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
}
));