linq 和lamba表达式

Linq To Sql中实现Left Join与Inner Join使用Linq语法与lambda表达式

当前有两个表，sgroup与sgroupuser，两者通过gKey关联，而sgroup表记录的是组，而sgroupuser记录是组中的用户，因此在sgroupuser中不一定有数据。需要使用Left Join获取数据：

Linq语法如下:

var sg = (from g in dc.sgroup
                     join gu in dc.sgroupuser on g.gKey equals gu.gKey into l
                     from lgu in l.DefaultIfEmpty()
                      select new { g, lgu }).ToList();

Lambda表达式如下：

var sg = dc.sgroup.GroupJoin(dc.sgroupuser, g => g.gKey, gu => gu.gKey, (g, gu) => new { g, gu }).Select(o=>o).ToList() ;

注意:

    Linq 与Lambda表达式取出的结果有所不同.Linq取出的结果的记录数与Sql中的Left Join的结果相同,而Lambda表达式取出的记录数是sgroup表中的记录数,sgroupuser对应的记录是以对象集合存在于结果中

附:

下面是Inner Join:

Linq语法如下:

var sg = (from g in dc.sgroup
                    join gu in dc.sgroupuser on g.gKey equals gu.gKey                  
                    select new { g, gu }).ToList();

Lambda表达式如下：

var sg = dc.sgroup.Join(dc.sgroupuser, g => g.gKey, gu => gu.gKey, (g, gu) => new { g, gu }).Select(o=>o).ToList() ;

注意:

   上面最后都用到了ToList()方法 , 用ToList()是为了一次性将数据取到本地.

egg:例子

using System;
using System.Collections.Generic;
using System.Text;
using System.Linq;
using System.Collections;

namespace fuck
{
    public delegate int del(int i);
    public delegate bool Mydetext();
    class Program
    {
        //聲明委託變量
        public Mydetext mytext;
        static void Main(string[] args)
        {
            #region 例子一
            //聲明委託變量
            del myDelegate = x => x * x; //lambda表達式也就是匿名委託的原理  
            //聲明委託變量
            del myDelegatetwo = delegate(int x)
                {
                    return x * x;
                };
            int i = myDelegate(5);
            int j = myDelegatetwo(5); //j = 25
            Console.WriteLine(i);
            Console.WriteLine(j);
            #endregion

            #region 例子二
            //聲明委託變量
            if (new Program() { mytext = new Mydetext(delegate() { Console.Write("Hello "); return false; }) }.mytext())
            {
                Console.Write("Hello ");
            }
            else
            {
                Console.WriteLine("World");
            }
            if (new Func<bool>(() => { Console.Write("Hello "); return false; }).Invoke())
            {
                Console.Write("Hello ");
            }
            else
            {
                Console.WriteLine("World");
            }
            if (new Program() { mytext = new Mydetext(() => { Console.Write("Hello "); return false; }) }.mytext())
            {
                Console.Write("Hello ");
            }
            else
            {
                Console.WriteLine("World");
            }
            #endregion

            #region 例子三
            List<int> ls = new List<int>();
            ls.Add(1);
            ls.Add(9);
            ls.Add(5);
            ls.Add(7);
            var chaojie = ls.Select((m, index) => new { m, index }).OrderByDescending(n => n.m).Take(1);
            //try 下面的和上面的效果是一樣子的   //這裡是這樣子理解的聲明一個匿名委託 return new { m, index };可以理解為查詢出m,和index
            var chaojietwo = ls.Select(delegate(int m,int index) { return new { index, m }; }).OrderByDescending(n => n.m).Take(1);
            //在這裡其實就是ls.Select(new {})  和例子四很像   delegate(int m,int index)的參數順序決定是值,還是索引/第一個是值,第二個是索引
            foreach (var w in chaojietwo)
            {
                Console.WriteLine("索引" + w.index + ",值" + w.m);
            }
            #endregion

            #region 例子四
            Dictionary<int,string> alt=new Dictionary<int,string>();
            alt.Add(1,"一");
            alt.Add(9,"九");
            alt.Add(5,"五");
            alt.Add(7,"七");
            var query = from qy in alt
                        select new {qy.Key,qy.Value };
            foreach (var q in alt)
            {
                Console.WriteLine("鍵" + q.Key + ",值" +q.Value);
            }
            #endregion

