Linq中的连接(join)

Linq中连接主要有组连接、内连接、左外连接、交叉连接四种。各个用法如下。

    注：本文内容主要来自《Linq实战》，本例中用到的对象请见文章底部。

1、 组连接

    组连接是与分组查询是一样的。即根据分组得到结果。 如下例,根据publisther分组得到结果。

    使用组连接的查询语句如下：

            //使用组连接
            var GroupQuery = from publisher in SampleData.Publishers
                             join book in SampleData.Books
                                  on publisher equals book.Publisher into publisherBooks
                             select new
                             {
                                 PublisherName = publisher.Name,
                                 Books = publisherBooks
                             };

    与上边等同的GroupBy语句如下：

            //使用Group
            var QueryByGroup = from book in SampleData.Books
                        group book by book.Publisher into grouping
                        select new
                        {
                            PublisherName = grouping.Key.Name,
                            Books = grouping
                        };

2、内连接

    内连接与SqL中inner join一样，即找出两个序列的交集。如下例找出book中的Publisher存在于SampleData.Publishers的资料。

    内连接查询语句如下：

            //join查询语句
            var joinQuery = from publisher in SampleData.Publishers
                            join book in SampleData.Books
                                on publisher equals book.Publisher
                            select new
                            {
                                PublisherName = publisher.Name,
                                BookName = book.Title
                            };

    与上边等同的查询操作符语句如下：

            //join操作符语句
            SampleData.Publishers.Join(
                SampleData.Books,               //join 对象
                publisher => publisher,         //外部的key
                book => book.Publisher,         //内部的key
                (publisher, book) => new        //结果
                {
                    PublisherName = publisher.Name,
                    BookName = book.Title
                });

3、左外连接

     左外连接与SqL中left join一样。如下例找出根据publisher中找出SampleData.Publishers中所有资料和book中存在于publisher的资料。

     左外连接查询语句如下：

            //left join, 为空时用default
            var leftJoinQuerybyDefault = from publisher in SampleData.Publishers
                                         join book in SampleData.Books
                                           on publisher equals book.Publisher into publisherBooks
                                         from book in publisherBooks.DefaultIfEmpty()
                                         select new
                                         {
                                             PublisherName = publisher.Name,
                                             BookName = (book == default(Book)) ? "no book" : book.Title
                                         };

     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)。

    我们可以不使用default关键字，但在要DefaultIfEmpty中给定当空时的默认对象值。语句如下：

            //left join, 为空时使用默认对象
            var leftJoinQuery = from publisher in SampleData.Publishers
                                        join book in SampleData.Books
                                          on publisher equals book.Publisher into publisherBooks
                                        from book in publisherBooks.DefaultIfEmpty(
                                        new Book { Title = "" }                         //设置为空时的默认值
                                        )
                                        select new
                                        {
                                            PublisherName = publisher.Name,
                                            BookName = book.Title
                                        };

4、交叉连接

    交叉连接与SqL中Cross join一样。如下例中找出SampleData.Publishers与SampleData.Books的交叉连接。

    交叉连接查询语句：

            var crossJoinQuery = from publisher in SampleData.Publishers
                                 from book in SampleData.Books
                                 select new
                                 {
                                     PublisherName = publisher.Name,
                                     BookName = book.Title
                                 };

    查询操作符语句：

            //不使用查询表达式
            SampleData.Publishers.SelectMany(publisher => SampleData.Books.Select(
                book => new
                {
                    PublisherName = publisher.Name,
                    BookName = book.Title
                }
                ));

 本像用到的对象：

  static public class SampleData
  {
    static public Publisher[] Publishers =
    {
      new Publisher {Name="FunBooks"},
      new Publisher {Name="Joe Publishing"},
      new Publisher {Name="I Publisher"}
    };

    static public Author[] Authors =
    {
      new Author {FirstName="Johnny", LastName="Good"},
      new Author {FirstName="Graziella", LastName="Simplegame"},
      new Author {FirstName="Octavio", LastName="Prince"},
      new Author {FirstName="Jeremy", LastName="Legrand"}
    };

    static public Subject[] Subjects =
    {
      new Subject {Name="Software development"},
      new Subject {Name="Novel"},
      new Subject {Name="Science fiction"}
    };

    static public Book[] Books =
    {
      new Book {
        Title="Funny Stories",
        Publisher=Publishers[0],
        Authors=new[]{Authors[0], Authors[1]},
        PageCount=101,
        Price=25.55M,
        PublicationDate=new DateTime(2004, 11, 10),
        Isbn="0-000-77777-2",
        Subject=Subjects[0]
      },
      new Book {
        Title="LINQ rules",
        Publisher=Publishers[1],
        Authors=new[]{Authors[2]},
        PageCount=300,
        Price=12M,
        PublicationDate=new DateTime(2007, 9, 2),
        Isbn="0-111-77777-2",
        Subject=Subjects[0]
      },
      new Book {
        Title="C# on Rails",
        Publisher=Publishers[1],
        Authors=new[]{Authors[2]},
        PageCount=256,
        Price=35.5M,
        PublicationDate=new DateTime(2007, 4, 1),
        Isbn="0-222-77777-2",
        Subject=Subjects[0]
      },
      new Book {
        Title="All your base are belong to us",
        Publisher=Publishers[1],
        Authors=new[]{Authors[3]},
        PageCount=1205,
        Price=35.5M,
        PublicationDate=new DateTime(2006, 5, 5),
        Isbn="0-333-77777-2",
        Subject=Subjects[2]
      },
      new Book {
        Title="Bonjour mon Amour",
        Publisher=Publishers[0],
        Authors=new[]{Authors[1], Authors[0]},
        PageCount=50,
        Price=29M,
        PublicationDate=new DateTime(1973, 2, 18),
        Isbn="2-444-77777-2",
        Subject=Subjects[1]
      }
    };
  }

*************转载：https://www.cnblogs.com/scottckt/archive/2010/08/11/1797716.html