ToList(）;
//開頭查用.StartWith
con.car.Where(r => r.name.StartsWith(TextBox1.Text)).ToList();
//結尾查用.EndWith
con.car.Where(r => r.name.EndsWith(TextBox1.Text)).ToList();
//最大值
con.car.Max(r => r.price * r.oil).ToString();
//最小值
con.car.Min(r => r.price).ToString();
//求和
con.car.Sum(r => r.price).ToString();
//平均值
con.car.Average(r => r.price).ToString();
//升序：
con.car.OrderBy(r => r.price).ToList();
//降序：
con.car.OrderByDescending(r => r.price).ToList();

//上一頁，下一頁，組合查詢：
int PageCount = 5;//每頁顯示條數
//上一頁，PageCount_Label.Text為當前頁碼
int pageNum = Convert.ToInt32(PageCount_Label.Text) - 1;
Repeater1.DataSource = con.car.Skip((pageNum - 1) * PageCount).Take(PageCount);
Repeater1.DataBind(); 
PageCount_Label.Text = pageNum.ToString();
//下一頁
int pageNum = Convert.ToInt32(PageCount_Label.Text) + 1;
Repeater1.DataSource = con.car.Skip((pageNum - 1) * PageCount).Take(PageCount);
Repeater1.DataBind();
PageCount_Label.Text = pageNum.ToString();
//組合查詢的點擊事件
List<car> list = con.car.ToList();
if (TextBox2.Text != "")
{
    List<car> list1 = con.car.Where(r => r.name.Contains(TextBox2.Text)).ToList();
    list = list.Intersect(list1).ToList();
}
if (TextBox3.Text != "")
{
    List<car> list1 = con.car.Where(r => r.oil == Convert.ToDecimal(TextBox3.Text)).ToList();
    list = list.Intersect(list1).ToList();
}
if (TextBox4.Text != "")
{
    List<car> list1 = con.car.Where(r => r.powers == Convert.ToInt32(TextBox4.Text)).ToList();
    list = list.Intersect(list1).ToList();
}
Repeater1.DataSource = list;
Repeater1.DataBind();