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

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. publicstring UserName
10. {
11. get;
12. set;
13. }
14. publicstring Sex
15. {
16. get;
17. set;
18. }
19. publicint Age
20. {
21. get;
22. set;
23. }
24. }

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;
        }
    }

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. var query1 =
10. from var1 in users
11. group var1 by var1.Sex;
12. string str = "";
13. foreach (var item in query1)
14. {
15. str += item.Key + "\r\n";
16. foreach (var user in item)
17. {
18. str += user.UserName + "--" + user.Sex + "--" + user.Age.ToString() + "\r\n";
19. }
20. }
21. textBox1.Text = str;
22. }

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 + "\r\n";

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

            }
            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

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. var query1 =
8. from var1 in users
9. group var1 by var1.Age into temp
10. orderby temp.Key descending
11. select temp;
12. string str = "";
13. foreach (var item in query1)
14. {
15. str += item.Key + "\r\n";
16. foreach (var user in item)
17. {
18. str += user.UserName + "--" + user.Sex + "--" + user.Age.ToString() + "\r\n";
19. }
20. }
21. 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 + "\r\n";

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

            }
            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：

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. var query1 =
8. from var1 in users
9. orderby var1.Age descending
10. select var1;
11. string str = "";
12. foreach (var item in query1)
13. {
14. str += item.UserName + "--" + item.Age + "\r\n";
15. }
16. 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 + "\r\n";
            }
            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：

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. var query = from re1 in arrOne
7. join re2 in arrTwo on re1 equals re2
8. select new { re1 = re1, re2 = re2 };
9. foreach (var item in query)
10. {
11. Console.WriteLine(item);
12. }
13. }

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：

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

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. var query = from re1 in arrOne
7. join re2 in arrTwo on re1 equals re2 into temgrp
8. from grp in temgrp.DefaultIfEmpty()
9. select new { re1 = re1, re2 = grp };
10. foreach (var item in query)
11. {
12. Console.WriteLine(item);
13. }
14. }

  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

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 }