C# Linq 和 SQL的左连接、右连接、内链接

在我们工作中表连接是很常用的，但常用的有这三种连接方式：左连接、右连接、内链接

在本章节中讲的是
1、如何在Linq中使用左连接，右连接，内连接。

2、三种连接之间的特点在哪？

3、Linq的三种连接语法是怎么样的呢（我觉得左右连接也就相当换个位置）

一、SQL Server 中的三种连接

首先我们示范以下SQL中的左连接，右连接，内连接，需要准备两张表：

CREATE TABLE [dbo].[Company](
    [Id] int identity Primary key,
    [Code] uniqueidentifier,
    [CompanyName] nvarchar(20) null,
    [Address] nvarchar(100) null
)
CREATE TABLE [dbo].[People](
    [Id] int identity Primary key,
    [CompanyCode] uniqueidentifier null,
    [Name] nvarchar(20) null,
    [Age] Int null,
)

数据准备

Company

 People

在SQL表连接 LEFT JOIN，RIGHT JOIN , INNER JOIN (JOIN)

--通过LEFT JOIN 关键字很明显看出是左连接
SELECT * FROM [People] p LEFT JOIN Company c on p.CompanyCode = c.Code

--通过RIGHT JOIN 关键字很明显看出是右连接
SELECT * FROM [Company] c RIGHT JOIN [People] p on p.CompanyCode = c.Code

--通过INNER JOIN 关键字很明显看出是内连接
SELECT * FROM [People] p INNER JOIN [Company] c on p.CompanyCode = c.Code

执行后的结果

各连接的特点：

左连接和右连接很好理解，看字面意思就可以知道左连接是按照左边的数据进行显示，如果右边的跟左边的匹配不到则右边的数据会为空，右连接相反

内链接也很好理解，看图可以看出来只要能匹配上的就显示不能匹配的就不显示数据

那么在C# Linq中我们如何实现表连接呢？

二、C# 中使用Linq实现集合之间的连接

同样先准备下类和数据源

Company和People 类

public class Company
{
    public int Id { get; set; }
    public Guid Code { get; set; }
    public string CompanyName { get; set; }
    public string Address { get; set; }
}

public class People
{
    public int Id { get; set; }
    public Guid CompanyCode { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
}

初始数据

List<Company> companyList = new List<Company>(){
    new Company() {
        Id=1,
        Code = new Guid("B0C5900C-DC15-4F9B-A2C0-25D79B2987E2"),
        CompanyName = "菜鸟公司",
        Address = "菜鸟地址",
    },
    new Company() {
        Id=2,
        Code = new Guid("FA46534F-E5FA-48DD-8C67-58A8FC5573FB"),
        CompanyName = "中级地址",
        Address = "中级地址",
    },
    new Company() {
        Id=3,
        Code = new Guid("8188B6E3-1FEA-462F-83A5-A6610996DB42"),
        CompanyName = "牛X公司",
        Address = "牛X地址",
    },
    new Company() {
        Id=4,
        Code = new Guid("64CD13EB-398A-4DC2-BFBF-F4032A40DC10"),
        CompanyName = "黑马公司",
        Address = "黑马地址",
    },
};


List<People> peopleList = new List<People>() {
    new People() {
        Id = 1,
        CompanyCode = new Guid("B0C5900C-DC15-4F9B-A2C0-25D79B2987E2"),
        Name = "Tony",
        Age = 18
    },
        new People() {
        Id = 2,
        CompanyCode = new Guid("B0C5900C-DC15-4F9B-A2C0-25D79B2987E2"),
        Name = "Ramon",
        Age = 18
    },
        new People() {
        Id = 3,
        CompanyCode = new Guid("FA46534F-E5FA-48DD-8C67-58A8FC5573FB"),
        Name = "Ronnin",
        Age = 18
    },
        new People() {
        Id = 4,
        CompanyCode = new Guid("8188B6E3-1FEA-462F-83A5-A6610996DB42"),
        Name = "Vi Vi",
        Age = 18
    },
        new People() {
        Id = 4,
        CompanyCode = new Guid("E0E04E6B-4E0F-4A83-AFDB-30427221448E"),
        Name = "Gary Chan",
        Age = 18
    },
};

Linq连接代码

{
    Console.WriteLine("-----------------------------------------------------LEFT JION---------------------------------------------------------");
    var qurey = from c in companyList
                join p in peopleList
                on c.Code equals p.CompanyCode into joinCompanyList
                from p in joinCompanyList.DefaultIfEmpty()
                select new
                {
                    Id = c?.Id,
                    CompanyName = c?.CompanyName,
                    CompanyAddress = c?.Address,
                    peopleName = p?.Name,
                    peopleAge = p?.Age
                };
    foreach (var item in qurey)
    {
        Console.WriteLine($"公司ID:{item.Id},公司名称:{item.CompanyName},公司地址:{item.CompanyAddress},个人名称:{item.peopleName},个人年龄:{item.peopleAge}");
    }
}
{

    Console.WriteLine("-----------------------------------------------------RIGHT JION---------------------------------------------------------");
    var qurey = from p in peopleList
                join c in companyList
                on p.CompanyCode equals c.Code into joinCompanyList
                from c in joinCompanyList.DefaultIfEmpty()
                select new
                {
                    Id = c?.Id,
                    CompanyName = c?.CompanyName,
                    CompanyAddress = c?.Address,
                    peopleName = p?.Name,
                    peopleAge = p?.Age
                };
    foreach (var item in qurey)
    {
        Console.WriteLine($"公司ID:{item.Id},公司名称:{item.CompanyName},公司地址:{item.CompanyAddress},个人名称:{item.peopleName},个人年龄:{item.peopleAge}");
    }
}

{

    Console.WriteLine("-----------------------------------------------------INNER JOIN---------------------------------------------------------");
    var qurey = from p in peopleList
                join c in companyList
                on p.CompanyCode equals c.Code
                select new
                {
                    Id = c?.Id,
                    CompanyName = c?.CompanyName,
                    CompanyAddress = c?.Address,
                    peopleName = p?.Name,
                    peopleAge = p?.Age
                };
    foreach (var item in qurey)
    {
        Console.WriteLine($"公司ID:{item.Id},公司名称:{item.CompanyName},公司地址:{item.CompanyAddress},个人名称:{item.peopleName},个人年龄:{item.peopleAge}");
    }
}

结果

注：上例中使用了DefaultIfEmpty操作符，它能够为实序列提供一个默认的元素。DefaultIfEmpty使用了泛型中的default关键字。default关键字对于引用类型将返回null,而对于值类型则返回0。对于结构体类型，则会根据其成员类型将它们相应地初始化为null(引用类型)或0(值类型)