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  • Linq入门演练---(1)基本用法-分组，排序,内连接

    这一节大家共同学习下LINQ的基本用法，主要包括LINQ的分组，排序，和内外连接。

    1.分组

       基本语法:  group element by key

                          element 表示查询结果返回的元素，key表示分组条件。group子句返回的类型为IGrouping<TKey,TElement>的查询结果。

                          IGrouping<TKey,TElement>可以看成一个hashtable内部嵌套一个list列表的数据结果. 其中TElement就是一个list

    我们看下面的一个事例，下面这个实例演示根据性别对用户进行分组

    代码

    [csharp] view plaincopyprint?
    1. publicclass User 
    2.     { 
    3.         public User(string userName,string sex,int age) 
    4.         { 
    5.             UserName = userName; 
    6.             Sex = sex; 
    7.             Age = age; 
    8.         } 
    9.  
    10.         publicstring UserName 
    11.         { 
    12.             get; 
    13.             set; 
    14.         } 
    15.  
    16.         publicstring Sex 
    17.         { 
    18.             get; 
    19.             set; 
    20.         } 
    21.  
    22.         publicint Age 
    23.         { 
    24.             get; 
    25.             set; 
    26.         } 
    27.     }  
    public class User
    {
        public User(string userName,string sex,int age)
        {
            UserName = userName;
            Sex = sex;
            Age = age;
        }

        public string UserName
        {
            get;
            set;
        }

        public string Sex
        {
            get;
            set;
        }

        public int Age
        {
            get;
            set;
        }
    }
    [csharp] view plaincopyprint?
    1. privatevoid button1_Click(object sender, EventArgs e) 
    2.         { 
    3.             User[] users = {  
    4.                            new User("刘德华","男",40), 
    5.                            new User("张学友","男",30), 
    6.                            new User("张曼玉","女",35), 
    7.                            new User("杨幂","女",53) 
    8.                            }; 
    9.  
    10.             var query1 = 
    11.                   from var1 in users 
    12.                   group var1 by var1.Sex; 
    13.  
    14.             string str = ""; 
    15.             foreach (var item in query1) 
    16.             { 
    17.                 str += item.Key + " "; 
    18.  
    19.                 foreach (var user in item) 
    20.                 { 
    21.                     str += user.UserName + "--" + user.Sex + "--" + user.Age.ToString() + " "; 
    22.                 } 
    23.               
    24.             } 
    25.             textBox1.Text = str; 
    26.  
    27.         }  
    private void button1_Click(object sender, EventArgs e)
        {
            User[] users = { 
                           new User("刘德华","男",40),
                           new User("张学友","男",30),
                           new User("张曼玉","女",35),
                           new User("杨幂","女",53)
                           };

            var query1 =
                  from var1 in users
                  group var1 by var1.Sex;

            string str = "";
            foreach (var item in query1)
            {
                str += item.Key + "
";

                foreach (var user in item)
                {
                    str += user.UserName + "--" + user.Sex + "--" + user.Age.ToString() + "
";
                }
             
            }
            textBox1.Text = str;

        }

    效果图

    以上是简单的分组，如果需要对分组结果进行排序，需要把分组结果保存到一个临时变量中

    语法： group element by key into  temp

       以下是按年龄从小到大的排序：

      代码

    [csharp] view plaincopyprint?
    1. User[] users = {  
    2.                           new User("刘德华","男",40), 
    3.                           new User("张学友","男",30), 
    4.                           new User("张曼玉","女",35), 
    5.                           new User("杨幂","女",53) 
    6.                           }; 
    7.  
    8.            var query1 = 
    9.                  from var1 in users 
    10.                  group var1 by var1.Age into temp 
    11.                  orderby temp.Key descending 
    12.                  select temp; 
    13.  
    14.            string str = ""; 
    15.            foreach (var item in query1) 
    16.            { 
    17.                str += item.Key + " "; 
    18.  
    19.                foreach (var user in item) 
    20.                { 
    21.                    str += user.UserName + "--" + user.Sex + "--" + user.Age.ToString() + " "; 
    22.                } 
    23.              
    24.            } 
    25.            textBox1.Text = str;  
     User[] users = { 
                           new User("刘德华","男",40),
                           new User("张学友","男",30),
                           new User("张曼玉","女",35),
                           new User("杨幂","女",53)
                           };

            var query1 =
                  from var1 in users
                  group var1 by var1.Age into temp
                  orderby temp.Key descending
                  select temp;

            string str = "";
            foreach (var item in query1)
            {
                str += item.Key + "
";

                foreach (var user in item)
                {
                    str += user.UserName + "--" + user.Sex + "--" + user.Age.ToString() + "
";
                }
             
            }
            textBox1.Text = str;

       效果图

     

