//使用组链接
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
};
//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
};
内链接与SqL中inner join同样，即找出两个序列的交集。以下例找出book中的Publisher存在于SampleData.Publishers的资料。ast
//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
};泛型
//join操做符语句
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
});
左外链接与SqL中left join同样。以下例找出根据publisher中找出SampleData.Publishers中全部资料和book中存在于publisher的资料。
//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操做符，它可以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)。
//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
};
var crossJoinQuery = from publisher in SampleData.Publishers
from book in SampleData.Books
select new
{
PublisherName = publisher.Name,
BookName = book.Title
};
//不使用查询表达式
SampleData.Publishers.SelectMany(publisher => SampleData.Books.Select(
book => new
{
PublisherName = publisher.Name,
BookName = book.Title
}
));
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] } }; }