Linq中连接主要有组连接、内连接、左外连接、交叉连接四种

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

}

));