    2.排序

        在linq中排序使用orderby

        语法 orderby elment  descending||ascending

          ascending表示升序，descending表示降序

        下面这个DEMO演示按年龄从大到小排序

        代码：

        

    [csharp] view plaincopyprint?
    1. User[] users = {  
    2.                            new User("刘德华","男",40), 
    3.                            new User("张学友","男",30), 
    4.                            new User("张曼玉","女",35), 
    5.                            new User("杨幂","女",53) 
    6.                            }; 
    7.  
    8.             var query1 = 
    9.                   from var1 in users 
    10.                   orderby var1.Age descending 
    11.                   select var1; 
    12.  
    13.             string str = ""; 
    14.             foreach (var item in query1) 
    15.             { 
    16.                 str += item.UserName + "--" + item.Age + " "; 
    17.             } 
    18.             textBox1.Text = str;  
    User[] users = { 
                           new User("刘德华","男",40),
                           new User("张学友","男",30),
                           new User("张曼玉","女",35),
                           new User("杨幂","女",53)
                           };

            var query1 =
                  from var1 in users
                  orderby var1.Age descending
                  select var1;

            string str = "";
            foreach (var item in query1)
            {
                str += item.UserName + "--" + item.Age + "
";
            }
            textBox1.Text = str;

      效果图

      

    3.内连接

        内连接查询就是根据一定的条件查询两个数据源中都存在的元素

       语法： join elment  in datasource on re1 equals re2

       以下代码查找两个数组中都存在的元素。

      代码：

    [csharp] view plaincopyprint?
    1. privatevoid button3_Click(object sender, EventArgs e) 
    2.         { 
    3.             //查询两个数组中都存在的元素 
    4.             int[] arrOne = { 10,20,30,6,8}; 
    5.             int[] arrTwo = { 10,4,6,8,90,3}; 
    6.  
    7.             var query = from re1 in arrOne 
    8.                         join re2 in arrTwo on re1 equals re2 
    9.                         select new { re1 = re1, re2 = re2 }; 
    10.  
    11.             foreach (var item in query) 
    12.             { 
    13.                 Console.WriteLine(item); 
    14.             } 
    15.  
    16.         }  
    private void button3_Click(object sender, EventArgs e)
        {
            //查询两个数组中都存在的元素
            int[] arrOne = { 10,20,30,6,8};
            int[] arrTwo = { 10,4,6,8,90,3};

            var query = from re1 in arrOne
                        join re2 in arrTwo on re1 equals re2
                        select new { re1 = re1, re2 = re2 };

            foreach (var item in query)
            {
                Console.WriteLine(item);
            }

        }

      查询结果：

    [csharp] view plaincopyprint?
    1. { re1 = 10, re2 = 10 } 
    2. { re1 = 6, re2 = 6 } 
    3. { re1 = 8, re2 = 8 }  
    { re1 = 10, re2 = 10 }
{ re1 = 6, re2 = 6 }
{ re1 = 8, re2 = 8 }

    4.左连接

    左连接和SQL中的左连接类似，返回第一个结果集中的所有元素。

    下面的代码演示使用左连接，左连接要用到defaultempty()方法，此方法从列表中获取指定元素，如果列表为空，返回默认值

    代码

    [csharp] view plaincopyprint?
    1. privatevoid button4_Click(object sender, EventArgs e) 
    2.       { 
    3.           //左连接 
    4.           int[] arrOne = { 10, 20, 30, 6, 8 }; 
    5.           int[] arrTwo = { 10, 4, 6, 8, 90, 3 }; 
    6.  
    7.           var query = from re1 in arrOne 
    8.                       join re2 in arrTwo on re1 equals re2 into temgrp 
    9.                       from grp in temgrp.DefaultIfEmpty() 
    10.                       select new { re1 = re1, re2 = grp }; 
    11.  
    12.           foreach (var item in query) 
    13.           { 
    14.               Console.WriteLine(item); 
    15.           } 
    16.       }  
      private void button4_Click(object sender, EventArgs e)
        {
            //左连接
            int[] arrOne = { 10, 20, 30, 6, 8 };
            int[] arrTwo = { 10, 4, 6, 8, 90, 3 };

            var query = from re1 in arrOne
                        join re2 in arrTwo on re1 equals re2 into temgrp
                        from grp in temgrp.DefaultIfEmpty()
                        select new { re1 = re1, re2 = grp };

            foreach (var item in query)
            {
                Console.WriteLine(item);
            }
        }

    查询结果

    [csharp] view plaincopyprint?
    1. { re1 = 10, re2 = 10 } 
    2. { re1 = 20, re2 = 0 } 
    3. { re1 = 30, re2 = 0 } 
    4. { re1 = 6, re2 = 6 } 
    5. { re1 = 8, re2 = 8 } 
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  • 原文地址：https://www.cnblogs.com/itjeff/p/3359540.html
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