            #region 例子五
            List<double> list = new List<double>();
            list.Add(2.55);
            list.Add(4.45);
            list.Add(12.35);
            list.Add(18.35);
            list.Add(17.35);
            list.Add(19.35);
            list.Add(6.355);
            list.Add(7.95);
            list.Add(4.35);
            var v = list.Select((m, index) => new { m, index }).OrderByDescending(n => n.m).Take(1);
            foreach (var w in v)
            {
                Console.WriteLine("索引"+w.index+",值"+w.m);
            }
            List<double> list2 = new List<double>();
            list2.Add(2.55);
            list2.Add(4.45);
            list2.Add(18.35);
            list2.Add(17.35);
            list2.Add(19.35);
            list2.Add(6.355);
            list2.Add(7.95);
            list2.Add(4.35);
            //其實這兩個是一個樣子的
            //var lt2 = list2.Where(a=>a==2.55);
            //lt2也可以寫成
            var lt2 = list2.Where(delegate(double ls11) { return ls11==2.55;});


           // var lt3 = (from z in list2
                      // select z).Where(y=>y>3);
            //其實下面這個和上面這個也是一樣子的
            var lt3 = (from z in list2
                       select z).Where(delegate(double y) { return y > 3; });
            Console.WriteLine("條件一");
            foreach (var w in lt2)
            {
                Console.WriteLine("值"+w);
            }
            Console.WriteLine("條件二");
            foreach (var w in lt3)
            {
                Console.WriteLine("值" + w);
            }
            #endregion
            Console.ReadKey();
           
        }
    }
}
效果

在公司的讲课

關於linq的最簡單例子
例子1
            var ss = from r in Am_recProScheme
                     select r;
            string sssql = "select * from Am_recProScheme";
例子2
            var ss = from r in Am_recProScheme
                     where r.rpId > 10
                     select r;          
            string sssql = "select * from Am_recProScheme where rpid>10";

關於linq的最簡單例子

簡單函數count min max sum 的應用

例子3      var ss = (from r in Am_recProScheme

                     select r).Max(p=>p.rpId);

           var ss = (from r in Am_recProScheme

                     select r).Sum(p=>p.rpId);

           var ss = (from r in Am_recProScheme

                     select r).Min(p=>p.rpId);

 

            string sssql = "select Max(reId) from Am_recProScheme";

            string sssql = "select Sum(reId) from Am_recProScheme";

            string sssql = "select Min(reId) from Am_recProScheme";

簡單order by應用

例子4            

             var ss = from r in Am_recProScheme

                     where r.rpId > 10

                     order by r.reId desc

                     select r;

                      

           

           string sssql = "select * from Am_recProScheme where rpid>10 order by rpId desc"



關於linq的最簡單例子

簡單函數like,in的應用

例子3       var ss = from r in Am_recProScheme

                     where r.SortsText.Contains("張");

                     select r;

            string sssql = "select * from Am_recProScheme where r.SortsText like ‘%張%‘";

           

            var ss = from r in Am_recProScheme

                     where （new int[]{24,25,26}）.Contains(r.rpId);

                     select r;

            string sssql = "select * from Am_recProScheme where r.rpId in (24,25,26)";

 

簡單查詢出某幾列應用

例子4            

             var ss = from r in Am_recProScheme

                      where r.rpId > 10

                     order by r.rpId desc

                     select new {r.rpId,r.rpName};

                      

           

           string sssql = "select rpId,rpName from Am_recProScheme where rpid>10 order by rpId desc"



關於linq的最簡單例子

簡單函數連表查詢的應用

我们在做SQL查询的时候经常会用到Inner Join,Left Join，连接方式的概念方面我想也不用给予太多解释，

我们今天的重点是让大家熟悉LINQ是如何使用Join来实现常用的表连接的。

创建测试用类：

class Customer
{
    public int CustomerId { get; set; }

    public string Name { get; set; }

    public int Age { get; set; }

}

class Product
{
    public int ProductId { get; set; }

    public string Name { get; set; }

    public string Origin { get; set; }

}

class Order
{
    public int OrderId { get; set; }

    public int CustomerId { get; set; }

    public List<Product> Products { get; set; }
}

我们用以下例子来熟悉 Join 关键字的用法。

1.Inner Join:

CreateEntities();
var query = from c in customers
            join o in orders on c.CustomerId equals o.CustomerId
            where o.OrderId == 2
            select c;
foreach (var customer in query)
{
    Console.WriteLine("Id:{0}, Name:{1}", customer.CustomerId, customer.Name);
}

　

