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] } }; }