标签：连接 默认 div book ros 判断 分组 左连接 sel
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。
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)) ? "" : book.Title //这里主要第二个集合有可能为空。需要判断 };
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 } ));
5.Linq实现左连接，写法如下
var leftJoinSql = from student in db.Student join book in db.Book on student.ID equals book.StudentID into temp from tt in temp.DefaultIfEmpty() select new { sname= student.Name, bname = tt==null?"":tt.Name//这里主要第二个集合有可能为空。需要判断 };
6.Linq实现右连接，写法如下
var rightJoinSql = from book in db.Book join stu in db.Student on book.StudentID equals stu.ID into joinTemp from tmp in joinTemp.DefaultIfEmpty() select new { sname=tmp==null?"":tmp.Name, bname=book.Name };
标签：连接 默认 div book ros 判断 分组 左连接 sel
原文地址：http://www.cnblogs.com/kongxiaoshuang/p/6782381.html
