C#进阶-LINQ表达式总结

LINQ(语言集成查询-language intergrated query)是一款很常用的扩展包，支持C#和Java，在系统进行查询数据的动作时，相较于执行数据库层面的SQL语句，后端层面的LINQ运行起来会更加高效稳定，可以极大缩短每次与数据库交互的时间，增加系统功能的稳定性，提高查询效率。本篇文章是LINQ的使用教程，用LINQ模仿对应SQL语句的执行效果。

目录：

章节一：基础语法Ⅰ(Select、OrderBy、Count、Average、Sum、Max、Min)

章节二：基础语法Ⅱ(Any、All、Single、First、Last、Skip、Take、Top)

章节三：分组查询 (GroupBy)

章节四：多表查询 Ⅰ(交集、并集、差集、去重)

章节五：多表查询 Ⅱ(Join连接查询)

章节六：投影

章节七：LINQ实现对集合的增删改查

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准备工作：

我们创建User对象和包含User对象的集合，后面以此为例。

class User
{
    public int id { get; set; } 
    public string name { get; set; } 
    public bool gender { get; set; }//male: true; female: fasle
    public int age { get; set; }
    public string occupation { get; set; } //职业
}

List<User> list = new List<User>()
{
    new User { id = 1, name = "Zhang Long", age = 38, gender = true, occupation = "Teacher"},
    new User { id = 2, name = "Zhang Jin", age = 18, gender = false, occupation = "Student"},
    new User { id = 3, name = "Zhang Shuai", age = 38, gender = false, occupation = "Teacher"},
    new User { id = 4, name = "Liu Guangzhi", age = 38, gender = false, occupation = "Doctor"},
    new User { id = 5, name = "Liu Ziming", age = 38, gender = true, occupation = "Doctor"},
    new User { id = 6, name = "Liu Shuai", age = 29, gender = false, occupation = "Doctor"},
    new User { id = 7, name = "Liu Jin", age = 21, gender = true, occupation = "Builder"},
    new User { id = 8, name = "Jiang Long", age = 38, gender = true, occupation = "Builder"},
    new User { id = 9, name = "Hu Ziming", age = 21, gender = true, occupation = "Student"},
    new User { id = 10, name = "Hu Jin", age = 21, gender = false, occupation = "Student"}
};

后面大家根据自己的需要跳转到各个章节学习！

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额外准备工作（章节四、五、七）：

class Salary
{
    public int id { get; set; } 
    public int user_id { get; set; } 
    public string name { get; set; } 
    public double salary { get; set; }
    public bool active { get; set; }
    public string occupation { get; set; } //职业
}

List<Salary> list = new List<Salary>()
{
    new Salary { id = 1,  user_id = 1, name = "Zhang Long", salary = 7800, active = true, occupation = "Teacher"},
    new Salary { id = 2,  user_id = 2, name = "Zhang Jin", salary = 1500, active = true, occupation = "Student"},
    new Salary { id = 3,  user_id = 3, name = "Zhang Shuai", salary = 8800, active = false, occupation = "Teacher"},
    new Salary { id = 4,  user_id = 4, name = "Liu Guangzhi", salary = 12800, active = true, occupation = "Doctor"},
    new Salary { id = 5,  user_id = 5, name = "Liu Ziming", salary = 13600, active = true, occupation = "Doctor"},
    new Salary { id = 6,  user_id = 6, name = "Liu Shuai", salary = 29000, active = false, occupation = "Doctor"},
    new Salary { id = 7,  user_id = 7, name = "Liu Jin", salary = 7000, active = true, occupation = "Builder"},
    new Salary { id = 8,  user_id = 8, name = "Jiang Long", salary = 8500, active = false, occupation = "Builder"},
    new Salary { id = 9,  user_id = 9, name = "Hu Ziming", salary = 2100, active = true, occupation = "Student"},
    new Salary { id = 10,  user_id = 10, name = "Hu Jin", salary = 1300, active = true, occupation = "Student"}
};