运行结果：

　　Id:1, Name:CA 　　

　上面这个是常见的内连接的例子，和SQL语法也很相似，但有以下几点要注意：

(1).连接条件： c.CustomerId equals o.CustomerId 只能使用 equals 不能用 =,==,等于 等表示。
以为LINQ的设计者认为 几乎所有的连接条件都是 = 条件不会出现 >,<,!= 等情况因此使用了个关键字来描述表连接条件。

(2).条件顺序：c.CustomerId equals o.CustomerId ，range variable: c 和b之前的顺序不能颠倒。

 

2.Group Join:

也许大家对Group Join的概念不太了解，没关系让我们通过例子来认识它：

CreateEntities();
var query = from c in customers
            join o in orders on c.CustomerId equals o.CustomerId into os
            select new { c, os };
foreach (var item in query)
{
    Console.WriteLine("Customer Id:{0}, Name:{1}", item.c.CustomerId, item.c.Name);
    foreach (var o in item.os)
    {
        Console.WriteLine("--Order Id:{0}", o.OrderId);
    }
}


结果：

Customer Id:1, Name:CA
--Order Id:1
--Order Id:2
Customer Id:2, Name:CB
--Order Id:4
Customer Id:3, Name:CC
--Order Id:3
Customer Id:4, Name:CD
Press any key to continue . . .

以上查询返回的结果：和Group By 返回的结果非常的相似：一个KEY对象对应一个集合。

要实现Group Join我们要引入一个关键字：into

但使用时要注意一下几点：

(1).使用into 关键字后 join 后面的 range variable：o 在后面的表达式块中就失去了作用域。

(2).range variable：os 通常情况下都是IEnumerable<T>类型的。

3.Left Join:

　Left Join 我们在SQL里经常用到，让我们来看看LINQ里怎么实现它：

CreateEntities();
var query = from c in customers
            join o in orders on c.CustomerId equals o.CustomerId into os
            from o2 in os.DefaultIfEmpty(
                new Order { OrderId = 0, CustomerId = 0, Products = new List<Product>() })
            select new { c, o2 };
foreach (var item in query)
{
    Console.WriteLine("Customer Id:{0}, Name:{1}--Order Id:{0}",
        item.c.CustomerId, item.o2.OrderId);
}

结果：

Customer Id:1, Name:1--Order Id:1
Customer Id:1, Name:2--Order Id:1
Customer Id:2, Name:4--Order Id:2
Customer Id:3, Name:3--Order Id:3
Customer Id:4, Name:0--Order Id:4
Press any key to continue . . .

我们可以看到Left Outer Join 的语法进一步的复杂化了，结果也有细微的不同。

(1).从语法上：

from o2 in os.DefaultIfEmpty(
                new Order { OrderId = 0, CustomerId = 0, Products = new List<Product>() })

主要区别在于以上者1句语句。查询方法DefaultIfEmpty 用于定义当查询记录为空时，预定义默认值。再从其集合中取出子元素。

(2).从结果上: 我们在遍历查询结果时可以发现Left Join相似于Inner Join结果都是“平面”的，然而Group Join返回的结果具有层次性。

 

题外：

由于C#是面向对象的，往往会通过对象与对象间的外系来实现数据间关系。有时表达2个之间的关系也可以不使用Join关键字，

因此Join关键字其实在实际LINQ查询表达式中用的不是很多。

2015-03-25



以下是linq連表的例子   相當於左連接
            var ls = (from i in gt.vendor_login
                  join j in gt.vendor on i.vendor_id equals j.vendor_id
                  into AAA
                  from j in AAA.DefaultIfEmpty()
                  select new myclass { AA = i.vendor_id, BB = j != null ? j.vendor_name_full : null }).ToList();
 
             // 相當於內連接
             var ls = (from i in gt.vendor_login
                  fromj in gt.vendor on i.vendor_id equals j.vendor_id
                   select new myclass { AA = i.vendor_id, BB = j != null ? j.vendor_name_full : null }).ToList();
          //類似於內連接
          ls = (from i in gt.vendor_login
                  from j in gt.vendor
                  where i.vendor_id==j.vendor_id
                  select new { i, j }).ToList().ConvertAll<VendorLoginQuery>(w => new VendorLoginQuery { login_id = w.i.login_id, vendor_id = w.i.vendor_id, login_ipfrom = w.i.login_ipfrom, login_createdate = w.i.login_createdate, vendor_name_full = w.j.vendor_name_full }).Skip(10).Take(100).ToList();